15.1) Cross-sensitivity of electrochemical sensors and how to handle it

At first there was the idea of eliminating the influencing agents. That was, and still is, done by means of an NO_x/SO_x filter to eliminate their respective interferences caused on the CO cell. Furthermore, the supplier of the sensors equipped the sensors directly with the respective filters. The problem that occurred when using filters is that they used to be clogged long before the sensors were consumed. That is a particular problem when the filter is incorporated in the sensors. Since they can not be replaced, the whole sensor is useless when the filter is gone.  

The need to eliminate the H₂ interferences on CO sensors was initiated by the TÜV in Germany. This is partly done using filters, but there are also sensors where the hydrogen concentration is measured independently of the CO and the compensation carried out electronically. These are then manufactured as 4-electrode sensors with a second sensing electrode.

was one of the first suppliers of instruments equipped with electrochemical sensors to introduce a cross-sensitivity calculation in their instruments.

When calibrating the instrument (GA-20plus, GA-21plus, GA-40plus, GA-40Tplus and GA-60), every influence detected is stored and goes into that particular calculation. That gives flue gas analyzers an outstanding performance in comparison to most other instruments, although it is often difficult to convince people that the new instrument is correct when they have been used to incorrect readings for years.

The main problem with this method of calculating and then compensating for the effects of other gases present is that the cross-sensitivities can, of course, only be calculated for the gases measured directly i.e. no cross-sensitivities can be calculated between gas components the respective instrument is not equipped to measure directly.

Particularly when testing for HCl, the cross-sensitivity can become a real problem due to the large response caused by SO₂. The HCl sensor has a 35% cross-sensitivity to SO₂ and a maximum operating concentration of 100 ppm. This means that a concentration of SO₂ in excess of about 250 ppm will cause the HCl sensor to go over-range. This is a simple sensor problem that can not be cured by any type of calculation or changes to the software. When the sensor exceeds the maximum range the response becomes unpredictable. The signal can climb to a high value or drop off without warning, thus invalidating the calculations carried out by the instrument. The only real way to avoid this is to know of the problem and to use filters to remove the SO₂ when testing for HCl.

At lower concentrations of SO₂ and, to a lesser extent, NO₂, there is no problem and the results shown by the analyser are trustworthy.

As more and more of the exotic gases are becoming interesting due to legislation, so the problem of cross-sensitivity is becoming more apparent. These gases often have a large number of serious cross-sensitivities to other components and it is no longer possible to simply assume that the effects are negligible.