May 7, 2018
What’s Wrong With Toxic Gas Detection?
By Michael D. Shaw
Detection of toxic gases is at the heart of efforts to better characterize the environment. However, its promise from the early 1970s has not been fully realized. Let’s take a look.
Practical detection, of both toxic/asphyxiant and combustible gases, dates back to 1816 and the Miner’s Safety Lamp. This lamp employed an ingenious flame arrestor, usually consisting of a screen with tiny holes, whereby flames cannot escape to propagate, in the presence of combustible gases. By the same token, in the presence of such gases, the flame will burn higher with a blue tinge. In the presence of asphyxiant gases, the flame would be extinguished, providing an early warning of imminent danger. Vegetable oil was the original lamp fuel of choice.
For a variety of reasons, these lamps did not provide the boon in safety originally hoped for, and gave way to electric lamps at the turn of the (19th to 20th) century.
The familiar canary in the coal mine (a so-called “sentinel species”), was introduced by Scottish physiologist John Scott Haldane circa 1913. While not quantitative, canary distress or death was a sure, reliable sign of imminent human danger. No special training was needed to use a canary, and no maintenance beyond its modest feeding was required.
In 1917, the first gas detector tube was developed at Harvard University, for measuring carbon monoxide–the most common canary killing culprit. These devices consist of a glass tube filled with a chemical reagent that turns color when an air sample containing the gas of interest is passed through it. In the original detector tubes, color comparison charts were provided to relate concentration with the extent of the color change. Various embodiments of hand pumps are used to draw the air sample through the reagent.
By the 1930s, detector tubes for many compounds were commercially available, and some years later, most tubes were converted to a length-of-stain concept. As such, the reagent color change will work its way through the column of reagent, based on the concentration of the analyte gas. A further refinement would see each tube, in a given production batch, being provided with a graduated scale, indicating gas concentration.
Although detector tubes are not particularly accurate (± 25%), they are easy to use, and require little operator training.
You might be noticing a pattern here: Ease of use is key; accurate and precise analytics, not so much.
Operating in a sort of parallel universe to toxic gas detection is the field of combustible gas detection. Many gases used in industry can be potentially explosive, and affected environments must be monitored. Typical alarm set points are at 10% and 20% of the lower explosive limit. Note that combustible gas detectors generally operate in the percent range, while toxic gas detectors operate in the parts-per-million or parts-per-billion range.
By the 1960s, instrumentation methods were starting to get established in certain aspects of gas detection. Unlike previous methods, however, some form of calibration would be needed. That is, the instrument must be challenged with a known standard. As you can well imagine, creating standards at ppm or ppb levels is far more challenging than doing the same at percent levels.
Yet, a false analogy was almost immediately established between toxic and combustible gas detection, presumably because they are both…gas detection. Indeed, I served on numerous standards committees in the 1980s, which were led by old time “gas detection” types, who–virtually without exception–were only knowledgeable in the less demanding field of combustible gas detection. This led to a series of issues which persist to this day:
1. While calibration of combustible gas detectors presents few problems, calibration of toxic gas detectors can be difficult–especially if you venture beyond the easy ones: carbon monoxide, hydrogen sulfide, and any other of the “common” toxic gases. In fact, such calibration can be cumbersome and expensive, and discourages many end-users. Nonetheless, proper calibration is essential to toxic gas detection.
2. Holding to the theme of “less demanding,” in toxic gas detection, there is still a disproportionate emphasis on portable and personal monitoring devices, even if continuous area monitors would be much more appropriate. But, you guessed it: Portable and personal units are cheaper and “easier.”
3. Since combustible gas detection is not concerned with an exposure over some duration of time, but rather must emphasize an instantaneous alarm to avert a possible disaster, there is no interest in data logging. Conversely, toxic gas detection is avidly concerned with exposure, and data logging should therefore be commonplace…only, it is not.
Where do we go from here? At Interscan, we do see a positive trend. Industries new to gas detection, such as food/agriculture, are faced with monitoring difficult compounds such as hydrogen peroxide and peracetic acid. We are most encouraged that these newcomers, armed with knowledge of wet chemistry analytical techniques, are well aware of the challenges. Could it be that a new wave of customers, with a new wave of analytical needs, will transform the gas detection industry?
We certainly hope so.