Response time

Rise time to 50% of final value, rise time to 90% of final value, and fall time to 10% of original value are typical specifications presented for gas detection instruments. 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.

Bear in mind that sensor rise and fall times are affected by many factors, including age; chemisorption and physisorption; cumulative exposure to target gas and interfering gases; and maintenance. It is also prudent to consider 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 x ⅛ inch (3.175 mm) ID 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.

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