As a percent of scale length (or percent of full scale). This rating method is most commonly employed with instruments equipped with an analog meter.Definitions
"Accuracy" as defined in the ANSI/ISA—51.1—1979 (R1993) standard Process Instrumentation Terminology is:
[T]he degree of conformity of an indicated value to a recognized accepted standard value, or ideal value.
Two common methods of rating or expressing accuracy are:
As a percent of scale length (or percent of full scale). This rating method is most commonly employed with instruments equipped with an analog meter.
As a percent of actual output reading. This rating method has become more popular for instruments provided with a digital meter.
The above referenced standard also notes that:
As a performance specification, accuracy (or reference accuracy) shall be assumed to mean accuracy rating of the device, when used at reference operating conditions. Accuracy rating includes the combined effects of conformity, hysteresis, dead band, and repeatability errors.
What Accuracy Is Not
In the world of gas detection instrumentation, as in other places, confusion exists. Consensus standards are helpful, of course, but cannot be expected to address all practical issues. Finally, critical terms can have very different meanings in contexts outside of gas detection.
Accuracy is not minimum detectability
"Minimum detectability" is simply the lowest meter reading or other type of instrument output that can be unambiguously discriminated from noise. [Some agencies set a standard that minimum detectability must be at least 2-2.5 times the noise level.]
Note that any data garnered at the level of minimum detectability will not be very accurate. For example, in a typical case, the minimum detectability of a particular instrument, provided with an analog meter, is given as 1% of full scale, and accuracy is ± 2% of full scale. Thus, for a 0-100 ppm scale, the minimum detectable reading of 1 ppm would actually be 1 ppm ± 2 ppm—hardly a useful measurement.
Similarly, on a digital unit, the minimum detectability of a particular instrument is often given as the least significant digit. On a commonly used 3˝ digit meter, for a range of 0-199.9 ppm, this would be 0.1 ppm. In this case, accuracy is specified at ± 2% of reading ± 1 least significant digit. Here, the minimum detectable reading of 0.1 ppm would actually be 0.1 ppm ± 0.002 ppm ± 0.1 ppm. Technically better than the analog example, but still of little value.
Even so, knowing the minimum detectability of an instrument can be helpful in situations when "go/no-go" readings are of interest. Given a properly calibrated instrument, the smallest observable response would be—by definition—the minimum detectable level, and would indicate at least the presence of the analyte in question (any interferences notwithstanding).
Of course, such practices should only be done when instruments with more appropriate sensitivity are not available.
Accuracy is not precision
"Precision" as defined in ASTM Standard D 1356—05 Standard Terminology Relating to Sampling and Analysis of Atmospheres is:
The degree of agreement of repeated measurements of the same property, expressed in terms of dispersion of test results about the mean result obtained by repetitive testing of a homogeneous sample under specified conditions.
A classic illustration shows a target with many arrows closely clustered around the bull's-eye. This scenario would be both accurate and precise. If many arrows were closely clustered far from the bull's-eye, the situation would be precise, but not accurate. If arrows were all over the target, those closest to the bull's-eye would be accurate, but the archery session was not precise.
However, if this understanding of precision holds in the fields of science, engineering, and statistics, as the term is used in the world of computing, precision can be either:
The number of significant decimal digits or bits by which a particular value is expressed. For example, a calculation which rounds to three digits is said to have a working precision or rounding precision of 3.
The units of the least significant digit of a measurement. For example, if a measurement is 25.371 meters then its precision is millimeters (one unit in the last place, or "ulp," is 1 mm)
Accuracy is not resolution
"Resolution" as defined in the ANSI/ISA—51.1—1979 (R1993) standard Process Instrumentation Terminology is:
The least interval between two adjacent discrete details which can be distinguished one from the other.
We can invoke another use of "resolution" to help visualize this concept. In the computer world, "resolution" is a measure of the sharpness of an image or of the fineness with which a device (such as a video display, printer, or scanner) can produce or record such an image. Resolution in this context is usually expressed as the total number or density of pixels in the image—typically as dots per inch or dots per millimeter.
For example, a 600-dpi (dots per inch) printer is one that is capable of printing 600 distinct dots in a line 1 inch long. Thus, it can print 360,000 dots per square inch.
Now, draw the analogy between all these "distinct dots" and the least significant digit on a digital meter. We referred earlier to a 3˝ digit meter, set up for a measuring range of 0-199.9 ppm. Our least significant digit here is 0.1 ppm. There is no way to read a digital meter beyond this least significant digit. Or, to put it another way, the resolution of the meter is 0.1 ppm.
Clearly, attempting to measure 0.1 ppm on this meter would not be accurate. As shown earlier, for an instrument accuracy specified at ± 2% of reading ± 1 least significant digit, a measurement at the level of resolution would render 0.1 ppm ± 0.002 ppm ± 0.1 ppm.
If we consider resolution as it might apply to an instrument equipped with an analog meter, best practice would dictate that even though an eagle-eyed observer might be able to discern elements between the actual divisions on the meter, resolution in this case will be limited to the smallest meter division. Analog meters are rarely provided with more than 100 divisions on the scale, yielding a typical 1% of full scale resolution.
Given a 0-100 ppm analog meter, on an instrument with a stated accuracy of ± 2% of full scale, a reading at the resolution point would be 1 ppm ± 2 ppm.
Further Issues
The preceding discussion left out, for reasons of clarity and brevity, certain other factors that should be considered in any rigorous presentation of accuracy as it applies to gas detection.
Absolute method, definitive method, and reference method
An analytical measurement is concerned with determining the amount of a given analyte in a defined mass or volume of the sample. If the analyte cannot directly be counted or measured, then a macroscopic parameter must be found which is functionally related to the amount (concentration) of the analyte.
Interscan, as a matter of course, has always included the disclaimer "Limited to the accuracy of the calibration standard" in any designation of the accuracy of our gas analyzers. This is an important point, since our instruments—and most other gas detection devices—must be calibrated against a known standard, commonly called a "span gas." Furthermore, for best performance, most gas detection instruments should also be zeroed with a zero gas.
In analytical work, this type of instrument is said to be a "reference method". This term is in contrast to an absolute method or a definitive method.
An "absolute method" is a method of of chemical analysis that bases characterization completely on standards defined in terms of physical properties.
A "definitive method" [per IUPAC Compendium of Chemical Terminology 2nd Edition (1997)] is a method of exceptional scientific status which is sufficiently accurate to stand alone in the determination of a given property for the certification of a reference material. Such a method must have a firm theoretical foundation so that systematic error is negligible relative to the intended use.
A "reference method" [per IUPAC Compendium of Chemical Terminology 2nd Edition (1997)] is a method having small, estimated inaccuracies relative to the end use requirement. The accuracy of a reference method must be demonstrated through direct comparison with a definitive method or with a primary reference material.
Commercial gas calibration standards are NIST-traceable, and can be obtained with stated accuracies of ± 2 percent, or even better in certain cases. While permeation devices are also NIST-traceable, and are often the only available standards for certain gases, since carrier gas flow rate and temperature of the permeation oven must be carefully controlled, additional sources of error are introduced.
Either way, there is more to be worried about in a gas concentration measurement than the inherent accuracy of the instrument itself.
Errors
If we wish to determine the overall accuracy of our gas detection measurement, we must then somehow combine all the known sources of error. For the most part, errors deriving from the instrument and errors deriving from the calibration method can be considered as being "additive."
An "additive error" is an error that is added to the true value, and does not depend on the true value itself. Thus, the result of the measurement is the sum of the true value and the additive error(s).
Consider the example of a 50 ppm reading taken on a digital instrument with a stated accuracy of ± 2% of reading ± 1 least significant digit, and a measuring range of 0-199.9 ppm. Also include the fact that you calibrated the instrument with a 50 ppm standard, having an accuracy of ± 2%.
This becomes 50 ppm ± 1 ppm ± 0.1 ppm ± 1 ppm. Simplifying this, the true value of your measurement lies somewhere between 47.9 and 52.1 ppm.
It is sometimes asked what concentration of span gas should be used to calibrate the instrument. Unfortunately, there is not a simple answer. Best analytical practice is to calibrate and measure at or very near the same point, but this is not easily achievable in all cases, and would defeat the purpose of any instrument with a reasonably useful measuring range.
In the real world, instrument linearity is specified, and a calibration curve, whereby a table or graph of the measured relationship of an instrument as compared over its range against a known source, can be provided. Moreover, instrument accuracy ratings include linearity, which is just a special case of conformity.
However, if a calibration standard is employed that forces the user to read the instrument in a very low region of the measuring range in order to make the calibration setting, the diminished accuracy of that reading will manifest itself as systematic error.
A "systematic error" is an error that is constant in a series of repetitions of the same experiment or observation.
Contrast this with a random error.
A "random error" is the fluctuating part of the overall error that varies from measurement to measurement.
Generally, systematic errors are more insidious than random errors, because their magnitude cannot be reduced by simple repetition of the measurement procedure. In this case, the calibration error derived from too low a calibration standard will also be a multiplicative error, as it is proportional to the true value of the quantity being measured, and will get worse, the higher the instrument reading.
Environmental effects, such as changes in ambient temperature and pressure will affect nearly all gas detection methods, and must be taken into account. Many analytical methods are also affected by changes in ambient humidity.
For highly reactive gases such as hydrazine and chlorine dioxide, chemisorption effects on all instrument wetted parts, including sample probes, can affect the measurements. Likewise, accumulated water and particulate in wetted areas can impair accuracy.
And, virtually unique to gas detection is the problem that other compounds can produce similar outputs on the instrument, causing so-called interferences. The prudent instrument user should seek out comprehensive interference data, as it would pertain to his application.
Finally, one could argue that a particular gas detection measurement suffers from some sampling error, in that the portion of air being sampled is not truly representative of the area in question. While there may be rare examples of this phenomenon occurring, the conditions producing such an anomaly would likely be known to the individual performing the measurement. In nearly all cases, though, we can assume that the gas molecules will properly diffuse throughout the environment, so that a sample taken anywhere can be considered representative.
Conclusion
There are many factors involved in obtaining accurate gas detection measurements, and they extend well beyond the instrument itself. Calibration standards, calibration methods, ambient conditions, chemisorption, entrained water and particulate, and interferences can all play a part in destroying accuracy.
At Interscan, we stand ready to work with you, to help you achieve the most accurate results—in a cost-effective manner—for your gas detection application.
Thanks for your question. The answer depends on what you mean by "double," and even on how we define "detector."
Usually, "detector" means the sensing element within the instrument. But, people sometimes refer to the entire instrument as the detector.
Now, if the sensor for carbon monoxide also responded to propane, this would not be desirable, as the propane would represent an interference, and cause errors in the CO measurement.
However, if you want to find an instrument that can measure both CO and propane, correctly, and without interferences, there are several suppliers (although not Interscan) who provide multi-gas units, that would employ an electrochemical sensor for CO, and a pellistor type sensor for hydrocarbons--and that would include propane.
If you will be using this type of instrument, make sure that the hydrocarbon sensor is calibrated for propane.
Background
In the continuing regulatory assault on ethylene oxide, more rules now apply. Fortunately, these latest statutes are not at all difficult to comply with. The new rule is called "National Emission Standards for Hospital Ethylene Oxide Sterilizers." It was published on 28 December 2007 as 40 CFR Part 63, with particular notice given to subpart WWWWW.
EPA is issuing national emissions standards for new and existing hospital sterilizers that emit hazardous air pollutants and are area sources within the meaning of Clean Air Act section 112(a)(2). The final rule is based on EPA's determination as to what constitutes the generally available control technology or management practices for the hospital sterilizer area source category.
This action is being finalized as part of EPA's obligation to regulate area sources listed for regulation pursuant to Clean Air Act section 112(c)(3).
Source Material
Here is a link to the final rule.
Here is a link to subpart WWWWW.
Here is a link to EPA's summary of the regulations.
Compliance
Our good friends at Honeywell Oxyfume® Sterilants have prepared an explanatory document, with guidelines on how to comply with the new rule. The guidelines would also apply to 100% EtO and cartridge systems.
The EPA has determined that Andersen sterilizers are effectively exempt from the rule, since in their method, sterilization bags are utilized, and these bags are always fully loaded—thus comprising "fully loaded sterilization units," as are required by Subpart WWWWW.
This ruling was made—as it must be—in response to an inquiry submitted by an Andersen customer.
When a gas concentration is measured or reported, we may talk in terms of ppm, but in reality we are measuring the partial pressure of the gas. (Refer to Dalton's Law of Partial Pressures)
Thus, any instrument reading is going to be affected by changes in ambient pressure. It is necessary to correct the readings, if the instrument will be used in an environment that is at a different atmospheric pressure from where the instrument was calibrated.
To correct for the influence of pressure, the value read on the instrument must be multiplied by the following correction factor:
|
Atmospheric pressure where calibrated |
It is the ratio that is important, not the units. Whether you are measuring atmospheric pressure in millibars, millimeters of mercury, inches of mercury, or kilopascals, it does not matter, so long as you are consistent.
For maximum accuracy, be aware that most barometers must also be corrected for temperature and gravity effects.
Finally, if aboard an aircraft, it is the cabin pressure that is of importance, rather than the exterior pressure.
"Dew Point" is defined as the temperature to which a given volume of air must be cooled at constant pressure and constant water vapor content in order for saturation to occur. If the air is cooled further, some of the moisture will condense.
The notion of "Saturated Vapor Pressure" can be understood by considering the process of evaporation in a closed container of water, with some air above the liquid. Evaporation will proceed until there are as many molecules returning to the liquid as there are escaping. At this point the vapor is said to be saturated, and the pressure of that vapor (usually expressed in millimeters of mercury [mmHg]) is called the saturated vapor pressure.
Since the molecular kinetic energy is greater at higher temperature, more molecules can escape the surface and the saturated vapor pressure is correspondingly higher. If the liquid is open to the air, then the vapor pressure is seen as a partial pressure along with the other constituents of the air.
For the sake of completeness, "Relative Humidity" is defined as the ratio of the partial pressure of water vapor in a gaseous mixture of air and water vapor, to the saturated vapor pressure of water—at a given temperature.
Relative humidity is expressed as a percentage and is calculated as follows:
Where
is the relative humidity of the gas mixture being considered
is the partial pressure of water vapor in the gas mixture
is the saturation vapor pressure of water at the temperature of the gas mixture
As you can see, while relative humidity might be a useful benchmark for weather reporting, dew point is the parameter of choice when one is concerned about condensation in a gas sample to be analyzed—especially one coming from a process stack.
Condensation can foul sampling lines, harm instruments, and in many cases can dissolve a portion of the very sample you are trying to analyze. For example, such gases as bromine, chlorine, chlorine dioxide, formaldehyde, hydrogen chloride, nitrogen dioxide, and sulfur dioxide are all quite soluble in water.
Given the dew point of the sample in question, you will immediately know that once it is brought to a temperature below this, condensation will occur.
OK, but how can this be determined for a typical stack sample?
In nearly all cases, previous analyses of the stack sample will have reported the percent of water contained therein, as this is important for process control purposes. Given that information, one need only consult a table showing the saturation vapor pressure of water as a function of temperature, and the dew point is instantly determined.
Let us consider the examples of 10 percent and 20 percent water in the sample:
1. First, download the table. Note that the spreadsheet is divided into six sections, separated by green bars. Each section provides vapor pressure data over a particular temperature range. All told, data is presented for temperatures from -10 deg C (14.0 deg F) to 114 deg C (237.2 deg F).
2. Assume that the gas sample in question is at atmospheric pressure—760 mmHg. By Dalton's Law of Partial Pressures, 10 percent water would indicate a vapor pressure of 76 mmHg.
3. Take a look at the table. Although 76 mmHg does not appear, 75.7 does, and this is close enough. The corresponding temperature is 46 deg C (114.8 deg F), and that is the dew point of this sample.
4. For 20 percent water, a vapor pressure of 152 mmHg would be indicated. By reference to the table, the dew point is seen to be somewhere between 60 and 61 deg C (140-141.8 deg F). Prudence would dictate using the higher temperature as the operational dew point.
Interscan is here to help you with all application and technical issues. Feel free to contact us at any time.
As you may know, surrogate gas calibration for chlorine dioxide is recommended because it is extremely difficult to generate a stable source of ClO2, especially for practical field use. Indeed, the use of chlorine as a surrogate for chlorine dioxide calibration is one of the very few instances whereby Interscan advocates such a method.
Pioneering work by NCASI, the The National Council for Air and Stream Improvement, had pegged the response ratio such that approximately 3 ppm of chlorine is required to cause a 1 ppm reading on an Interscan chlorine dioxide analyzer.
Subsequent efforts have refined the number a bit. We now advise our customers that:
2.8 ppm of chlorine will show a 1 ppm reading on a chlorine dioxide analyzer, and this ratio is stated with an accuracy of ±10%
In many cases, the terms "gas detector" and "gas monitor" are used interchangeably, and this can lead to confusion.
When the word "detector" is mentioned in the context of a detector tube, it is clear that we are referring to short term or even one-shot grab sample measurements. Moreover, when someone speaks of monitoring a work area for the presence of a toxic gas, we can certainly infer that the effort is going to be long term, if not 24/7 continuous.
However, as we said, "detector" and "monitor" are often used interchangeably. Thus, it may not be that helpful to simply put a name on a given piece of equipment. Instead, you should look at it functionally.
Is the piece of equipment intended for long term or short term service? Generally, a battery-powered piece of equipment is intended for short term survey work, while line-powered units are intended for continuous application. Also, an instrument intended for continuous use will normally have a more robust appearance.
While guidelines may exist that mandate monitoring of an area for any number of toxic compounds, no official government recommendations are made as to the detailed design of such a monitoring system.
In practice, the design of a toxic gas area monitoring system results from a collaboration between the end-user, consultant(if any), monitoring system vendor, and sometimes the regulatory agency involved.
Saying that, a few fundamental design parameters can be mentioned here:
The location and number of detection points must first and foremost consider the people working in the area, and where they will be located.
In applications whereby leaks from equipment or upsets in a process could cause problems, points should be located near likely leak points, with ventilation characteristics taken into account.
Alarm functionality should serve the dual purposes of warning the affected employees in the area, as well as shutting down the process causing the alarm situation.
In certain cases, emergency ventilation might be utilized to drive the offending toxic out of the area. It is prudent in these cases to sample key intake registers—thus sampling the oldest or most stale air in the monitored area.
"Minimum Detectability" is one of those instrumentation terms that is used frequently, but is seldom defined. Indeed, even though you will encounter this term on many data sheets, its definition does not appear in any of the usual learned references, including Process Instrumentation Terminology, ANSI/ISA—51.1—1979(R1993) and Standard Terminology Relating to Sampling and Analysis of Atmospheres, ASTM D 1356 - 05.
However, the ASTM standard does provide us with...
Method Detection Limit: The minimum concentration of an analyte that can be reported with a 99% confidence that the value is above zero, based on a standard deviation of greater than seven replicate measurements of the analyte in the matrix of concern at a concentration near the low standard.
Simplifying this, we can say that "Minimum Detectability" is the lowest concentration of analyte that can be unambiguously discriminated from noise. [Some agencies set a standard that minimum detectability must be at least 2-2.5 times the noise level.] Fair enough, but how can this be utilized in occupational health or process gas detection?
First of all, it is important to note that any data garnered at the level of minimum detectability will not be accurate. For example, in a typical case, the minimum detectability of a particular instrument is given as 1% of full scale, and accuracy is ± 2% of full scale. Thus, for a 0-100 ppm scale, the minimum detectable reading of 1 ppm would actually be 1 ppm ± 2 ppm—hardly a useful measurement.
Similarly, on a digital unit, the minimum detectability of a particular instrument is often given as the least significant digit. On a commonly used 3˝ digit meter, for a range of 0-199.9 ppm, this would be 0.1 ppm. In this case, accuracy is specified at ± 2% of reading ± 1 least significant digit. Here, the minimum detectable reading of 0.1 ppm would actually be 0.1 ppm ± 0.002 ppm ± 0.1 ppm. Technically better than the analog example, but still of little value.
Even so, knowing the minimum detectability of an instrument can be helpful in situations when "go/no-go" readings are of interest. Given a properly calibrated instrument, the smallest observable response would be—by definition—the minimum detectable level, and would indicate at least the presence of the analyte in question (any interferences notwithstanding).
Of course, such practices should only be done when instruments with more appropriate sensitivity are not available.
The dictionary defines "danger" as "the state of being exposed to harm : liability to injury, pain, or loss." Typically, gas detection is deployed in areas that contain potential danger.
This danger may result from some sort of upset condition, as in a leak—representing an immediate toxic or explosive risk—or it could be more insidious, as in a long-term relatively low-level exposure to some toxic compound, devastating only in its cumulative effects.
Although many compounds list both occupational exposure limits and lower/upper explosive limits, in practice, there are few common hazardous gases that will present themselves as both toxic and combustible dangers. One of these is ethylene oxide (EtO). Even though the widest use of this chemical is in chemical syntheses, in terms of numbers of people affected, the health care related applications are of the greatest interest.
A recent case serves to illustrate the dangers of EtO, and what can happen if those concerned—both on the user and regulatory sides—lose sight of this.
EtO is an essential sterilant, used on the many medical devices that cannot take the heat of steam. Despite the introduction of numerous processes that promised to replace EtO, none has. Indeed, Johnson & Johnson is still one of the world's biggest health care consumers of EtO even though it also markets a sterilizer touted as a partial EtO replacement.
On 19 August 2004, at a contract sterilization facility in Ontario, California, mishandled EtO caused an explosion, that resulted in injuries to four employees, and damage to the facility that disrupted normal operations for nine months.
Brilliant and detailed investigative work on this incident was performed by the U.S. Chemical Safety and Hazard Investigation Board (CSB).
CSB's webpage on the investigation is here
A very informative video, featuring forensic-quality animation, is available for live-streaming.
DVD copies of this program, and others created by CSB, are available on request.
After studying CSB's materials, one will note that:
1. In general, all concerned seemed to be detached from the reality the EtO is explosive. During a maintenance procedure, a key aeration phase was bypassed, and shatter-resistant glass was not utilized in the control room windows overlooking the process. Indeed, every injury was caused by flying glass.
2. System designers were under the quite mistaken belief that monitoring pressure can substitute for gas detection.
3. In its zeal to protect us from "evil" EtO, California and certain other states continue to require the treatment of backvent EtO emissions, even though the Federal EPA (hardly an anti-Green organization) eliminated this requirement in 2001. That catalytic oxidizers—featuring live flames—are needed in this process doesn't seem to bother anyone.
Therefore, in the minds of regulators, the very questionable "hazard" of venting EtO to the atmosphere, as could have been treated by the much safer chemical scrubber method, is somehow more important than the obvious risks of employing the catalytic method, which, in fact, catalyzed this explosion.
Lest we forget, the biggest danger involved in this entire enterprise is that if not properly sterilized, the medical devices can cause serious illness or death to hapless patients! Indeed, if EtO were used more, and the various alternative methods were used less, we could probably put a dent in the more than 100,000 deaths caused by in-hospital infections, that occur every year in the United States.
It is also well worth remembering that with technological solutions do come some potential dangers. No one argues that we should return to the horse and buggy, even if the gasoline in automobiles is dangerous. Users of gasoline are cautious, and understand the hazards involved.
Similarly, sterilization is a boon that drives beneficial invasive medical care. EtO and other potentially dangerous chemicals can be handled safely, but only if both the users and the regulators understand the dangers, and impose logic, right reason, context, and perspective on the methods involved.
Most codes and standards—at least regarding allowable concentrations of various substances—are promulgated by domestic jurisdictions, whether at the state/province or national governmental level.
However, there are numerous international organizations that publish recommendations on best practices and safe handling of chemicals, including:
International Program on Chemical Safety (IPCS)
National Fire Protection Association (NFPA)--which despite the name is truly international
International Occupational Hygiene Association
Center for Chemical Process Safety
Definitions
A sample draw system uses a pump to draw sample back into the instrument, where it enters the sensor for analysis, and is then exhausted to the atmosphere or vent line.
Gas detection instruments with diffusion sensors do not use a pump to pull the sample to the sensor. Instead, they rely on the inherent movement of the air to direct a sample to them. In portable/survey applications, the sensors are usually located integral to the analyzer enclosure, but can also be deployed remotely on an umbilical electrical line. In multipoint systems, they are mounted remotely in the field, and connected electrically to the analyzer control panel, or SCADA (Supervisory Control and Data Acquisition) system. In some cases, wireless communication can be used.
Historical Background
Operating a gas sensor in the diffusion mode gained popularity with the advent, some years ago, of "catalytic" type combustible gas sensors. These sensors are packaged in explosion-proof housings (required by where they must be installed), and a signal is delivered back to the control panel. The package is called a sensor "head."
Clearly, there are good reasons why you would not want to bring a potentially combustible mixture back into an instrument cabinet. However, among the many detection points of a typical combustible application, there may be locations that are simply not suited to a remote sensor, either because one cannot be mounted close enough, or maintenance would be difficult or impossible at that particular spot.
Thus, an intrinsically safe or explosion-proof pump would be employed to direct sample back to the diffusion sensor head, along with some sort of special appurtenance to adapt the diffusion head to a flow-though sampling arrangement.
For the most part, widespread combustible gas detection preceded toxic gas detection in the marketplace. Unfortunately, because of the superficial similarity of the two applications (you are detecting "gas" in both of them), analogies were drawn between the two, and a great deal of confusion was created, much of which persists to this day. In fact, there are more differences than similarities:
Combustible gas detection deals with concentrations in percent levels, while toxic gas detection is in the realm of parts-per-million—10,000 times more sensitive, and much more demanding!
Notwithstanding certain chemicals that might poison a combustible gas sensor, there is usually no worry about interferences (other gases present in the environment that might affect the results).
While calibration of toxic gas sensors can be a significant and expensive issue, calibration of combustible gas sensors is generally straightforward, using stable gas blends, available in pressurized cylinders.
Toxic gases are usually much more reactive and unstable than combustible gases (assuming no ignition source). Many difficulties stem from this, including calibration standard stability and degradation of the sample to be analyzed.
Given their higher range of measurement, combustible gas sensors need calibration less frequently than toxic gas sensors.
Putting it another way, in the far less demanding world of combustible gas detection, diffusion sensors are an appropriate methodology.
Diffusion Sensors in Toxic Gas Applications
Now we can consider how diffusion sensors stack up in the rigorous and unforgiving world of toxic gas detection. First, we must note the inherent problems:
Because they rely on air movement rather than actively pulling a sample, the response time is usually slower.
Calibration is often more difficult, requiring special accessories to convert a diffusion mode of operation to flow-through for calibration purposes. Moreover, the implied equivalence between calibrating via flow-through and monitoring under diffusion is not always well-documented.
Calibration can be further complicated if adjustments must be made at the control panel, while gas is being applied to the remote diffusion sensor head.
In certain applications, interfering gases may be present. The sample draw approach allows for a chemical scrubber to be placed upstream of the sensor, to absorb the interferent. This is dicey at best with a diffusion sensor.
All gas sensors measure partial pressure, and a sample actively brought to the sensor is at a slightly elevated pressure, while a diffusion sensor operates at ambient pressure. As such, the output sensitivity of sample draw sensors is usually higher than diffusion sensors. This can be important for the many toxic gases with low regulatory levels.
In view of the foregoing, we can conclude that the more a toxic gas detection application matches the diminished rigor of a combustible gas application, the more it will be suitable for diffusion sampling. But how many toxic gases will have the necessary characteristics of...
a) Being chemically stable
b) Having a comparatively high regulatory concentration level
c) Presenting applications whereby interferences might not be an issue
The answer, of course, is not too many. I can only think of carbon monoxide (CO), certain alcohols, and a limited number of other organics. But, even CO applications can be problematical, as you will see.
A Carbon Monoxide App That Stymied Diffusion Sampling
CO monitoring is to be done at many points within a large enclosed area, that is a testing facility for all sorts of construction vehicles. Because there are no columns in the entire structure and extremely high vehicles will be in the facility, sampling must be done at ceiling level—44 feet (13.4 m) above the ground.
While remote diffusion sensors COULD be mounted to the ceiling, who would want to calibrate and maintain them up there? And, make no mistake: they would have to be maintained. Sometimes, the smoke can be intense, so that even if the diffusion head were protected with a sintered metal filter cap, it would have to be cleaned regularly. Not a good fit.
Our approach was to run tubing to the ceiling level for each point, going back to a central panel. All calibration and maintenance is performed at the panel. Normally, we use end-of-line filters at the sampling point, but these would not be appropriate in this app. So, we arranged in-line filters at ground level, and provided each sample line with an instrument air blowback feature, to take care of any particulate clogging problems.
The fact is, there are MANY apps that aren't simple enough to be handled properly with diffusion sampling. Sad to say, that doesn't prevent some vendors from shoehorning your app into their limited product offerings.
| Note: All exposure limits cited in this article are current as of 31 December 2006 |
Ultimately, the entire matter of where to set the instantaneous concentration alarm(s) is tied into what is expected by the regulatory agency. In the United States, for most workplace environments, it is the Occupational Health and Safety Administration (OSHA).
OSHA has published Permissible Exposure Limits, or PEL's, for many toxic substances. Three types of PEL's are defined, although not for every substance.
• Most common is the 8-hour time weighted average (TWA) value, referring to the maximum daily exposure, based on an 8-hour day/40-hour workweek. Note that the only practical way to document compliance with a TWA standard is via a data acquisition/archiving/reporting system, such as Interscan's Arc-Max®.
• For some airborne contaminants, an "excursion value" is set, generally referring to the maximum allowable concentration averaged over a particular 15-minute time period. Limits on the number of these excursions during an 8-hour workday, as well as a mandatory time interval between such exposures, may also be set. This parameter is sometimes called a Short-Term Exposure Limit (STEL).
• In certain cases a "ceiling value" is designated, as the instantaneous concentration which should never be exceeded during the workday. A provision may apply in some cases whereby if instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time over a working day. To add to the confusion, occasionally a single value is promulgated as a "STEL/ceiling."
One more term is used: "Action Level." This is usually one-half of the allowable 8-hour TWA for the substance in question. The only significance of the action level is as a benchmark during the initial screening of a workplace. If concentration levels of the chemical in question are greater than or equal to this action level, then regular monitoring will have to be done. For certain compounds, such as ethylene oxide, OSHA has formulated detailed guidelines on the initial monitoring process.
Note that there is no logical reason to set an instantaneous alarm for a toxic gas monitoring system at the action level. This number is already one-half of a value that is allowable for an 8-hour time-weighted average exposure. In fact, setting alarms at the action level, unless there is a purpose behind it that has nothing to do with regulatory compliance, is to be avoided, as it tends to upset personnel needlessly, and may require more frequent instrument maintenance and calibration.
OSHA's PEL's are derived from many sources, including the National Institute of Occupational Safety and Health (NIOSH), the American Conference of Governmental Industrial Hygienists (ACGIH), and the American National Standards Institute (ANSI). NIOSH and ACGIH also publish standards for airborne contaminants, that are not always in harmony with the OSHA regulations, but are often helpful, especially in matters that OSHA does not address.
The instrument user is well-advised to make himself familiar with the applicable regulations. Of course, we at Interscan can provide guidance. Please contact our sales department for any applications assistance.
Now, let's consider a few specific examples.
Carbon Monoxide (CO)
OSHA specifies an 8-hour TWA of 50 ppm. Setting the system alarm at 50 is not recommended, unless the workplace concentration is always teetering just below this level, and compliance with the 50 ppm standard is truly an issue.
Although OSHA does not define a STEL or ceiling value for CO, NIOSH does, and it is 200 ppm. Absent the condition of meeting the 50 ppm standard, a more prudent course would be to set alarm-1 (the warning alarm level) at 100 ppm and alarm-2 (the danger alarm level) at 200 ppm.
All Interscan continuous monitoring systems are provided with at least two adjustable alarm levels, and our portable units are equipped with a single adjustable alarm set point. In this example, a single alarm-equipped unit should be set between 100 and 200, so a logical choice would be 150 ppm.
Chlorine (Cl2)
OSHA does not specify an 8-hour TWA, but denotes a ceiling value of 1 ppm. ACGIH lists an 8-hour TWA of 0.5 ppm. Here, there is but a 0.5 ppm difference between ACGIH's TWA and OSHA's ceiling, which is probably why OSHA does not bother with a TWA. In this case, set points of 0.5 ppm for alarm-1 and 1 ppm for alarm-2 would be a reasonable idea. A single-alarm equipped unit should be set somewhere between 0.5 and 1, probably closer to 0.5.
Ethylene oxide (EtO)
OSHA calls out an 8-hour TWA of 1 ppm, with a STEL of 5 ppm. ACGIH has no STEL, but concurs with the 1 ppm TWA. We would recommend settings of 2 ppm (or 2.5 ppm) and 5 ppm, for alarm-1 and alarm-2, respectively. A single-alarm equipped unit should be set at 2 or 3 ppm.
There is no conceivable reason to set any alarm at the action level of 0.5 ppm, since this value is of no significance once monitoring is formally underway.
Formaldehyde (HCHO)
OSHA specifies an 8-hour TWA of 0.75 ppm, and a STEL of 2 ppm. ACGIH defines a STEL/ceiling of 0.3, with no 8-hour TWA, and NIOSH posts an 8-hour TWA of 0.016 ppm. Inasmuch as the U.S. Environmental Protection Agency's Integrated Risk Information System (IRIS) classifies formaldehyde as a probable human carcinogen, the ultra-low numbers proffered by ACGIH and NIOSH surely reinforce the notion that exposure should be as low as possible. American workplaces are legally required to meet the OSHA limits. Still, if a workplace can be kept at lower levels, so much the better.
Our recommendation would be for settings of 1 ppm and 2 ppm, for alarm-1 and alarm-2, respectively, keeping a special eye on how close the TWA is getting to 0.75 ppm. A single alarm unit should be set to 1 ppm, again keeping a special eye on how close the TWA is getting to 0.75 ppm. For those occupancies that can be better controlled, alarm settings can be put lower, but more instrument maintenance may be required.
Hydrogen chloride (HCl)
No American agency specifies a time-weighted average exposure recommendation. Rather, only ceiling values are proffered. ACGIH sets theirs at 2 ppm, while OSHA and NIOSH have theirs at 5 ppm. The International Agency for Research on Cancer (IARC) denotes HCl as "IARC-3: Unclassifiable as to carcinogenicity in humans." Similarly, the ACGIH calls it "Not classifiable as a human carcinogen." Typically, this designation is used for compounds that "could" be carcinogenic, but of which there is insufficient data to draw this conclusion.
While the OSHA level has the force of law in the United States, prudence would dictate a nod to the lower ACGIH concentration. Thus, we would recommend that a single alarm unit be set to 2.5 or 3.0 ppm. A two-alarm unit should have alarm-1 at 1.9 ppm, and alarm-2 at 4.0 or 4.5 ppm. Hydrogen sulfide (H2S) OSHA does not specify an 8-hour TWA, but instead defines a ceiling concentration of 20 ppm. An additional provision allows an "acceptable maximum peak above the acceptable ceiling concentration for an 8-hour shift" of 50 ppm, for a maximum duration of 10 minutes once, and only if no other measured exposure (presumably above the ceiling value) occurs. ACGIH lists a TWA of 10 ppm, subject to change. Clearly, these regulations were written to respond to differing situations faced by various industries that have hydrogen sulfide exposure. For those that actually need the 50 ppm loophole, alarm-1 should be set at 20, and alarm-2 should be set at 50. For those operating at lower levels, settings of 10 and 20 respectively would be advised. Settings for a single alarm unit would be problematical for the high concentration scenario, while a setting of 20 ppm would suffice for the lower level applications. Sulfur dioxide (SO2) OSHA has published an 8-hour TWA of 5 ppm, with no ceiling value, and ACGIH has a TWA of 2 ppm. with a 5 ppm STEL. No doubt, 5 ppm should be one of the alarm set points on a dual alarm unit, and should be the setting on a single-alarm equipped instrument. Depending on individual circumstances, an alarm-1 setting of 2 and alarm-2 setting of 5 seem appropriate. In conclusion, there is no pat answer to the alarm setting question, as individual applications will vary, and although OSHA standards have the force of law, the other learned institutions may disagree on allowable levels. With practical experience that is simply unmatched in the industry, Interscan stands ready to help you with all aspects of your toxic gas monitoring applications.
Interscan provides detailed sensor response data in our Tech Center.
Rise time to 90% of final value, rise time to 50% of final value, and fall time to 10% of original value are given for all gases, and specialized sensor types for hydrazine and hydrogen sulfide. It is noted there that the 50% figure is useful when considering how fast an instrument will respond in a critical (alarm) situation, whereby "full" response is not as important as the "step" response to an immediately hazardous concentration of toxic gas.
It is further noted that sensor rise and fall times are affected by many factors, including age, chemisorption, cumulative exposure to target gas and interfering gases, and maintenance. Data is also provided to help the user determine the lag time caused by interconnect tubing used to draw sample in from remote points. Fortunately, even 100 feet (30.48 m) of typical ¼ inch (6.35 mm) OD tubing introduces a lag time of only 29 seconds.
It is important to temper the sometimes misplaced zeal for fast response times, and zero lag intervals.
When real-time gas detection instruments were first introduced, it was natural to compare their relatively instant response with the predominant wet chemical or detector tube methods. As more instruments came on the market, using different operating principles, one of the specifications that was inevitably compared was response time. "Lag time," used in a context where there is no interconnect tubing to a remote point, refers to whatever inherent system delay exists, before the sample gets to the detector. Generally invoked for instrument methods that have quick detector response times, but plumbing issues that slow the overall response, this definition of lag time was judged to be "more fair" to such techniques, but becomes a pointless distinction to an instrument user.
Moreover, lightning fast response and fall times may look good on a brochure or website, but offer few real-world advantages, beyond allowing a particular type of detector to be used in a stream-switching context with more points than a slower responding detector might allow. Precious few situations exist whereby an instrument that responds a few seconds quicker can be said to offer any advantage. Nearly all sample-draw direct-reading toxic gas analyzers—such as Interscan's—respond fast enough for 99% of applications. Saying that, if one is considering a diffusion sensor approach (no sample-draw pump) one definitely SHOULD take into account the inherent lag time of such devices. There are many factors to consider in this case, including ventilation characteristics, measuring range, and reactivity of the target gas.
As to the matter of lag time due to interconnect tubing, this is of practical significance only in stream-switching installations. There is a valid concern here that since data is not continuous for all points all the time, the newest and most updated sample should be quickly presented to the instrument. However, in a continuous monitoring installation, few situations are so critical that adding 30 or so seconds of lag time would matter. And, if it does matter, steps can be taken to minimize even this small amount of lag time.
Please remember that as important as instantaneous high concentration alarms may be, most occupational health monitoring is ultimately more concerned with long-term exposure, based on an 8-hour time-weighted average. Even the NIOSH IDLH (Immediately Dangerous to Life and Health) levels are based on a 30-minute exposure, and best practices for any monitoring system would take into account either the short term exposure limits of 15 minutes, or the ceiling value (if the target gas has one) that requires the fastest monitoring.
In other words, diligent applications engineering and proper design of the gas detection system in the first place will make any discussion of lag times caused by interconnect tubing to be moot.
EPA's Method 21, entitled "Determination Of Volatile Organic Compound Leaks," calls for a portable instrument to be used for this purpose, and details certain specifications and calibration requirements.
"Volatile Organic Compounds," better known as VOC's, have been defined by EPA to include in effect "any volatile compound of carbon" that is not specifically exempted. Here's a link to EPA regulations. Enjoy!
It is true that most Method 21 work is done using infrared, photoionization, or flame ionization detectors, since these techniques can handle a broad range of organic compounds. But, if you don't need to measure a broad range of compounds, and if your requirements include formaldehyde, ethylene oxide, HCN, or propylene oxide, then our portable analyzers will more than do the trick. Bear in mind that in most cases, you should use the UL-Classified intrinsically safe version of the analyzer, and these are readily available.
Unlike photoionization or flame ionization units (if not equipped with chromatographic columns), Interscan gas analyzers are quite specific for the target compound, and often boast greater sensitivity, as well. Portable infrared analyzers offer excellent specificity, but are cumbersome and expensive, and often lack sufficient sensitivity.
Finally, Interscan analyzers are usually priced more attractively than alternative products.
We welcome your inquiries!
First, let's define the terms:
"Continuous" monitoring means that each sampling point is being monitored all the time, and in most cases, the concentration displayed reflects what is truly happening at the point in real time. There are certain exceptions to this involving instrument techniques that run batch analysis, but that is beyond the scope of this discussion.
"Stream-switching" monitoring means that one detector is sampling several sampling points on a sequential basis. As such, the concentration data for a particular point is displayed and recorded in real time only when that point is being actively analyzed.
Stream-switching was first used with laboratory and process gas chromatographs, owing to their high cost, and the understandable desire to amortize this cost over many sample inputs. In fact, automated sequential valves, accommodating as many as 24 inputs, were introduced into chromatography applications early on.
When it became popular to monitor vehicular tunnels and parking garages for carbon monoxide, early techniques included non-dispersive infrared and catalytic oxidation, using Hopcalite®. Since these installations usually featured many sampling points, and the detector technology was expensive, stream-switching became the sampling method of choice. However, no sequential valve intended for chromatograph application would suffice for the long tubing runs, larger tubing diameters, and pressure drops involved. Instead, pneumatic systems utilizing solenoid valves, timers, and high-capacity pumps were developed for this purpose.
Cost savings will accrue with stream-switching in two ways: One detector can be shared over many points, eliminating the need for purchasing several detectors, and only one detector has to calibrated and maintained, reducing labor costs. Of course, the long sampling lines will have to be checked periodically, as will the end-of-line filters at each remote point.
It should be noted that for the purposes of ventilation control in a tunnel or parking garage, stream-switching is a valid approach. Carbon monoxide, in the concentrations generally encountered here, is not immediately life-threatening, and acknowledging that stream-switching may only provide useful data for each point a few times per hour, this is still satisfactory for ventilation control.
But, monitoring carbon monoxide for ventilation control, in an occupancy where few if any people are situated for a time period approaching an 8-hour workday, is hardly representative of the bulk of occupational safety toxic gas detection applications. Indeed, it is in the typical workplace application that the deficiencies of stream-switching come to light.
Many toxic gases encountered in the workplace have extremely low allowable limits, and many have been assigned "ceiling values," that should never be exceeded. Prudence would dictate that such compounds should be continuously monitored in areas frequented by employees. One could argue that absent a ceiling value or short term exposure limit (15-minute average), all that is needed is to maintain an 8-hour time-weighted average below the allowable limits (TLV®-TWA or OSHA PEL). This assumes, of course, that while a particular sampling point is off-stream, no hazardous release would occur.
Clearly, we cannot assume that a hazardous release will not take place, but even if we could, by what mathematical theory can we calculate an 8-hour time-weighted average at a particular sampling point, if that sampling point is off-stream more than it is on-stream? In a four-point stream-switching system (and that would be a small one) each point is off-stream 75% of the time. In a ten-point stream-switching system, each point is off-stream 90% of the time. Data acquisition systems are widely available to handle intermittent data, but what value should they store for the off-stream time?
The most rational approach would be to store the average value calculated during the on-stream time, but that is at best an approximation, and could easily understate or overstate the true conditions. Thus, your 8-hour time-weighted average calculation for a particular point is based on little data, and mostly approximations. Not good. And remember, we have ignored the problem of a hazardous release occurring while a particular point is off-stream. Since the points are off-stream most of the time, it is likely that a hazardous release WILL occur while a point is off-stream. Not good, again.
There is little doubt that continuous, dedicated monitoring at all points is the preferred approach for occupational safety and defensive archiving purposes, although it can become expensive, given increased hardware and maintenance costs. Fortunately, there are ways to control the expense.
1. A dedicated continuously monitoring Interscan system, given the less expensive technology of the electrochemical sensors and attendant electronics, is far less expensive than even stream-switching systems utilizing other technologies, such as non-dispersive infrared and photoionization (often employed with a gas chromatograph).
2. In most cases, a relatively small number of points will suffice. Regrettably, inflated sampling point counts have been touted by certain stream-switching vendors, in an effort to make the high cost of their systems seem more attractive, being amortized over many points.
3. In applications where many points really are required, Interscan will suggest a cost-effective and safe combination of stream-switching for less critical areas, and continuous monitoring for the rest of the facility. Interscan does have extensive experience in both stream-switching and continuous multi-point applications, with installations dating back to the late 1970's.
We welcome the opportunity of working with you.
As if it weren't bad enough that clueless environmental authorities try to impose their ignorance on the field by using so-called "metric" units of gas concentration, another trap may be lurking for you.
Everyone around the world agrees that a part-per-million (ppm) equals 10-6. Unfortunately, since everyone around the world does not agree with what a "billion" is, they can't agree on what a part-per-billion (ppb) is, either.
In what is termed the American system, 109 is a billion. But, in the European system, 109 is a milliard (sometimes called thousand million). In the Euro system, a billion is 1012. Ironically, the French used what is now the American system as far back as the seventeenth century, before switching to the Euro system in 1948. Are you confused yet?
While I have never encountered the term parts-per-milliard to signify 10-9, it certainly could happen. Imagine if that were also abbreviated "ppm"!
However, the unit µl/M3 [microliter per cubic meter] has been published in numerous official Korean documents, and was used to avoid the potential parts-per-billion confusion. This interesting unit does define 10-9 in a non-ambiguous fashion.
The good news is that American cultural hegemony has already affected the numbering system, and, as you might expect, money was involved.
We prevailed upon (at least) the Brits that 109 dollars be called a billion dollars, and this was agreed to in 1974, under Prime Minister Harold Wilson. This usage has also spread into common parlance, and according to the The Times of London style guide,
[a] "billion [is] one thousand million, not a million million"
Even though our definition of a billion as 109 is supposed to apply in official government statistics worldwide, the Korean example is clearly an exception, and is probably not unique.
Our recommendation is to ask what someone means by parts-per-billion, in any international dealings.
For any gas to be detectable by an electrochemical sensor, it must first be "electroactive." As defined by the International Union of Pure and Applied Chemistry (IUPAC) Analytical Chemistry Division Commission on Electroanalytical Chemistry, an electroactive substance is... In voltammetry and related techniques, a substance that undergoes a change of oxidation state, or the breaking or formation of chemical bonds, in a charge-transfer step.
Most alcohols are electroactive. Indeed, considerable work has been done in the development of alcohol fuel cells, and several alcohol breath meters used by police agencies are based on electrochemical sensors.
In the toxic gas detection field, sensitivity to alcohols has also been exploited, but usually for dubious purposes. For example, certain brands of so-called formaldehyde detectors are really little more than re-labeled alcohol detectors, that have much greater response to the alcohols, than to formaldehyde. [In contrast, the Interscan formaldehyde sensor has good rejection to alcohol interferents, especially the ones that are typically encountered in formaldehyde monitoring applications.]
In the application of monitoring ethylene oxide (EtO), certain manufacturers have foolishly and irresponsibly instructed their customers to bump-test (challenge a gas detector with something other than a legitimate calibration standard) their instruments with alcohol. This is most often done by simply taking a rag or swab that has been wetted with alcohol, and placing it close to the sensor inlet.
The theory is that since it is "too difficult" to obtain the proper EtO calibration standard, the alcohol bump-test will at least assure the user that the monitoring instrument is still working. Unfortunately, this practice is dangerous and is to be avoided because:
As to how alcohols affect the Interscan electrochemical sensor, it all comes down to the most basic rule of toxicology, as articulated in the sixteenth century by a Swiss chemist named Paracelsus: "The dose makes the poison."
Parts-per-million level concentrations of alcohol would simply be detected, and would react away, generally without incident. However, in many cases where our sensors are exposed to alcohol, the concentrations are not low. Rather, the concentrations are at levels in excess of 500 ppm, and even higher. (Bear in mind that the OSHA PEL for isopropyl alcohol is 400 ppm.)
At such relatively high concentrations, an Interscan sensor—intended for operation at ppm and sub-ppm levels—will surely react to the alcohol, but this overexposure will cause the instrument to go off-scale, until all the alcohol reacts away. Moreover, certain reaction products could build up on the sensing electrode, creating a long-term "background" effect, that can markedly affect sensor performance.
With prolonged or repeated exposure to macro levels of alcohols, an Interscan sensor can be effectively destroyed, and be rendered essentially useless for detection of the target toxic compound. This is why we have always warned our customers about alcohol bump-testing, and provide means for instrument shut-off and automatic re-start, should alcohol be used to scrub down work areas, that are being monitoring for EtO.
Be concerned, then, that alcohol contamination can void the sensor warranty, but be more concerned that exposing the sensor to macro levels of alcohol will compromise your monitoring program.
Current occupational exposure limits for EtO mandate monitoring for it at 1 ppm or less. How a sensor might respond to perhaps 1000 ppm or more of alcohol is in no way related to its low-level EtO performance. As such, a false sense of security will almost surely be instilled. Worse, proper calibration might be performed infrequently or not at all.
Even if the bump-test procedure were somehow refined into a sort of surrogate calibration method, based on reproducible sensor response data, it is still not appropriate. Analytical best practices demand that an instrument be calibrated with the target compound to be detected. Imagine defending a lawsuit on EtO exposure, and having to admit that the instrument was "calibrated" with an alcohol.
"Minimum Detectability" is one of those instrumentation terms that is used frequently, but is seldom defined. Indeed, even though you will encounter this term on many data sheets, its definition does not appear in any of the usual learned references, including Process Instrumentation Terminology, ANSI/ISA—51.1—1979(R1993) and Standard Terminology Relating to Sampling and Analysis of Atmospheres, ASTM D 1356 - 05.
However, the ASTM standard does provide us with...
Method Detection Limit: The minimum concentration of an analyte that can be reported with a 99% confidence that the value is above zero, based on a standard deviation of greater than seven replicate measurements of the analyte in the matrix of concern at a concentration near the low standard.
Simplifying this, we can say that "Minimum Detectability" is the lowest concentration of analyte that can be unambiguously discriminated from noise. [Some agencies set a standard that minimum detectability must be at least 2-2.5 times the noise level.] Fair enough, but how can this be utilized in occupational health or process gas detection?
First of all, it is important to note that any data garnered at the level of minimum detectability will not be accurate. For example, in a typical case, the minimum detectability of a particular instrument is given as 1% of full scale, and accuracy is ± 2% of full scale. Thus, for a 0-100 ppm scale, the minimum detectable reading of 1 ppm would actually be 1 ppm ± 2 ppm—hardly a useful measurement.
Similarly, on a digital unit, the minimum detectability of a particular instrument is often given as the least significant digit. On a commonly used 3˝ digit meter, for a range of 0-199.9 ppm, this would be 0.1 ppm. In this case, accuracy is specified at ± 2% of reading ± 1 least significant digit. Here, the minimum detectable reading of 0.1 ppm would actually be 0.1 ppm ± 0.002 ppm ± 0.1 ppm. Technically better than the analog example, but still of little value.
Even so, knowing the minimum detectability of an instrument can be helpful in situations when "go/no-go" readings are of interest. Given a properly calibrated instrument, the smallest observable response would be—by definition—the minimum detectable level, and would indicate at least the presence of the analyte in question (any interferences notwithstanding).
Of course, such practices should only be done when instruments with more appropriate sensitivity are not available.
As discussed in the Calibration Basics Knowledge Base article, Interscan's gas analyzers, and virtually all other direct-reading gas analyzers are not absolute methods. Rather, they employ "relative" [or "reference," but not necessarily EPA Reference] methods. That is, methods that produce some output that must be calibrated against a known standard.
Generally, these units must also be zeroed against a known zero gas.
Ideally, a zero gas should contain none of the particular chemical you will be testing for. However, "none" is a metaphysical concept, and in analytical chemistry, all parameters must be measured. Thus, a zero gas will be certified to contain less than a certain amount of compounds of interest.
For example, Scott-Marrin—a highly-respected specialty gas supplier—specifies its best grade of ultrapure air, verified with state-of-the-art analytical technology, at...
Total Hydrocarbons < 0.01 ppm CO < 0.01 ppm NOx < 0.001 ppm SO2 < 0.001 ppm N2O < 0.001 ppm
Consider an application for carbon monoxide (CO) monitoring, using an EPA designated reference method. From the above figures, you already know that your zero gas will be introducing a 0.01 ppm error into the mix.
The lowest range offered by Thermo Scientific's Model 48i gas filter correlation CO analyzer is 0-1 ppm. Thus, if you choose to operate in this range, a 1% systematic [and additive] error is unavoidable.
This also means that operation at a more rational 0-10 ppm or 0-50 ppm range for CO measurement would introduce an insignificant error from the zero—assuming that the same high quality zero gas were to be used.
Two questions now come to the fore:
1. What if you want to measure a gas at low concentration levels that is not mentioned in a zero gas specification? After all, the five entities listed above barely scratch the surface of common applications, even if they do address certain major ones.
2. What means does the specialty gas supplier employ to zero the instruments they themselves use to certify the zero gas standards?
The answer to the first question is that you must contact the gas supplier and describe your application, indicating the target gas, and the lowest measurement concentration level. Ask the supplier to state its guaranteed not-to-exceed maximum concentration of that target gas in the zero standard. In some cases it may not be possible to provide such a guarantee, especially if no suitably sensitive and accurate analytical technique exists for the target gas.
Contrary to popular opinion, there are not good, sensitive, and accurate wet chemical methods for all pollutant compounds of interest.
Getting an answer to the second question is your follow-up: Just how does the supplier do its analytical work for the target compound, and just how does it zero the instrument involved?
In other words, ask the tough questions! If you don't like the answers, find another supplier.
As always, Interscan is here to help you with all application and technical issues. Feel free to contact us at any time.
| Note: All exposure limits cited in this article are current as of 31 December 2006 |
Ultimately, the entire matter of where to set the instantaneous concentration alarm(s) is tied into what is expected by the regulatory agency. In the United States, for most workplace environments, it is the Occupational Health and Safety Administration (OSHA).
OSHA has published Permissible Exposure Limits, or PEL's, for many toxic substances. Three types of PEL's are defined, although not for every substance.
• Most common is the 8-hour time weighted average (TWA) value, referring to the maximum daily exposure, based on an 8-hour day/40-hour workweek. Note that the only practical way to document compliance with a TWA standard is via a data acquisition/archiving/reporting system, such as Interscan's Arc-Max®.
• For some airborne contaminants, an "excursion value" is set, generally referring to the maximum allowable concentration averaged over a particular 15-minute time period. Limits on the number of these excursions during an 8-hour workday, as well as a mandatory time interval between such exposures, may also be set. This parameter is sometimes called a Short-Term Exposure Limit (STEL).
• In certain cases a "ceiling value" is designated, as the instantaneous concentration which should never be exceeded during the workday. A provision may apply in some cases whereby if instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time over a working day. To add to the confusion, occasionally a single value is promulgated as a "STEL/ceiling."
One more term is used: "Action Level." This is usually one-half of the allowable 8-hour TWA for the substance in question. The only significance of the action level is as a benchmark during the initial screening of a workplace. If concentration levels of the chemical in question are greater than or equal to this action level, then regular monitoring will have to be done. For certain compounds, such as ethylene oxide, OSHA has formulated detailed guidelines on the initial monitoring process.
Note that there is no logical reason to set an instantaneous alarm for a toxic gas monitoring system at the action level. This number is already one-half of a value that is allowable for an 8-hour time-weighted average exposure. In fact, setting alarms at the action level, unless there is a purpose behind it that has nothing to do with regulatory compliance, is to be avoided, as it tends to upset personnel needlessly, and may require more frequent instrument maintenance and calibration.
OSHA's PEL's are derived from many sources, including the National Institute of Occupational Safety and Health (NIOSH), the American Conference of Governmental Industrial Hygienists (ACGIH), and the American National Standards Institute (ANSI). NIOSH and ACGIH also publish standards for airborne contaminants, that are not always in harmony with the OSHA regulations, but are often helpful, especially in matters that OSHA does not address.
The instrument user is well-advised to make himself familiar with the applicable regulations. Of course, we at Interscan can provide guidance. Please contact our sales department for any applications assistance.
Now, let's consider a few specific examples.
Carbon Monoxide (CO)
OSHA specifies an 8-hour TWA of 50 ppm. Setting the system alarm at 50 is not recommended, unless the workplace concentration is always teetering just below this level, and compliance with the 50 ppm standard is truly an issue.
Although OSHA does not define a STEL or ceiling value for CO, NIOSH does, and it is 200 ppm. Absent the condition of meeting the 50 ppm standard, a more prudent course would be to set alarm-1 (the warning alarm level) at 100 ppm and alarm-2 (the danger alarm level) at 200 ppm.
All Interscan continuous monitoring systems are provided with at least two adjustable alarm levels, and our portable units are equipped with a single adjustable alarm set point. In this example, a single alarm-equipped unit should be set between 100 and 200, so a logical choice would be 150 ppm.
Chlorine (Cl2)
OSHA does not specify an 8-hour TWA, but denotes a ceiling value of 1 ppm. ACGIH lists an 8-hour TWA of 0.5 ppm. Here, there is but a 0.5 ppm difference between ACGIH's TWA and OSHA's ceiling, which is probably why OSHA does not bother with a TWA. In this case, set points of 0.5 ppm for alarm-1 and 1 ppm for alarm-2 would be a reasonable idea. A single-alarm equipped unit should be set somewhere between 0.5 and 1, probably closer to 0.5.
Ethylene oxide (EtO)
OSHA calls out an 8-hour TWA of 1 ppm, with a STEL of 5 ppm. ACGIH has no STEL, but concurs with the 1 ppm TWA. We would recommend settings of 2 ppm (or 2.5 ppm) and 5 ppm, for alarm-1 and alarm-2, respectively. A single-alarm equipped unit should be set at 2 or 3 ppm.
There is no conceivable reason to set any alarm at the action level of 0.5 ppm, since this value is of no significance once monitoring is formally underway.
Formaldehyde (HCHO)
OSHA specifies an 8-hour TWA of 0.75 ppm, and a STEL of 2 ppm. ACGIH defines a STEL/ceiling of 0.3, with no 8-hour TWA, and NIOSH posts an 8-hour TWA of 0.016 ppm. Inasmuch as the U.S. Environmental Protection Agency's Integrated Risk Information System (IRIS) classifies formaldehyde as a probable human carcinogen, the ultra-low numbers proffered by ACGIH and NIOSH surely reinforce the notion that exposure should be as low as possible. American workplaces are legally required to meet the OSHA limits. Still, if a workplace can be kept at lower levels, so much the better.
Our recommendation would be for settings of 1 ppm and 2 ppm, for alarm-1 and alarm-2, respectively, keeping a special eye on how close the TWA is getting to 0.75 ppm. A single alarm unit should be set to 1 ppm, again keeping a special eye on how close the TWA is getting to 0.75 ppm. For those occupancies that can be better controlled, alarm settings can be put lower, but more instrument maintenance may be required.
Hydrogen chloride (HCl)
No American agency specifies a time-weighted average exposure recommendation. Rather, only ceiling values are proffered. ACGIH sets theirs at 2 ppm, while OSHA and NIOSH have theirs at 5 ppm. The International Agency for Research on Cancer (IARC) denotes HCl as "IARC-3: Unclassifiable as to carcinogenicity in humans." Similarly, the ACGIH calls it "Not classifiable as a human carcinogen." Typically, this designation is used for compounds that "could" be carcinogenic, but of which there is insufficient data to draw this conclusion.
While the OSHA level has the force of law in the United States, prudence would dictate a nod to the lower ACGIH concentration. Thus, we would recommend that a single alarm unit be set to 2.5 or 3.0 ppm. A two-alarm unit should have alarm-1 at 1.9 ppm, and alarm-2 at 4.0 or 4.5 ppm. Hydrogen sulfide (H2S) OSHA does not specify an 8-hour TWA, but instead defines a ceiling concentration of 20 ppm. An additional provision allows an "acceptable maximum peak above the acceptable ceiling concentration for an 8-hour shift" of 50 ppm, for a maximum duration of 10 minutes once, and only if no other measured exposure (presumably above the ceiling value) occurs. ACGIH lists a TWA of 10 ppm, subject to change. Clearly, these regulations were written to respond to differing situations faced by various industries that have hydrogen sulfide exposure. For those that actually need the 50 ppm loophole, alarm-1 should be set at 20, and alarm-2 should be set at 50. For those operating at lower levels, settings of 10 and 20 respectively would be advised. Settings for a single alarm unit would be problematical for the high concentration scenario, while a setting of 20 ppm would suffice for the lower level applications. Sulfur dioxide (SO2) OSHA has published an 8-hour TWA of 5 ppm, with no ceiling value, and ACGIH has a TWA of 2 ppm. with a 5 ppm STEL. No doubt, 5 ppm should be one of the alarm set points on a dual alarm unit, and should be the setting on a single-alarm equipped instrument. Depending on individual circumstances, an alarm-1 setting of 2 and alarm-2 setting of 5 seem appropriate. In conclusion, there is no pat answer to the alarm setting question, as individual applications will vary, and although OSHA standards have the force of law, the other learned institutions may disagree on allowable levels. With practical experience that is simply unmatched in the industry, Interscan stands ready to help you with all aspects of your toxic gas monitoring applications.
The answer to your question is YES.
You are correct in noting that the response of our instruments is linear. More than that, electrochemical voltametric sensors (such as we use) are inherently linear, with no electronic compensation required.
However, for optimum accuracy in most applications, it is best to calibrate the instruments at a concentration somewhere around 50% of the scale range or higher, if you can. This is because there will always be various sources of error beyond the sensor electrochemistry, and prudent analytical technique would frown on calibrating an instrument at 1% of the scale range for readings at 90% of the scale.
Furthermore, for optimum accuracy, if for some reason you must operate toward the bottom of the range, ideally you should calibrate close to this level.
Saying that, for most customers, calibration at 50% of the range or higher should be quite satisfactory.
For more information, please consult our articles on accuracy and minimum detectability.
This is a follow-up to our earlier article on Using Common Sense and Science in Expressing Gas Concentrations, inspired by a real-life adventure of one of our sales engineers.
As our hero was slogging through some e-mail inquiries, he came upon two units of measurement he had not seen before:
ppmv and µg/Nm3
ppmv
ppmv is simply parts-per-million by volume, and since that is always the way parts-per-million is figured for gas measurement, it is just a more pedantic (or self-consciously complete) way of rendering "ppm."
To be rigorous, the correct textbook definition of parts per million would have it--
mass of one component in milligrams/total mass of the solution in kilograms
Thus, strictly speaking, ppm should be figured as mass, not volume.
In the practical sense, though, this definition is most often applied to liquid solutions, even if two or more gases (and we are usually referring to a pollutant gas in air) also comprise a solution.
The reason that gas parts-per-million is always parts-per-million by volume is that traditionally, gases have been handled by volume or pressure, but usually not by mass. While the "high-loading" balance technique (whereby a cylinder is weighed to milligram resolution, and the target gas is added by mass or weight) is sometimes employed to make calibration gas blends, the gold standard is still a glass flask. Here, a volume of target gas is injected into a flask of known volume, containing the balance gas. The operation must be done at a known and constant temperature and pressure.
µg/Nm3
µg/Nm3 means micrograms per normal cubic meter (Nm3). The "normal" cubic meter is defined as being at 0°C (273.15°K) and 101.325 kPa or 760 mmHg (i.e. 1 atmosphere of absolute pressure). However, this notation is no longer appropriate unless the specific reference conditions are explicitly stated, since there are currently many different definitions of what constitutes standard reference conditions.
| Temperature | Absolute pressure | Relative humidity | Publishing or establishing entity |
|---|---|---|---|
| °C | kPa | % RH | |
| 0 | 100.000 | IUPAC (present definition) | |
| 0 | 101.325 | IUPAC (former definition), NIST, ISO 10780 | |
| 15 | 101.325 | 0 | ISA, ISO 13443, EEA, EGIA |
| 20 | 101.325 | EPA, NIST | |
| 25 | 101.325 | EPA | |
| 25 | 100.000 | SATP | |
| 20 | 100.000 | 0 | CAGI |
| 15 | 100.000 | SPE | |
| °F | psia | % RH | |
| 60 | 14.696 | SPE, OSHA, SCAQMD | |
| 60 | 14.73 | EGIA, OPEC, EIA | |
| 59 | 14.503 | 78 | Army Standard Metro |
| 59 | 14.696 | 60 | ISO 2314, ISO 3977-2 |
Here are the full names of the entities listed in the above table--
| IUPAC | International Union of Pure and Applied Chemistry |
| NIST | National Institute of Standards and Technology |
| ISA | ICAO's International Standard Atmosphere |
| ISO | International Organization for Standardization |
| EEA | European Environment Agency |
| EGIA | Electricity and Gas Inspection Act (of Canada) |
| EPA | U.S. Environmental Protection Agency |
| SATP | Standard Ambient Pressure and Temperature |
| CAGI | Compressed Air and Gas Institute |
| SPE | Society of Petroleum Engineers |
| OSHA | U.S. Occupational Safety and Health Administration |
| SCAQMD | California's South Coast Air Quality Management District |
| OPEC | Organization of Petroleum Exporting Countries |
| EIA | U.S. Energy Information Administration |
| Std. Metro | U.S. Army's Standard Metro (used in ballistics) |
As you can see, by using the unit µg/Nm3, you are bound to be misunderstood—if not in the definition of normal or standard conditions, then by the difficulties inherent in using mass/volume units rather than parts-per-million.
A detector tube is a graduated glass tube filled with a chemical reagent that will produce a color change, when exposed to the gas in question. It is used with a hand pump that will draw a sample into the tube.
The tubes are generally supplied in packages of ten, and are sealed at both ends. In operation, the tips are broken off, and the tube is inserted into the hand pump. Depending on the manufacturer, the pump utilizes either a bellows or piston design, drawing a 100 milliliter sample through the tube.
As the sample works its way up the tube toward the pump, it reacts with the reagent such that the length of the color change produced is proportional to concentration. The point where this reaction stops is read off against graduated markings on the tube.
Detector tubes are easy to use, are relatively inexpensive, and the method is intrinsically safe, allowing it to be deployed in all occupancies. [ISA-RP12.6 defines intrinsically safe equipment as "equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration."]
Since detector tubes are available for hundreds of compounds, and have been around in a practical format since the 1930s, they are familiar to virtually anyone working in gas detection. But, seventy or so years after their introduction, there are now scores of gas detection instruments available. Thus, one might well ask when it is appropriate to use detector tubes.
We should first consider some of the disadvantages of detector tubes:
Tubes are not very accurate. At best, expect ± 20%.
Tubes can be quite temperature sensitive, and although correction data is customarily provided by the manufacturer, in practice it is not frequently applied. This is usually because the tube data sheet, supplied in the package, is not brought out into the field by the user. Out of sight, out of mind.
While the detector tube catalogs tout availability for hundreds of compounds, there are far fewer unique reaction chemistries. As such, tubes are prone to interferences from other gases. To their credit, the manufacturers document these interferences—often far better than instrument manufacturers do. However, the information provided may not prove terribly helpful.
Even if you are told that compound "x" may interfere with the measurement of your target compound, you may have no idea if compound "x" is present, or at what concentration it may be present. Furthermore, a specific interference ratio is seldom given, and since the method is based on colorimetric reactions, certain compounds can bleach out the color change that should otherwise occur.
Many tubes require that multiple pump strokes (creating multiples of the basic 100 mL sample size) be taken, to achieve the desired sensitivity. Errors can occur either from the failure to keep track of the number of strokes, from not allowing sufficient time for each sample to work its way through the tube before taking another pump stroke, or from "pre-cleanse" layers of the tube (intended to remove water vapor or other chemicals) being used up as they are taxed beyond their capacity.
Owing to their basis in colorimetric reactions, the tubes have a shelf life, and many of them have to be stored at cold temperatures.
Although so-called long-term detector tubes are available for a small number of compounds, the far more common standard tubes can only give the user a grab-sample "snapshot" of the air, and can never substitute for real-time monitoring.
For the most part, detector tubes are best used when a quick and dirty test will suffice. Examples would include:
Testing around plumbing components where a leak is suspected, and it is known what compound would be leaking.
Cursory evaluation of hazardous material spill situations, especially when more appropriate instruments are not available.
Cursory evaluation of nuisance odor complaints, especially if there is a suspect compound.
Back in the day, the biggest advantage of tubes was that they were cheap. However, at upwards of $60.00 per package (March, 2007), and considering that inexpensive instruments are now available for many of the most common toxic compounds, this advantage is rapidly disappearing.
As suggested by the "quick and dirty" examples above, tubes have a place for one-shot evaluations, but are simply not appropriate for routine or prolonged use in the same location. After all, once a toxic gas problem has been identified, it must either be eliminated or continuously monitored.
A simple rule of thumb would be to question any routine or repeated use of detector tubes. There will almost always be a better way to fulfill the gas detection application.
Introduction
It is quite unlikely that you will ever use an absolute method for gas detection. Rather, you will employ any one of dozens of "relative" [or "reference," but not necessarily EPA Reference] methods—that is, methods that produce some output that must be calibrated against a known standard. Then, its display can be directly read out in units of concentration, usually parts-per-million (ppm).
Even though proper calibration is 90% of successful gas detection, it is a subject that has been neglected—often purposely—by the majority of instrument manufacturers. There's a good reason for this, of course: Proper calibration can often be difficult and expensive. But, we're getting a bit ahead of ourselves.
Gas Blends in Cylinders
Early occupational health toxic gas detection focused on carbon monoxide (CO) and hydrogen sulfide (H2S). The calibration standards were supplied as gas blends in cylinders, and in the case of CO, at least, things worked out pretty well. This is because CO is not very reactive, and, within reason, maintains a stable concentration in the cylinder, as the pressure drops with use.
On the other hand, H2S is very reactive, and the original simplistic techniques used to create the cylinder gas blends could not provide a stable product. The problems observed with H2S blends were soon seen in blends for many other toxics. To make matters worse, improper analogies were drawn between experiences in combustible gas detection and toxic gas detection, establishing a false sense of security about poorly prepared gas blends.
In fact, other than the obvious point that both combustible and toxic gas detection get involved with detecting gases, the two fields of endeavor could not be more different.
The combustible gases of interest are nearly all stable (unless they are ignited by some external source), while nearly all toxic gases are unstable, and in many cases are extremely reactive. Most importantly, though, combustible gas detection is done in percent level concentrations, while toxic gas detection is done in parts-per-million, and even parts-per-billion concentrations—10,000 and 10 million times lower, respectively!
Fortunately, calibration gas blending technology has improved, encompassing specialized techniques for passivating the cylinders, as well as logging experience to determine how long a blend must age to become stable, and how long stability can be guaranteed. Much of the technological development has been done with aluminum cylinders, since this material seems to be less prone to wall effects and unwanted chemical reactions than steel.
Interscan can recommend good gas blend suppliers, but no matter what company you choose, the following points are important:
Order the blend so that the concentration is about 50% of the instrument's measuring range.
Ensure that the blend's analysis is ± 2% accurate (or better).
Insist on NIST-traceability.
Obtain a written guarantee as to how long the blend will be stable.
Since most of the cost of the blend is in the analysis labor, order the largest cylinder you can use. Stay away from disposab