This is a question customers ask all the time. Why do you sometimes see negative readings on your gas monitor?
All electrochemical or catalytic bead gas sensors can be prone to both positive and negative drift due to environmental factors like changes in temperature and humidity. However, these are not the most common causes of negative sensor readings.
Zeroing in a Contaminated Atmosphere
Negative sensor readings most commonly occur when your instrument has been “zeroed” in a contaminated atmosphere, where small levels of the sensor’s target gasses are present. When the instrument is later in a clean-air environment, the sensors will show a negative reading that corresponds to the concentration of the contaminant that was present when the device was zeroed. For example, if there is 5 PPM carbon monoxide present when the sensor is zeroed, the reading will be -5 PPM when the sensor is returned to clean air.
Negative Cross Interference
Negative readings may also occur when the sensor is exposed to a gas that produces a negative cross interference. If a sulfur dioxide sensor, which typically has a -100% cross interference to nitrogen dioxide, is exposed to 2 PPM NO2, the resultant sulfur dioxide reading on your instrument will be -2 PPM.
So, does this mean that you should avoid using sensors that have negative cross interferences to each other in the same instrument? Absolutely not! If you have NO2 and SO2 present in the same atmosphere, the only way that you can understand the true concentration of each gas is by using both sensors in a multi-gas detector.
In the example that we used above, if your atmosphere contained 2 PPM SO2 along with the 2 PPM NO2, the resultant SO2 reading due to the negative cross interference would be zero. The only way that you could know that you have 2 PPM SO2 present is by recognizing the presence of the NO2 gas and understanding its effect on the SO2 sensor. Eliminating one of the sensors from the instrument does not eliminate the hazard — instead, you would be exposed to it without knowing.
Customers will sometimes say that they had never seen a negative reading on an instrument until they changed monitors and now see them all the time. This is because some manufacturers mask negative readings because they believe they confuse users. When negative readings are masked, all negative readings are displayed as zero. This can actually prevent you from seeing and recognizing the hazards that exist, putting you in danger.
If an H2S sensor has an offset of -10 PPM due to sensor drift or a false zero operation and negative readings have been masked by the manufacturer, exposure in a true concentration of +10 PPM will still produce a zero reading and a concentration of +20 PPM would only be displayed as 10. This situation would be easier to recognize if the negative reading was displayed in the first place.
So, while negative readings are puzzling and uncomfortable to most gas monitor users, they are not always a bad thing. If you understand the circumstances that cause the negative readings, you will get more information from your instrument and have a better understanding of the environment you are working in.
In situations like these, having a strong knowledge of gas detection can make all the difference. Check out our training courses, which cover everything from sensor technology to creating reports with gas detection data.