Excerpt: Cross-sensitivity compensation in electrochemical gas sensors.

Introductory text

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

The term cross-sensitivity describes the fact that a sensor reacts not only to the target parameter, but also to other influencing parameters. To put it another way: A sensor with cross-sensitivity does not possess perfect selectivity. This is especially challenging for gas sensors, as the measurement of a specific gas concentration should ideally be possible in a gas matrix of any complexity – with hundreds of gases and vapours potentially interfering with the selectivity. It is therefore not surprising that almost all measurement principles used in gas sensors show a cross-sensitivity to accompanying gases. For example, paramagnetic measuring instruments for oxygen also react to nitrogen dioxide, and ammonia and carbon dioxide act as interfering parameters in chemiluminescent methods for the determination of nitrogen oxides. The electrochemical gas sensors as used in Testo measuring instruments are also not free from cross-sensitivities.

Fig. 1: Electrochemical gas sensors in the measuring instrument testo 340

Cross-sensitivities in electrochemical gas sensors and compensation strategies

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:

Fig. 2: Electrochemical sensor for CO and other gases (schematic representation)

Catalyst materials

The most important approach is, as already mentioned, a targeted selection of catalyst materials and mixtures for the electrode and of the correspondingly suitable electrolyte. All in all, the technology in commercially available electrochemical gas sensors has reached a well developed level. In detail however, further progress can be achieved. As an example, a newly available, CO-insensitive SO₂ sensor is described on page 5.

Bias voltage

The selection of a suitable bias voltage for the working electrode can also lead to an improvement of the selectivity. This method is used, for example, in NO sensors. The working electrode uses graphite as a catalyst material, and an additional bias voltage of 300 mV over the reference electrode, which is also integrated into the sensor. Here too, aqueous sulphuric acid is used as an electrolyte. The electrochemical potential of this system allows it to demonstrate NO – however not, or at least hardly, the accompanying gases NO₂ and CO, which gives electrochemical gas sensors a comparatively high level of selectivity.

Filters

Many electrochemical gas sensors use chemical filters against cross-influences. In order to fulfil the filter function, the filter material must retain the the interfering accompanying gases while allowing the target gas to permeate unhindered.

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