Tutorial: Infrared gas detectors

08/16/2007


In many process plant applications, there are very compelling reasons to detect and measure the presence of various gasses in the atmosphere or process gas stream. An obvious example would be escaping methane that could cause a fire or explosion, or accumulations of carbon monoxide that could asphyxiate an employee. One type of instrumentation for this purpose is a point infrared gas detector. There are many configurations using this versatile technology for a wide variety of applications.

The basic concept of the technology is very simple: Infrared radiation (IR) passing through a test sample will be affected by various gas components. Specific gasses absorb or reflect IR in predictable and measurable ways. An instrument using this approach generates IR with known characteristics and shines the radiation at a detector across an open space. In some cases the open space will be very small and inside a single enclosure. In other situations, the two devices may be more than 100 ft. apart.

Some approaches split the beam, either at the source or the detector, into a reference beam and a measurement beam. Any gas between the two will affect what strikes the detector. The ability to analyze the data using various sensor technologies, optics, and electronics is what separates manufacturers and differentiates products.

The variety of detectable gasses is broad, covering a range of flammable and toxic substances common in process plant environments. For example: acetone, ammonia, butane, carbon monoxide and dioxide, ethanol, isobutane, MEK, nitrous oxide, phosgene, propane, refrigerants, styrene, toluene, vinyl chloride, xylene, and many more. Virtually all hydrocarbons are detectable.

There are limitations:

Some gasses either don’t affect the sensor at all (e.g., hydrogen) or can influence sensitivity.
Water and water vapor absorbs IR and can cause sensitivity to drift if there is no compensation.

A given unit cannot differentiate between a wide variety of products. For example, a single instrument can’t be expected to give you component proportions from a mixture of ammonia, methane, phosgene and styrene. If you need to detect multiple gasses, you may need multiple sensors. However, given that there is a limit to what will be present in any single plant environment, this is usually not a major issue.

Different units have different sensitivities to different products. Choose a unit that has the appropriate sensitivity for the specific products that concern you.

If a gas detector is intended to be part of a larger safety system, the design has to have appropriate certifications for the environment and expected products. Safety issues related to gas detectors include the same elements as any hazardous area sensors, but also detection thresholds, response times, etc.

Given the critical nature of these applications, discuss your needs thoroughly with any prospective supplier.

IR gas detectors are available from a variety of manufacturers, including:

Comag IR
Detector Electronics Corporation
Delphian Detection Technology
General Monitors
Honeywell Analytics
KT Sensor
Sierra Monitor

You can also search online at the Control Engineering Supplier Search .

—Peter Welander, process industries editor, PWelander@cfemedia.com , Control Engineering Weekly News





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