In a recent article entitled “LED-Driven Infrared Sensors: Shining New Light on LEL Gas Measurement for Oil and Gas and Confined Space Entry Applications” published in Industrial Hygiene News, Bryan Bates, CEO of GasClip Technologies, touted the benefits of using low-power infrared sensors for detecting combustible gases over the industry standard catalytic bead sensing technology.  While there can be no doubt that the comparatively minimal power consumption, ability to detect gas in an inert atmosphere and the immunity from poisons of IR sensors provides tangible benefits in some applications, the infrared technology does come with some significant shortcomings.

First and foremost, the infrared sensor has no ability to detect hydrogen – Period.   If the sensor is being used in any application to detect combustible gas on a general basis and there is any possibility that hydrogen may be encountered, the user of the instrument will be left unprotected.  Bates acknowledges this limitation but promotes the cross interference of the typical carbon monoxide sensor to hydrogen, which is typically on the order of 40 – 60 percent as the cure for this ill.  When did relying on the shortcomings of one sensor to make up for the shortcomings of another become a recommended or acceptable practice?  The cross interference of hydrogen on the CO sensor is very common, and although the response stated here is typical, it can vary greatly from sensor to sensor.  What happens if a given sensor has a significantly less level of interference?   Hydrogen interference on carbon monoxide sensors results in false CO alarms which leads the user to a lack of trust and confidence in the monitor.  Once this occurs repeatedly, the user is as likely to turn the monitor off or not use it at all as he/she is to heed its warnings, regardless of the level of training that they may have.

A key benefit of the catalytic bead gas sensor is that it the only sensing technology which detects gas based on the hazard of the gas itself – it detects combustible gas by burning it.   The catalytic bead sensor is capable of detecting virtually any combustible gas, because it is combustible.  By contrast, the detection capability of the infrared sensor is limited by the IR adsorption characteristics of the gas and the bandwidth of the IR filter in the sensor.   Many combustible gases are simply undetectable by this type of low power infrared sensor.  Examples of non-detectable combustible gases include acetylene, acrylonitrile, aniline and carbon disulfide.  Bates acknowledges the limitation of the sensor for detecting acetylene, a very common hazard in hot work and confined space entry applications, and again promotes the interference of the carbon monoxide sensor as the solution to this pitfall.  The argument given above with respect to hydrogen remains the same and gets broad when you take into account that the CO sensor may not have a cross interference to a given combustible gas that is simply not within the scope of detection of the infrared sensor.  

The response of the catalytic bead sensor to combustible gas is inherently linear and there is relatively close correlation in the response from one gas to another with respect to the calibration gas.  Response factors for catalytic bead sensors to various gases are typically less than two.  The response of the infrared sensor is non-linear and only becomes linear once the sensor is characterized to a particular gas.  Response factors between gases also vary greatly and can be greater than a factor of ten in some cases.   For example, an infrared sensor that is characterized to methane will provide a non-linear response to pentane or propane and accordingly the response factor will only be accurate at one point on the response curve.  If a gas with a response factor of 10 or greater is encountered, the instrument would produce a false alarm when the actual gas concentration was just 1-percent of the LEL.  The chart below highlights the difference in gas response factors between catalytic bead and low-power infrared combustible gas sensors.

Catalytic bead sensors are relatively unaffected by varying environmental conditions such as temperature and pressure.  These factors greatly affect the performance of the infrared sensor and therefore the sensor must be characterized for these environmental effects in order to provide an accurate and reliable response.

As stated above, the benefits of the infrared technology for detecting combustible gases in some applications are undeniable.  However, before dismissing the long-standing, industry standard catalytic bead technology, you must make sure that your application is appropriately matched to the technical capabilities of the sensor.   Otherwise, the risk you have exposed yourself to may be far greater than the reward.

Dave