Thermal Conductivity Gas Detection with the VQ31MB

The VQ31 series devices include two matched elements used for detecting gases in a colume concentration of 0 to 100% by using the recommended bridge circuit and the mounting arrangement as shown in Figure 1 and 2 respectively.

The VQ31M is provided with braided leads and the VQ31MB has extended pins for direct attachment to PCBs.

The elements function based on the thermal conductivity principle.

The sensing unit is exposed to the test atmosphere and the supply of the reference element is performed by sealing in reference air.

Bridge circuit

Figure 1. Bridge circuit

Recommended mounting arrangements

Figure 2. Recommended mounting arrangements

Operation of the VQ31

The device response is based on the difference between the thermal conductivity of the reference air and the test atmosphere. When the thermal conductivity of the test atmosphere is higher than that of reference air, more heat is lost by the sensing element to the surroundings in comparison with the reference element.

Due to this high heat loss, the sensing element is cooled and its resistance is reduced. A gas mixture’s thermal conductivity is based on temperature and the individual thermal conductivity of the constituents of the mixture. The key temperature is the operating temperature of the sensing element (typically a maximum of 500°C at the recommended bridge supply voltage).

It is possible to operate the sensor at any bridge voltage up to the standard bridge supply, that value inclusive, with the sensing element operating at temperatures up to and inclusive of the typical maximum. The thermal conductivity of common gases relative to air is shown in Table 1 at various temperatures. It is important to note that while operating the sensor in a constant voltage mode, the response is non-linear to high gas concentrations.

Table 1. Thermal conductivities of common gases relative to air

Element temperature 0 °C 100 °C 200 °C 300 °C 400 °C 500 °C 600 °C
Air 1.000 1.000 1.000 1.000 1.000 1.000 1.000
Nitrogen 0.996 0.993 0.997 0.999 0.998 0.994 0.988
Oxygen 0.987 1.026 1.049 1.062 1.065 1.062 1.056
Argon 0.686 0.687 0.682 0.674 0.663 0.650 0.636
Carbon dioxide 0.621 0.745 0.832 0.893 0.933 0.959 0.975
Water vapour 0.969 1.076 1.192 1.303 1.404 1.493 1.574
Methane 1.244 1.500 1.723 1.911 2.066 2.192 2.296
Ethane 0.742 1.027 1.271 1.474 1.638 1.769 1.874
Propane 0.619 0.874 1.092 1.271 1.415 1.529 1.619
Carbon monoxide 0.961 0.962 0.970 0.975 0.976 0.974 0.970
Ethylene 0.703 0.970 1.195 1.378 1.524 1.637 1.727
Acetylene 0.886 1.040 1.155 1.240 1.301 1.344 1.376
Hydrogen 7.371 6.918 6.692 6.548 6.435 6.336 6.252
Helium 5.972 5.681 5.492 5.338 5.197 5.062 4.939
Ammonia 1.082 1.295 1.493 1.670 1.824 1.955 2.069

Electrical Specifications

The following data relates to the VQ31 series operating in the recommended circuit shown.

  • Continuous operation
  • Bridge supply - 2.0 to 3.5 V
  • Bridge power consumption:
    • at 3.5V is 0.35W max
    • at 2.0V is 0.28W
  • Typical response to methane in air
    • at 3.5V is 2.5 mV/%
    • at 2.0V is 1.0 mV/%
  • Response time is a function of the mounting type

Mechanical Specifications

  • Shock test - 250g, 5blows in each plane
  • Vibration test - 20g, 24cycles from 100 to 3200Hz

The outline of the VQ31 series is given in Figures 3 and 4.

Outline of VQ31M

Figure 3. Outline of VQ31M

Outline of VQ31MB

Figure 4. Outline of VQ31MB

Marking

A unique serial number on the can of both the reference and sensing elements denote each element. The serial number is engraved in red on the sensing element and black on the reference element. The sensing element also carries a red circular label on the base, which identifies the device type.

Important Notes

Points to be noted include the following:

  • Operation may be under either direct diffusion or flow conditions in appropriate mountings
  • With open-circuit conditions at the bridge output.
  • The response time is a function of the mounting time used.

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