Catalytic Combustible Gas Sensors (Pellistors)
These gas sensors are affected by poisons and inhibitors. Catalytic combustible gas sensors are susceptible to a number of different compounds which can result in irreversible poisoning or reduce the sensitivity of the sensor to measure and respond to different gases. The nature of the poison could be physical- soot or other bi-products of combustion which block the pores in the bead or chemical with molecules forming strong bonds with the catalyst and reducing activity.
Poisons such as silicon, lead, sulphur or phosphorus based compounds can result in immediate sensor failure. The most common chemicals that can poison catalytic sensors are those containing silicon, such as common oil, and lubricants and silicone compounds used in machinery.The simple act of cleaning your instrument with a silicon based cleaning product or hand cleaner can kill the sensor.
Inhibitors such as sulphur compounds (H2S), halogenated hydrocarbons and chlorinated compounds reduce sensor response (ability to measure). Compounds and chemicals used in common fire extinguishers and Freon used in refrigeration can cause the sensor to malfunction. Inhibitors slow the sensor response and in some cases reduce the sensor’s ability to measure some gases. This becomes readily apparent when instruments experiencing inhibitors cannot measure methane but can measure pentane. High concentrations of flammable/combustible gases (above the LEL) can have a poisoning effect, due to the build-up of carbon soot. Molecules with high carbon content such as aromatic compounds can be a particular concern.
If you have or suspect you have any poison or inhibitor in your working environment where your gas detection device is used then you must put into action testing procedures to make sure your instruments are working correctly. These testing procedures include bump testing and calibration.
“Bump Testing” the instrument prior to use is necessary as some airborne containments may inhibit or poison the catalyst, so the sensor may not be capable of measuring gas. Use methane to bump test the combustible gas sensor even if other gases are used to calibrate. This is because methane is difficult to burn and will show the signs of the sensor malfunction before other test gases. A “Bump Test”, in these circumstances would allow a low concentration of flammable gas to flow across the sensor, and if the audible and visual alarms are activated this would indicate that the sensor is “seeing gas “. The procedure is simple to perform and only requires a cylinder of bump test gas and a regulator, and can be performed by field operators. Note: If the instrument fails the “Bump Test ‘’ it should be placed out of service and sent for repair/calibration.
Calibration should continue on your scheduled time periods provided you are bump testing prior to use. Technicians should test the instrument first before calibrating and record all sensor response data. This will indicate if the sensor is losing response between calibration periods and allow you to determine if the calibration period should be shortened or lengthened. Bump testing and calibration is always important but particularly critical when inhibitors or poisons are present.