Thermal conductivity sensors are devices that quantify gases such as methane, helium and hydrogen, with percentage volume ranging from 0 to 100% and with varying thermal conductivities corresponding to a reference gas such as air. The difference in thermal conductivities can change according to the temperature of the gas.
The operating temperature of sensors running on SGX voltages ranges from 400 to 500°C. The temperature of the gases to be detected by the sensor will be eventually close to the operating temperature. In addition, the thermal conductivities of the detectable gases, except carbon dioxide and argon will be greater when compared to air. The graph below shows the difference in thermal conductivities of different gases.
Figure 1. Relative Thermal Conductivity of Gas with Temperature
The graph indicates that sensor signals of low density gases such as helium and hydrogen will be greater than that of other gases. The difference in thermal conductivities could be up to a factor of 6.
Like the compensators present in catalytic pellistor sensors, two matched inert beads are provided in the thermal conductivity sensor. The beads are welded on the header consisting of a metal can. One of the beads is sealed in the metal can or a chamber inside the certified head, and the other bead is exposed to the target gas.
The sensor is arranged in a Wheatstone bridge circuit and supplied with a constant voltage. Under atmospheric conditions, the beads release heat creating a set difference in resistances. This difference provides a bridge signal that can be nullified within the circuit.
Upon exposing the detecting elements to the target gas, a rise or drop in bead temperature occurs owing to the loss of heat from the detector bead, which subsequently results in the formation of resistance. The difference in bead temperature corresponds to the difference in thermal conductivities of target gases with respect to the reference gas. However, the sealed bead does not undergo any changes. The rise or drop in resistance is demoted by the positive/negative results of the bridge circuit.
The output derived based on the target gas, operating voltage and the type of sensor can be range from 50 to 100mV positive for pure argon gas, and 100 to 700mV negative for pure gases such as helium and methane.
The signal output is observed to be non-linear with reference to the increasing gas concentrations. The non-linearity can be analyzed with the help of look-up tables or curve fitting methods such as quadratic response.
Resolution and Accuracy
In general, thermal conductivity sensors measure gases of volume percentage between 0 and 100%. The resolution of the sensor, however, is sufficient to extend over such a huge LEL range. The accuracy of output derived from the curve fitting determines the resolution of sensor.
The devices carrying the thermal conductivity sensor should meet the specified standards of EN 61779 Parts 3 and 5.
- The sensor operation does not harm the environment and does not require an oxygen supply as there are no catalytic processes involved. However, the sensor response may vary with changes in bead temperature in case of decomposition of species on the detecting bead.
- It is necessary to thoroughly seal the compensating element to avoid signal loss when the target gas reaches the element.
- The rate of heat loss from the bead will be affected with varying levels of pressure and humidity of target gases.
About SGX Sensortech (IS)
SGX Sensortech is a market leader in innovative sensor and detector devices that offer unrivalled performance, robustness and cost- effectiveness.
SGX have been designing and manufacturing gas sensors for use in industrial applications for over 50 years, offering excellent applications support for an extensive range of gas sensors and the expert capability for custom design or own label.
As an independent OEM supplier of gas sensors, we pride ourselves on providing customers with unrivalled product reliability and personal product support via specialist engineers.
SGX gas sensors are built to the highest standards with all pellistor and infrared gas sensors achieving ATEX and IECEx certification, SGX gas sensors are also UL and CSA approved.
Our product portfolio has continued to expand in technology and detectable gases used in a wide range of applications including:-
- Oil and gas
- Confined space entry
- Indoor air quality
- Industrial area protection
- Leak detection
This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.
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