One of the analogies we hear from time to time is describing something as a three-legged stool. Rather than something to sit on, this illustration helps us understand a concept that depends on three elements working together to make it effective. With the stool, taking away any one leg causes it to collapse, regardless of how strong the other two legs are.
Such is the case with fire and gas detection. An effective gas detector has to deliver in three areas: accuracy, response speed and measurement range. We discuss these elements in detail in my article in automation.com’s Instrumentation & Sensors eNewsletter for August, 2019, Specifying Fire and Gas Detection Systems to Improve Operations.
Accuracy relates to the trustworthiness of the reading to reflect the severity of a hazard, while fast response enables early warning. A highly accurate instrument that does not respond quickly to a dangerous situation is unsuited for service in a safety instrumented function. Similarly, a device whose reading cannot be trusted but responds quickly on exposure to an agent is of little use as a safety device. Finally, the device must respond within the relevant range of the target gas. A good sensor strikes a fine balance between accuracy and speed, while rejecting false trips and alerting when dangerous concentrations are measured.
The article examines all three areas, but one often misunderstood is accuracy. Gas detectors looking for flammable or toxic gases must measure small amounts of the subject product accurately. In fact, well designed sensors should have their highest accuracy in the low end of their range so they may provide early warning of incipient leaks.
As a result, accuracy becomes a key consideration when setting alarm limits. If the range of plus or minus is too wide, alarm limits cannot be set at the most desirable points. For instance, if the nominal alarm level is 40 % LEL (lower explosive level) and the device accuracy is ±3 % LEL, the instrument should be configured to alarm at 37 % LEL. The sensor’s accuracy dictates how the alarm level is set, rather than the ideal value, which can put it perilously close to where false alarms can sound, disrupting production.
To improve operations, alarm levels should be set as close to ambient conditions as possible without causing false alarms. Emerson’s Net Safety Millennium II SC310 Catalytic Bead and SC311 Point Infrared Combustible Gas Sensors are designed to provide exactly that kind of performance. The SC310 and SC311 provide versatile, robust and field-proven performance in a rugged package designed for the most extreme industrial environments. For flammable gases, % LEL detection is fast and reliable with simplified field serviceability using the M2B, M21 or M22 universal transmitters.
You can find more information like this and meet with other people looking at the same kinds of situations at Emerson Exchange and in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Flame & Gas Group and other specialty areas for suggestions and answers.
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