Posted in | Gas Sensors

Combustible Gas Detection with the VQ21 / VQ21TB

SGX Sensortech supplies an extensive range of combustible gas sensors or pellistors functioning based on the catalytic oxidation principle.

The VQ21T Series belong to the VQ21 range of low voltage, high power, poison resistant detectors for general purpose applications, especially for fixed point systems where the considering power consumption is not an overriding factor.

It may be used for detection of most flammable vapours and gases and especially can sense methane in air mixtures with concentrations from 0.1% up to 5% (100%LEL).


The VQ21 is an enhanced poison-resistant sensor having two matched elements used for detecting combustible gases especially methane in air mixtures in concentrations from 0.1% upwards. Carbon dioxide or water vapour interference is absent. The least sensitivity is 10mV/% methane. The VQ21 is so designed that it can be used in air where silicone and other poisoning agent traces may be present.

Key Features

The key features of the VQ21 are:

  • Poison resistant
  • Low voltage
  • For fixed systems

Electrical Specifications

The data provided below is related to the VQ21 operating in the recommended circuit shown in Figure 1.

  • Operation - continuous
  • Bridge supply -2.0+ 0.1 V
  • Bridge power consumption - 0.75W max
  • Typical average sensor current - 300 mA
  • Minimum sensitivity - 10 mV/% methane
  • Linearity - linear up to 5% methane
  • Response time to register 1 1/4% in a 21/2% concentration is 2s
  • Maximum methane concentration is 5 %

Bridge circuit

Figure 1. Bridge circuit


The recommended mounting arrangements are shown in Figure 2

Recommended mounting arrangements

Figure 2. Recommended mounting arrangements

The outline is shown in Figure 3.

Outline (All dimensions without limits are nominal)

Figure 3. Outline (All dimensions without limits are nominal)


Each element is identified with a distinct serial number engraved on the compensator and detector can. On the detector and compensator, the serial number is written in red and black respectively. Also, the detector has a red circular label on the base identifying the device type.

Important Points to Remember

  • Operation may be under either diffusion or direct flow conditions in appropriate mountings
  • With open-circuit conditions at the bridge output.
  • The response time is a function of the utilized mounting type.
  • In case the VQ21 is exposed to over 5% methane concentration, the instrument calibration of the instrument must be checked.
  • The element supply is done as a matched pair with a trimming resistor R of the correct value which is supplied where required. The trimming resistor is to be linked across the compensator element as shown below.
  • The poisoning resistance by silicone compounds partly depends on the housing type in which the elements are mounted.
  • The response to some heavy flammable vapours, especially aromatic substances with side chains, may be less than that calculated from the conversion (K) factors listed in the preamble.


The VQ21T Series is especially resistant to poisoning by highly serious poisons, which includes the complete range of silicones. The VQ21T Series will offer longer life in comparison with a standard VQ1 pellistor. A typical response of a VQ21T sensor when exposed to hexamethyldisiloxane vapour is shown in Figure 4.

ypical response of vq21t and vq21 when exposed to Hexamethyldisiloxane atmospheres

Figure 4. Typical response of vq21t and vq21 when exposed to Hexamethyldisiloxane atmospheres

Operating Principle

The detector includes a fine platinum wire coil implanted in a porous ceramic bead impregnated with a noble metal catalyst. The coil behaves like an electrical heater and as a resistance thermometer.

The bead mounting is performed on a header with connecting leads and enclosed by a metal can with the end open to the atmosphere. In case a flammable gas is present when the bead is heated to 400–500°C the gas will oxidise and the resultant release of energy will heat the bead still further. An increase in coil temperature is sensed as an increase in coil resistance.

The coil temperature is impacted by ambient temperature and by changes in the thermal conductivity of the air caused by the possible presence of non-flammable gases such as carbon dioxide. A second inert bead is used in order to compensate for temperature changes not caused by the oxidation of the flammable gas.

This compensator includes a ceramic bead made on a similar platinum coil and instead of having a catalyst the treatment of the ceramic surface is done so that oxidation cannot take place. The two beads mounted normally on different headers are then used in a circuit that determines the resistance difference.

As the two beads have a different colour normally, their emissivity and slope resistances are different. Hence to obtain the best temperature performance it is important to connect a suitable fixed resistor in parallel with the compensator to correct for its higher slope resistance.

SGX Sensortech pellistors are supplied as a set of compensator and detector elements along with resistor, all three matched together perfectly to obtain the best performance.

Operation Procedure

The sensor output is based on a complicated combustion process established around the sensing beads. It is imperative that the gas layers around the beads are not disturbed, or else the bead temperature and the device output will change. Hence it is important to mount these devices in a

draught-free environment. Ideally, it is important to mount the elements symmetrically side by side with the open ends exposed to the gas being monitored. The elements are provided with ‘O’ rings so that gas-tight seals may be made between the mounting and the elements.

Similar to other sensors functioning on the catalytic oxidation principle, the sensor should be separated from the gas being monitored by a suitable flame arrestor. It is suggested that the compensator and the detector be operated in a Wheatstone Bridge circuit so that a difference signal may be obtained. Figure 5 shows the circuit.

Typical response of VQ21 to 20% LEL of various gases and vapours (relative to methane) as the bridge supply voltage is varied

Figure 5. Typical response of VQ21 to 20% LEL of various gases and vapours (relative to methane) as the bridge supply voltage is varied


This information is related to the device continuously operating in the recommended bridge circuit and mounted in a standard SGX Sensortech test block. Each element pair has a unique serial number.

Important Notes

  • The sensitivity is influenced partially by the gas flow to the elements. Consequently, it is impacted by the geometry of the mounting arrangements. The quoted figures were obtained in SGX Sensortech's standard test block, details of which are available on request.
  • The response time t50 is defined as the time for the sensor to reach 50% of final signal level. The response time is based on the type of mounting used and hence it may differ in use from the quoted values.
  • If the sensor has been exposed to greater than 100% LEL gas (while powered up), the instrument’s calibration should be checked. Operating the sensor in such high gas concentrations can cause carbon deposition on the detector, with a consequent change in sensitivity and zero offset.
  • Shock and vibration test results are based on customer mounting conditions and application.
  • The elements are provided as a matched pair with a selected trimming resistor R. This resistor must be arranged in parallel with the compensator as shown.

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