|Pellistors are solid-state devices used to detect gases which are either combustible or which have a significant difference in thermal conductivity to that of air. The detecting elements consist of small ‘pellets’ of catalyst-loaded ceramic whose resistance changes in the presence of the target gas hence the term ‘pellistor’ being a combination of ‘pellet’ and ‘resistor’.
The earliest form of catalytic gas sensor used heated bare coils of platinum wire to burn the gas. The heat generated by the burning process produced a change in the resistance of the coil. This change was measured using a simple Wheatstone bridge circuit.
One year ago, SGX SensorTech introduced the VQ548MP, a MEMS Pellistor. The MEMS pellistor structure consists of a pair of planar metal heaters coated with a layer incorporating a noble metal catalyst for Detector structure and with inert layer for Compensator structure. This heater pair deposited onto membranes, optimally designed for thermal insulation, allows operating power as low as 60 mW per structure (120 mW for the pair) in continuous operation. (The meander acts both as an electrical heater and as a resistance thermometer. The pellistor dual- structure is die attached on a PCB and electrically connected with wire bonding.) The sensor chip is surrounded by a plastic can equipped with flame arrestor towards the end open to the atmosphere.
Today, SGX has introduced a new version of the MEMS Pellistor family, the VQ548MP-DA device. While the VQ548MP will still be available, SGX has had the device re-certified to meet new standards: Ex-da-IIC. The testing is being completed and the certifying body done close to the SGX manufacturing site in Poland: ATEX Notified Body- NB1461. http://obac-cert.co.uk/accreditations/product-certification-en. Samples of these devices are available to qualifying customers, as well as copies of the final certification documents.