14
Gas Detection
Choosing the Best Technologies for Combustible Gas and VOC Measurement
Most multi-sensor gas detectors include sensors used to measure the four most commonly encountered hazards: oxygen deficiency (and enrichment), LEL combustible gas, carbon monoxide and hydrogen sulphide. However, in some cases, these basic sensors are not capable of measuring the atmospheric hazards that are actually present.
The key to success is understanding the monitoring environment, and the specific benefits and limitations of the sensors selected.
Hundreds of thousands of these basic “four-gas” atmospheric monitors are used every day. The sensors utilised in these portable gas detectors are extremely good at detecting what they are designed to measure. As good as the sensors are, however, they still have limitations. It is critically important for instrument users to understand what the sensors in their instrument cannot properly measure as well as what they can.
In most cases the types of sensors installed in these basic instruments are well suited to the hazards to be measured. However, specific conditions and hazards may require the use of more specialised sensors, or a specialised calibration strategy that will provide more accurate readings for the gases actually present. Combustible gas sensors are particularly subject to limitations that can materially affect their ability to detect certain types of combustible gases and vapours. The good news is that there is an extremely wide range of technologies and types of sensors available for use in portable multi-sensor instruments.
Just because one type of sensor does not work for a particular gas does not mean there are no alternatives.
The only limitation is that the instrument must be sufficiently flexible to make use of the most appropriate detection technologies. Oxygen, carbon monoxide and hydrogen sulphide sensors are designed to measure a single type of gas. There is very little ambiguity in the readings these sensors provide.
The only gas an oxygen sensor responds to is oxygen. Electrochemical sensors designed to measure a particular gas may not be quite so specific.
Although sensor manufacturers design their products to minimise responsiveness to gases other than the one they are supposed to measure, no design is perfect.
For instance, CO sensors may also respond to hydrogen as well as to the vapours produced by alcohol, solvents and other volatile organic chemicals (VOCs).
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Since most interfering effects are positive, the possibility that the sensor may occasionally provide higher than actual readings for CO is generally not regarded as a safety concern. It just means that workers leave the affected area a little sooner.
Similarly, hydrogen sulphide sensor readings can be affected by exposure to degreasers and solvents such as methanol and citrus oil cleaners. The sensor with the most important limitations is the traditional “catalytic” or “pellistor” type percent LEL combustible gas sensor.
In spite of the millions of combustible sensor equipped atmospheric monitors in service around the world, there is still a lot of misinformation and misunderstanding when it comes to the performance characteristics and limitations of this very important type of sensor.
Understanding how combustible sensors detect gas is critical to correctly interpreting readings, and avoiding misuse of instruments that include this type of sensor.
Figure 1: Combustible pellistor type sensor showing housing and detached flame arrestor
AET Annual Buyers’ Guide 2012
www.envirotech-online.com How Combustible Sensors Detect Gas
“Pellistor” type LEL sensors detect gas by catalytically oxidising or “burning” the gas on an active bead or “pellistor” located within the sensor. The heating effect on the bead is proportional to the amount of combustible gas present in the atmosphere. Catalytic-bead sensors respond to a wide range of ignitable gases and vapors, but are unable to differentiate between different combustible gases. They provide one signal based on the total heating effects of all the gases capable of being oxidised that are present in the vicinity of the sensor.
The heating effect or “relative response” of the sensor varies from gas to gas. Generally speaking, the larger the molecule, the lower the relative response.
Pellistor type sensors generally include a flame arrestor that can slow, reduce or prevent larger hydrocarbon molecule from entering the sensor (Figure 1).
Small combustible gas molecules like hydrogen (H2), methane and propane (C3H8) diffuse through the flame arrestor very rapidly. The larger the molecule, the slower it diffuses through the flame arrestor into the sensor.
Saturated hydrocarbons larger than nonane (C9H20) are unable to penetrate the flame arrestor at all in appreciable quantities. Traditional pellistor type LEL sensors should not be used to measure hydrocarbon gases larger than nonane in size.
To put this in perspective, less than 4% of the molecules in a bucket of diesel fuel are small enough to pass through the flame arrestor and enter the sensor.
This is one of the reasons that pellistor LEL sensors show such a low response when exposed to the vapors of “heavy” fuels such as diesel, kerosene, jet fuel and heating oil.
Although most VOC vapours are combustible, the toxic exposure limits are much lower than the flammability limits.
For example, for diesel fuel 10% LEL is equal to about 600 ppm vapour. However, the TLV (Threshold Limit Value) for diesel vapour is only 15 ppm (as an 8 hour TWA). If you wait for the combustible gas alarm to go off at 10% LEL you could potentially exceed the toxic exposure limit by 40 times!
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