In our new blog series, we’re investigating the core principles of gas detection, and how they function to create the comprehensive gas safety systems we know today. First up: combustible gas & catalytic sensors!
History of Combustible Gas Sensors
The earliest forms of a combustible gas detector were used in the mines, to detect ‘firedamp’. This was the name given to flammable gases, primarily methane; found where the coal was mainly bituminous. The device could be used to estimate the levels of combustible gases up to an accuracy of 25–50%, depending on the user’s experience, training, age, colour perception.
Obviously, with such a huge margin for error, more accurate detectors needed to be developed, leading to all but scrapping this type of optical detector in favour of the catalytic sensor. It does still bear resemblance to the early detector, as it relies on the combustion of gas and the resulting water and Carbon Dioxide.
The Catalytic Sensor
Nowadays, nearly all modern combustible gas sensors are a form of the electro-catalytic sensor. The sensor is a very small bead, made of an electrically heated platinum wire coil. It is first covered with a ceramic base such as alumina and then given a final outer coating of Palladium or Rhodium catalyst dispersed in a substrate of Thoria.
When a combustible gas and air mixture passes over the hot catalyst surface, combustion occurs and the heat involved increases the temperature of the sensors bead. This alters the resistance of the Platinum coil and can be measured by using the coil as a temperature thermometer in a standard electrical bridge circuit.
The resistance change is directly related to the gas concentration in the surrounding atmosphere and can be displayed on a meter or some similar indicating device, giving an accurate indication of the concentration of gas present.
Response to Combustible Gases
Due to the nature of the gas, the catalytic sensor must be kept in a strong metal casing behind a flame arrestor, allowing the gas and air mixture to diffuse into the housing and onto the sensor, but preventing any flame spreading to the outside atmosphere.
The flame arrestor slightly reduces the speed of the sensor’s response, but in most cases, the electrical output will give a reading seconds after the gas has been detected.
However, because the response curve is considerably flattened as it approaches the final reading, the response time is often specified as the time to reach 90% of its final reading; known as the T90 value. T90 values for catalytic sensors are typically between 20 and 30 seconds.
Temperature Stability in Catalytic Sensors
Thermally-matched beads ensure stability when the ambient conditions may vary in the environment. They are located in opposing arms of a Wheatstone bridge electrical circuit, where the ‘sensitive’ sensor (usually known as the ‘s’ sensor) will react to any combustible gases present, whilst a balancing, ‘inactive’ or ‘non-sensitive’ (n-s) sensor will not.
The non-sensitive sensor is either coated with a film of glass or de-activated entirely so that it will act only as a compensator for any external temperature or humidity changes.
Poison-resistant sensors also improve temperature stability, as these have better resistance to degradation by substances such as silicones, Sulphur and lead compounds which can rapidly de-activate (or ‘poison’) other types of catalytic sensor.
Calibration of Catalytic Sensors
The biggest cause of failure for catalytic sensors is degradation from being exposed to certain poisons. With this in mind, it’s vital that your catalytic sensors are not only calibrated on installation but also checked regularly and re-calibrated as necessary, using an accurately calibrated standard gas mixture so that the zero and ‘span’ levels can be set correctly on the controller.
Want to know more about calibration? Discover the importance of integrating regular calibration into your maintenance schedule.
While catalytic sensors are not the only type of detection for combustible gases, they provide a simple, reliable, low-cost solution for your gas safety needs. Use the DSA Suppliers’ many years of experience to help you choose the right combustible gas detection system for your unique environment.