1. Zero reading in polluted atmosphere
Negative sensor readings occur more often when the instrument is "zeroed" when a small amount of the sensor's target gas is present in a polluted atmosphere. When the instrument is later placed in a clean air environment, the sensor will show a negative reading, corresponding to the contaminant concentration at the time the device was zeroed. For example, if the carbon monoxide concentration is 5 PPM when the sensor is zeroed, the reading will be -5 PPM when the sensor is returned to clean air.
2. Negative Cross Interference
Negative readings can also occur when the sensor is placed in a gas that produces negative crosstalk. If the SO2 sensor, which normally has -100% cross-interference to NO2, is placed in 2 PPM of NO2, the SO2 reading on the instrument will be -2 PPM.
So, does this suggest that you should avoid using sensors that have negative cross-interference with each other in the same instrument? no! If both NO2 and SO2 are present in the atmosphere, the way to tell you the true concentration of each gas is to use both sensors in a multi-gas detector.
In the example we used above, if the atmosphere contains 2 PPM of SO2 and 2 PPM of NO2, the SO2 reading will be zero due to negative cross-interference. The way to know that it contains 2 PPMSO2 is to confirm the presence of NO2 and understand its effect on the SO2 sensor. Removing a sensor from an instrument doesn't eliminate the danger -- instead, you're exposed to it without knowing it.
Customers sometimes say they haven't seen negative readings on the instrument before, and then they changed the tester and they kept getting negative readings. This is because some manufacturers have blocked negative readings for fear of user confusion. When negative readings are masked, all negative readings are displayed as zero. In this way, you are effectively putting yourself in danger without being aware of the danger that exists.
If the H2S sensor is set with an offset of -10 PPM due to reading drift or false zeroing and the manufacturer masks the negative reading, the exposure reading will still show zero at a true concentration of 10 PPM, which is At 20 PPM, the exposure reading shows 10. This condition is easier to identify if a negative reading is displayed in the first place.
