VQ21T Series Combustible Gas Sensor Elements To obtain the best temperature performance it is To be read in conjunction withIntroduction to Pellistor Gas Sensor and Pellistor Application Notes 1, 2, 3, 4, 6 and 7. sometimes necessary to connect a suitable fixed resistor in parallel with the compensator to correct for its higher slope FEATURES resistance. SGX Sensortech pellistors are supplied as a set of Highly poison resistant detector and compensator elements together with a resistor, Low voltage all three carefully matched together to obtain the best For fixed systems performance. INTRODUCTION GENERAL DATA SGX Sensortech supplies a wide range of combustible Gases detected Flammable gases and vapors gas sensors (pellistors) working on the catalytic oxidation principle. The VQ21T Series are members of the VQ21 Expected operating life >5 years range of high power, low voltage poison resistant detectors for general purpose application particularly for fixed point systems Bridge supply voltage 2.0 + 0.1 V where the consideration of power consumption is not an Sensor current 300 mA typical overriding factor. It may be used to detect most flammable gases and vapours and, in particular, can detect methane in Sensor power consumption 750 mW max air mixtures with concentrations from 0.1% up to 5% Minimum sensitivity (note 1) 15 mV/% (100%LEL). SGX Sensortech made several major improvements to Linearity linear up to 3% CH4 the structure of pellistor beads to reduce the susceptibility to Response time (t ) (note 2) 2 secs Typical catalyst poisoning which was a major failing of early pellistor 50 structures. The structure of the VQ21 range represents the Max. gas concentration (see note 3) 100% LEL most poison resistant one available from any source. The VQ21T Series is particularly resistant to poisoning by the Operational temperature -40C to +70C most serious poisons - the whole range of silicones. Storage temperature +10C to +25 C The VQ21T Series will provide several orders of magnitude longer life when compared with a standard VQ1 pellistor. A Pressure 1 Atm. +/-10% typical response of a VQ21T sensor when exposed to hexamethyldisiloxane vapour is shown on page 3. Humidity 0-80%RH (Non condensing) Orientation Any PRINCIPLE OF OPERATION Shock test (see note 4) 250 g, 5 blows in each plane The detector consists of a fine coil of platinum wire Vibration test (see note 4) 20 g, 24 cycle, 100 to 3200 Hz embedded in a porous ceramic bead impregnated with a noble metal catalyst. The coil acts as an electrical heater and as a resistance thermometer. The bead is mounted on a Operation header with connecting leads and is surrounded by a metal The output of a sensor is dependent on a complex can with the end open to the atmosphere. If a flammable gas combustion process established around the sensing beads. is present when the bead is heated to about 400C to 500C It is imperative that the layers of gas around the beads are the gas will oxidise and the resultant release of energy will not disturbed, otherwise the bead temperature and hence heat the bead still further. This increase in temperature is device output will change. Therefore the devices must be detected as an increase in resistance of the coil. The mounted in a draught-free environment. Ideally, the elements temperature of the coil is also affected by ambient should be mounted symmetrically side by side with the open temperature and by variations in thermal conductivity of the ends exposed to the gas being monitored. air caused by the possible presence of non-flammable gases The elements are supplied with O rings so that gas- such as carbon dioxide. To compensate for temperature tight seals may be made between the elements and the changes not caused by the oxidation of the flammable gas a mounting. As for all sensors operating on the catalytic second, inert bead is used. This compensator consists of a oxidation principle, the sensor should be separated from the ceramic bead made on a similar platinum coil and instead of gas being monitored by a suitable flame arrestor. It is containing a catalyst the ceramic surface is treated so that recommended that the sensor pair are run in a Wheatstone oxidation cannot take place. The two beads are mounted on Bridge circuit so that a difference signal may be obtained. A separate headers, and known as the sensor pair which are suitable circuit is shown on page 2. In use, the bridge supply then used in a circuit which detects the difference and change voltage should not be allowed to vary by more than the in their resistance when a flammable gas atmosphere is specified 0.1 V, or the output in clean air may change in present. sympathy. Copyright SGX Europe Sp. z o.o. 2012-2018 VQ21T Ser Issue 6, March 2018 RECOMMENDED BRIDGE CIRCUIT MARKING 3. If the sensor has been exposed to greater than 100% LEL gas (while powered up), the instruments calibration Each pair of elements carries a unique serial number. should be checked. Operating the sensor in such high gas For devices supplied with PCB legs the number is written concentrations can cause the deposition of carbon on the in red on the side of the detector can and in black on the detector, with a consequent change in sensitivity and zero compensator can. offset. Running the device in clean air for an extended period will usually restore normal operation. PRECAUTIONS 4. Shock and vibration test results are dependent on Do not apply more than 50% of the maximum bridge customer mounting conditions and application. voltage to either element or otherwise there may be adverse effects on the stability of sensitivity and zero offset. 5. The elements are supplied as a matched pair with a selected trimming resistor R. This resistor must be fitted in Although the VQ21T Series possesses a high degree of parallel with the compensator as shown. resistance to poisons, it will be affected by prolonged exposure. Additional protection may be provided by the use of activated carbon cloth filters, but users should be aware VARIANTS AND ORDERING that, firstly, these filters will prevent high molecular weight INFORMATION flammable gases from reaching the detector, and, The VQ21T Series is available with a number of options. secondly, these filters require regular replacement to The variants are identified by adding suffices added to the maintain their effectiveness. basic type number. When ordering, the full type number with suffices must be used. WARNING OPTIONS INCLUDED To ensure safe operation and to satisfy the requirements of the recognised approval authorities. SGX recommends that a suitable sintered metal flame arrestor PCB Hydrogen Type Number be used with the device. Legs mods. VQ21TB VQ21TSB NOTES 1. The sensitivity is partially influenced by the flow of gas Other variants are available. Potential users are encouraged to the elements. Consequently, it is affected by the to discuss their applications in confidence with SGX sales geometry of the mounting arrangements. The quoted and application engineers who will be able to suggest the figures were obtained in SGX standard test block, details most appropriate and economical solution to their particular of which are available on request. requirements. 2. The response time t50 is defined as the time required for the sensor to reach 50% of final signal level. The response time is dependent on the type of mounting used and therefore it may differ in use from the quoted values. Copyright SGX Europe Sp. z o.o. 2012-2018 VQ21T Ser Issue 6, March 2018