Leak Detection

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.