Excerpt: Cross-sensitivity compensation in electrochemical gas sensors.
Introductory text H2>
The reliability and accuracy of Testo emission measurement technology has earned it a very good reputation among customers worldwide. Typical applications are the adjustment and monitoring of heating systems as well as measurements on combined heat and power plants, engines or turbines. Depending on the fuel and the plant settings, the gas matrix in these jobs is fairly well known.
In addition to this however, emission measuring instruments from Testo are used for monitoring the most diverse processes in which the gas composition can vary considerably. This white paper concerns itself with the issue of the possible cross-sensitivity of gases occurring here, and how to deal with it.
Cross-sensitivities in gas sensors H2>
Cross-sensitivities in electrochemical gas sensors and compensation strategies H2>
The functional principle of an electrochemical gas sensor is explained in the diagram in Fig. 2. The gas to be measured, for example carbon monoxide (CO), must pass through a diffusion barrier (a capillary or membrane), and in the case of some sensor types a chemical filter, and then reaches the so-called working electrode. This “floats” in an electrolyte, i. e. in an acidic or alkaline, aqueous solution. The gas molecule triggers a chemical reaction at the working electrode and ions, for example protons (H+), are formed, which reach the counter-electrode, where they react with the oxygen present as a solution in the electrolyte. At the same time, an electrical current is created which is diverted to an external circuit and serves as a measure of the gas concentration present. The third electrode (reference electrode) is used to stabilize the sensor signal.
In order for these chemical reactions to take place at the electrodes, they must contain a noble metal (e.g. platinum) as a catalyst. The choice of suitable catalyst materials for electrodes is limited, and the corresponding materials demonstrate their catalytic effects with different gases. By mixing different catalysts, the selectivity over a specific gas can be increased. However, it is unavoidable that electrochemical gas sensors show cross-sensitivities. A platinum electrode, for example, possesses a high catalytic activity and in a gas sensor for CO filled with aqueous, diluted sulphuric acid, will also demonstrate the cross-gases NO, NO₂, SO₂ and H₂.
So how then can these undesired cross-sensitivities in gas sensors and gas measuring instruments be minimized, in order to achieve a reliable and accurate display of gas concentration even in unknown and complex gas mixtures? Various strategies come into play:
Catalyst materials H2>
Bias voltage H2>
- CO sensor with H₂ compensation
- Compensation of cross-sensitivities
- Peculiarities of gas measuring instruments and gas sensors from Testo
- Limits to the compensation of cross-sensitivities
- Success in the further development of the SO₂ sensor