GAS SENSOR TECHNOLOGIES
Catalytic sensors (sometimes called a bead or pellistor type) have been mainly used for combustible gases including ammonia and are the most popular sensor for this application. The Gas Sensor functions by burning the gas at the surface of the bead and measuring the resultant resistance change in the bead (which is proportional to concentration). These are relatively low-cost, well established and understood and they have a good life span, up to 5 years. The response time is about 20-30 seconds. They can be subject to poisoning in certain applications but not generally in refrigeration and are more effective at gas levels of 2,000PPM up to 100% LEL. They are used mainly with combustible gases and are therefore suited to ammonia and the hydrocarbon refrigerants at high concentrations. They do see all combustible gases but they respond at different rates to each and so can be calibrated for particular gases. There are ammonia specific versions.
Electrochemical cells are used mainly for toxic gases and are suitable for ammonia but not for the other refrigerants. These generally consist of two/three electrodes immersed in an electrolyte medium. An oxidation / reduction reaction generates an electric current that is proportional to gas concentration. They are very accurate (0.02PPM) and tend to be used principally for toxic gases which cannot be otherwise detected or where high levels of accuracy are needed. Electrochemical Gas Sensors were relatively expensive with a short life span. However Gas Sensors are now available to cover the key range of 0-1000 PPM and with a longer lifetime of about 3 years in clean air with 3 to 4 years not unusual. Exposure to large ammonia leaks or constant background ammonia will shorten the sensor life. These are ideal for ammonia in the key range of 0-1,000PPM. They are subject only to rare cross interference. They may react to sudden large humidity changes but quickly settle.
Infrared technology utilises the fact that most gases have a characteristic absorption band in the infrared region of the spectrum and this can be used to detect them. Infrared Gas Sensors when first introduced were specific to a single gas and therefore not suitable for applications involving monitoring more than one gas. They were very selective and accurate – reading down to one part per million. Infrared was typically used where a high level of accuracy and specificity is required. This very precision in performance ensured that they were expensive. However the specificity became a disadvantage in machinery rooms as phase out resulted in mixed gas installations needing a different model for each gas, which was a very expensive solution. New models were developed based on broad infrared wavelength monitoring that could detect a mixture of gases. This however reduced the specificity and accuracy.
Refrigerant specific units may be used if a possibility of cross interference exists.
The semi-conductor functions by measuring the resistance change (proportional to the concentration) as gas is absorbed on to the surface of a semi-conductor, which is normally made from metal oxides. These can be used for a wide range of gases including combustible, toxic and refrigerant gases. It is claimed that they perform better than the catalytic type in the detection of combustible gases at low concentrations, up to 1,000PPM. These are low-cost, long life, sensitive, stable, resistant to poisoning and can be used to detect a large range of gases including all the CFC, HCFC, HFC refrigerants, ammonia and hydrocarbons. However, they are not selective and are not suited to detecting a single gas in a mixture or for use where high concentrations of interfering gases are likely to be present. Cross interference can be minimised by using filtered Gas Sensors and by calibrating for the specific gas and incorporating a delayed response. Semi-conductors for halocarbons can be used to detect simultaneously more than one gas or a mixture. This is particularly useful in monitoring a plant room with a number of different refrigerants.