01 February 2003
Point infrared gas detector design guide
By Dave Opheim
Users of flammable gas detectors are finding point infrared (PIR) combustible gas detectors work well for lower explosion limit hydrocarbon vapor measurement and are less costly than traditional catalytic combustible gas detectors. While many fixed and portable gas detector manufacturers are adding PIR gas detection instruments to their repertoire, technical evaluators of today's PIR gas detectors market find not all are created equal.
WHY POINT INFRARED?
Historically, detecting combustible gases involved sensors based on catalytic (pellistor) hot-wire bead technology. While they generally work well, pellistor-based sensors are susceptible to undetected failures-primarily catalyst poisoning and flame arrestor plugging, either of which prevents gas sensing.
The only way to detect these failures is to periodically check the sensor with test gas, so it's possible covert failure could remain undetected for months-calibration frequency for catalytic sensors is typically every 30 to 90 days.
The absence of covert failure modes is one advantage infrared gas detectors have over catalytic sensor technology. Other advantages are reduced calibration frequency, longer service life, and superior resistance to the effects of contaminants.
HOW PIR GAS DETECTORS WORK
All infrared gas detectors use variations on the basic measurement scheme. An infrared illuminates a volume of gas that has diffused into a measurement chamber. The gas absorbs some of the infrared wavelengths as the light passes through it, while other wavelengths pass through completely unattenuated. The amount of absorption relates to the concentration of the gas.
You can measure absorption by a set of optical sensors and subsequent electronics. The change in intensity of the absorbed light compares with the intensity of nonabsorbed light. The microprocessor computes and reports the gas concentration from the absorption.
The most common failure mode of infrared gas detectors is insensitivity or signal drifting caused by water and water vapor. Water absorbs the monitored infrared wavelengths within PIR gas detectors and must be rejected from the device optics. Effective control techniques include heated optics to help control condensation, avoiding use of sintered metal flame arrestors to comply with explosionproof design (these filters are prone to blockage and generally cannot be cleaned), and self-draining optics protection to ensure windblown rain won't accumulate in the optics and impede operation. In heavy gas detector applications, installation near grade level is typical to increase exposure to saturation from deflected rainwater.
A good design withstands direct exposure to high-pressure water without device failure. It should be easy to remove the optical protection system so you can inspect and clean it. It will use a multilayer baffle set to achieve a labyrinth chamber protection system. Provide conformal coating of PC boards, especially for humid, tropical settings.
Not all PIR detectors deliver the same sensitivity. Generally, the longer the optical path, the better the sensitivity. For this reason, folded path optical designs are useful in that the measurement path can be close to double that of the overall length of the instrument. This design requires the use of mirrors, which if implemented properly offer effective and robust performance. Sapphire-coated mirrors offer maximum resistance to degradation.
|Infrared measurement scheme|
TYPICAL PERFORMANCE REQUIREMENTS
Users install PIR gas detectors primarily for early detection and warning of the presence of a dangerous leak of combustible gas. Detector performance should comply with ISA-12.13.01-2001, Performance Requirements for Combustible Gas Detectors, to ensure adequate capability. This standard addresses speed or time required to reach 60% of full scale output (T60), which must be within 12 seconds. The optimum design achieves a balance between speed of response and good optics protection.
The accuracy performance test requires exposing the detector to five gas concentrations up to 100% full scale concentration. In each case, the concentration indicated must not vary from the known test gas concentration by more than ±3% of full scale gas concentration or ±10% of applied gas concentration, whichever is greater. PIR gas detectors are generally factory calibrated to methane gas because it ensures proper sensitivity to methane as well as an abundance of sensitivity to other hydrocarbon vapors. IT
Dave Opheim is gas detection products marketing manager at Detector Electronics Corp. in Minneapolis.
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