Why do I see a negative reading on my gas detector?
1. Zero readings in polluted atmosphere
When the instrument is "zeroed" when there is a little concentration of the sensor's target gas in a contaminated atmosphere, the sensor results are more adverse. The sensor will display a negative value, which corresponds to the pollutant concentration at the time the instrument was zeroed, when the instrument is later put in a clean air environment. When the sensor is returned to clean air, for instance, if the carbon monoxide concentration is 5 PPM when it is zeroed, the reading will be -5 PPM.
2. Negative cross-interference
When the sensor is submerged in a gas that produces negative cross-interference, negative readings can also happen. If there are 2 PPM of NO2 in the environment where the SO2 sensor typically exhibits a -100% cross-interference with NO2, the SO2 reading on the instrument will be -2 PPM.
Does this imply that it is best to avoid employing sensors in the same instrument that negatively interfere with one another? no! Using both sensors in a multi-gas detector will allow you to determine the true concentration of each gas in the atmosphere if NO2 and SO2 are both present.
In the example we presented above, the reading for SO2 would be zero due to negative cross-interference if the atmosphere contains 2 PPM of SO2 and 2 PPM of NO2. Confirming the existence of NO2 and comprehending how it affects the SO2 sensor will allow you to determine if you have 2 PPMSO2. The risk is not reduced when a sensor is removed from an instrument; rather, you expose it without realizing it.
Customers occasionally claim that their meter had never displayed a negative reading before, but after changing the detector, it continued to do so. This is due to the fact that some manufacturers conceal negative readings out of concern for users' confusion. Negative readings are displayed as zero when they are muffled.That way, you can actually put yourself at risk by not being aware of the danger that exists.
The exposure reading will still show zero when the true concentration is 10 PPM if the H2S sensor is set with an offset of -10 PPM because of reading drift or a false zero operation, and the manufacturer masks negative readings. The exposure readout displays 10 at 20 PPM. If a negative reading is initially displayed, this condition is simpler to spot.
