Each gas has a known thermal conductivity - how well heat transfers through it. Thermal conductivity is measured with a sensor that employs four matched filaments that change resistance according to the thermal conductivity of the gas passing over it.
The thermal conductivities of some gases can be found in the table below.
Thermal conductivities of common gases
The sensor uses four matched filaments that change resistance according to the thermal conductivity of the gas passing over it. These four 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, VOUT is zero and the bridge is considered balanced. When zeroing, the reference gas is passed over all the filaments, the resistances will be the same (because filaments are matched) and the bridge is balanced. When the sample gas is passed over half of the bridge, then VOUT’s 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 the elements. 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 gas to be measured is introduced into the sample gas stream, the two Katharometer elements in this gas 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 measure gas content 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 linearises the curve.
Download the full discussion note below for equations for the 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 a 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:
WARNING: Many sensors may not be used to measure gas/air or gas/oxygen mixtures which are capable of ignition.
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.