Gases all have their own thermal conductivity that helps you to understand how well heat moves through it.
Thermal conductivity is measured using a sensor that employs four matched filaments that change their resistance according to the thermal conductivity of the gas passing over it.
The thermal conductivities of several common gases are listed in the table below.
Thermal conductivities of common gases
Principle of Operation for thermal conductivity analysis
The sensor uses four matched filaments that change resistance according to the thermal conductivity of the gas passing over it. The sensors and the filaments are connected in a ‘wheatstone bridge’ configuration as shown below in Figure 1.
Figure 1. Wheatstone Bridge of the thermal conductivity detector
When all four resistances are the same, the VOUT is zero and the bridge is considered balanced. When ‘zeroing’ the reference gas is passed over all of the filaments. The resistances will be the same because the filaments are matched and the bridge is balanced. When the sample gas is passed over half of the bridge, then the VOUT value correlates to the content of the sample gas in the reference.
The detector is a four element ‘katharometer’ having two elements situated in the reference gas and two elements in the sample gas shown in Figure 2 below.
Figure 2. Cut-away view of the thermal conductivity sensor.
The four elements are electronically connected in a bridge circuit and a constant current is passed through the bridge to heat them. If each element is surrounded by the same gas, then the temperature – and hence the resistance of each element – will be similar and the bridge circuit will be balanced.
Figure 3. Electrical diagram of the thermal conductivity sensor.
When the sample gas is introduced into the sample gas stream, the two katharometer elements in the stream will be cooled to a greater extent than the two elements in the reference gas. The bridge circuit will be unbalanced, producing a signal voltage related to the makeup of the sample gas. This relationship is non-linear. As a result, the Systech Illinois 542 programmable gas analyzer is calibrated at zero, mid-span, and high span. And the software mathematically linearizes the curve.
Download the full discussion note linked below for equations showing the relation between bridge voltage output and thermal conductivity.
Measure the gas sample content of a sample/reference mixture by comparing the thermal conductivity of the mixture with that of the reference.
For example, hydrogen has a thermal conductivity which is approximately seven times greater than that of nitrogen, so small changes are readily detected. All other common gases have thermal conductivities similar to nitrogen, so the method of measurement is fairly selective.
Helium is the only other gas with a thermal conductivity comparable with that of hydrogen.
Other gases that may be measured using this technique are:
- Carbon dioxide
- Sulphur dioxide
WARNING: once again, many sensors can’t be used to measure gas/air or gas/oxygen mixtures if they are potentially flammable.
The Systech Illinois 542 gas programmable gas analyzer is used by industrial gas companies, metal heat treating companies, and furnace manufacturers.
Applications range from high purity gas production to furnace atmospheres.
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