Sensor Cross Sensitivity
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 unwanted effect; this may be a positive response, negative response or inhibition.
Positive Response
A positive response means that sensors respond to not just the target gas but another gas as well. This response could give the user the impression that there is target gas present when there isn’t or more target gas present than is 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 has a positive response to H2. Therefore, if the CO sensor saw 200ppm of H2, then the CO sensor will show around 100ppm.
Negative response
A negative response is when sensors produce a reduced response to the target gas if exposed to a gas that causes an adverse reaction. 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 has been reduced because of this negative effect. A couple of examples of a negative responding situation: An SO2 sensor has a negative response to NO2. Therefore if an SO2 sensor sees 10ppm of NO2 at the same time as 10ppm of SO2, the net result could read 0ppm reading or even a negative value, depending on the instrument.
Inhibition
Inhibition is similar to a negative response; however, what 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. For example: 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.
Dangers of Cross sensitivity
An example of a dangerous situation is when a multigas detector is simultaneously monitoring NO2 and SO2 gases. NO2 sensors can have a negative response to SO2 gas and vice versa; some SO2 sensors can have a negative response to NO2 gas. It is possible that the instrument will read 0ppm for both gases when in fact, both gases are present.
Making good use of known Cross Sensitivity
There are more types of gases in the world than there are different types of gas sensors. It can be advantageous to use a sensor of one kind to detect another gas. One example is to use a CO or an SO2 sensor to detect Phosphine fumigation gas. Both the CO and SO2 sensors have high cross-sensitivity to PH3 gas and are suitable for high ppm level PH3 monitoring.
Problematic Cross Sensitivity
CO sensors are highly responsive to H2, Acetylene, Ethanol and Alcohol. Some safety offices are tasked to maintain site breathing apparatus and gas detection equipment in the same office. The gas detectors are likely to have CO readings when alcohol is used to clean the face masks. The CO sensors can take a long time to recover after alcohol exposure. Since the onset of Covid19, some users have experienced an increase in false CO readings due to alcohol-based hand sanitisers and cleaners.
Advantage of Cross Sensitivity
Not all calibration gases are readily available for the calibration of gas sensors. One example is Formaldehyde. Formaldehyde calibration gas is expensive to produce and is not readily available in disposable gas cylinders like most other calibration gases. However, some formaldehyde gas sensors are cross sensitive to CO gas and can therefore be calibrated using low cost, readily available CO gas as an alternative calibration gas.
Summary
It should be noted that cross-sensitivity is not the same for all sensors and changes between sensor manufacturers. It is important to note that the level of cross-sensitivity changes as sensors age. Some sensors are manufactured with inbuilt filters to minimise the effects of cross-sensitivity, and these filters break down over time, changing sensor performance. The sensors respond to gases that can be the target gas and respond to other gases called cross-sensitivity. However, the sensor is unable to analyse or differentiate between gas types to which it responds.
Positive Response
A positive response means that sensors respond to not just the target gas but another gas as well. This response could give the user the impression that there is target gas present when there isn’t or more target gas present than is 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 has a positive response to H2. Therefore, if the CO sensor saw 200ppm of H2, then the CO sensor will show around 100ppm.
Negative response
A negative response is when sensors produce a reduced response to the target gas if exposed to a gas that causes an adverse reaction. 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 has been reduced because of this negative effect. A couple of examples of a negative responding situation: An SO2 sensor has a negative response to NO2. Therefore if an SO2 sensor sees 10ppm of NO2 at the same time as 10ppm of SO2, the net result could read 0ppm reading or even a negative value, depending on the instrument.
Inhibition
Inhibition is similar to a negative response; however, what 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. For example: 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.
Dangers of Cross sensitivity
An example of a dangerous situation is when a multigas detector is simultaneously monitoring NO2 and SO2 gases. NO2 sensors can have a negative response to SO2 gas and vice versa; some SO2 sensors can have a negative response to NO2 gas. It is possible that the instrument will read 0ppm for both gases when in fact, both gases are present.
Making good use of known Cross Sensitivity
There are more types of gases in the world than there are different types of gas sensors. It can be advantageous to use a sensor of one kind to detect another gas. One example is to use a CO or an SO2 sensor to detect Phosphine fumigation gas. Both the CO and SO2 sensors have high cross-sensitivity to PH3 gas and are suitable for high ppm level PH3 monitoring.
Problematic Cross Sensitivity
CO sensors are highly responsive to H2, Acetylene, Ethanol and Alcohol. Some safety offices are tasked to maintain site breathing apparatus and gas detection equipment in the same office. The gas detectors are likely to have CO readings when alcohol is used to clean the face masks. The CO sensors can take a long time to recover after alcohol exposure. Since the onset of Covid19, some users have experienced an increase in false CO readings due to alcohol-based hand sanitisers and cleaners.
Advantage of Cross Sensitivity
Not all calibration gases are readily available for the calibration of gas sensors. One example is Formaldehyde. Formaldehyde calibration gas is expensive to produce and is not readily available in disposable gas cylinders like most other calibration gases. However, some formaldehyde gas sensors are cross sensitive to CO gas and can therefore be calibrated using low cost, readily available CO gas as an alternative calibration gas.
Summary
It should be noted that cross-sensitivity is not the same for all sensors and changes between sensor manufacturers. It is important to note that the level of cross-sensitivity changes as sensors age. Some sensors are manufactured with inbuilt filters to minimise the effects of cross-sensitivity, and these filters break down over time, changing sensor performance. The sensors respond to gases that can be the target gas and respond to other gases called cross-sensitivity. However, the sensor is unable to analyse or differentiate between gas types to which it responds.