Working in Technical Support, one of the most common questions from customers is for bespoke configurations of toxic gas sensors. This frequently leads to an investigation into the cross sensitivity of the different gases that the sensor will be exposed to.
Cross sensitivity responses will vary from sensor type to sensor type, and suppliers often express the cross sensitivity in percentages while others will specify in actual parts-per-million (ppm) levels.
Cross sensitivity is a sensor’s reaction to other gases which can “interfere” with how the sensor reacts. Exposing a sensor to a gas that is not the target gas can cause an undesirable effect; this may be a positive response, negative response or inhibition.
A positive response means that sensors respond to not just the target gas but to another gas as well.
This could give the user the impression that there is target gas present when there isn’t, or that there is more target gas present than is really the case. This may cause the user to induce ventilation, evacuate from the area or take another corrective action to remove the gas hazard, which may not be necessary.
A good example of a positive responding situation:
- A CO sensor which has a positive response to H2 at a rate of 60%. Therefore, if the CO sensor saw 200ppm of H2 then the CO sensor will show around 120ppm (i.e. 60% of 200ppm).
A negative response is when sensors produce a reduced response to the target gas if also exposed to a gas that causes a negative response. If this happens, the user may be exposed to the target gas and not know they are at risk, or it may reduce the level of gas seen on the instrument display as it is been reduced because of this negative effect.
A couple examples of a negative responding situation:
- An SO2 sensor has a -120% response to NO2. Therefore if an SO2 sensor sees 5ppm of NO2 at the same time as 5ppm of SO2, the reading would be reduced by 6ppm showing a 0ppm reading or even a negative value, depending on how your instrument works.
- If the same sensor sees 2ppm NO2 then the reading would be reduced by 2.4ppm, giving you a reading of only 2.6ppm SO2 when exposed to 5ppm SO2.
Inhibition is similar to a negative response; however what actually happens is the sensor will not respond at all to the target gas if exposed to the inhibitor at the same time, or the sensor may take hours if not days to recover before responding to the target gas again.
- a Cl2 sensor is inhibited by H2S
- SO2 sensors can be inhibited by NH3 and take many hours to recover before responding again to SO2.
Therefore when using your multitoxic gas instrument make sure you are aware of the sensor cross sensitivities, as it may very well save your life.