Due to environmental parameters such as temperature and humidity changes, all electrochemical or catalytic gas sensors may be sensitive to both positive and negative drift. These are not, however, the most prevalent causes of adverse sensor measurements.
Negative sensor measurements happen more frequently when your device is “zeroed” in a contaminated atmosphere where there are tiny amounts of target gasses of the detectors.
If the tool is later in a clean air setting, the sensors will display an adverse reading corresponding to the contaminant concentration current during the zeroing procedure. For example, when the sensor is zeroed, if there is 5 PPM of carbon monoxide, the reading will be -5 PPM when the sensor returns to clean air.
Negative measurements can also happen when a gas that generates an adverse cross-interference is subjected to the sensor. If a sensor of sulfur dioxide, which typically has a cross-interference of -100 percent with nitrogen dioxide, is subjected to 2 PPM NO2, the resulting reading of sulfur dioxide will be-2 PPM.
If you have NO2 and SO2 in the same atmosphere, having both sensors is the only way you can comprehend the real concentration of each gas. In the instance we used above, if your atmosphere included 2 PPM SO2 together with 2 PPM NO2, the resulting reading of SO2 would be zero owing to adverse interference.
Only by acknowledging the existence of the NO2 gas and understanding its impact on the SO2 sensor can you understand that you have 2 PPM SO2 present. Eliminating one of the instrument’s sensors does not eliminate the danger to which it is subjected.
Sometimes we’re going to say they’ve never seen an adverse reading on a tool before, but they’ve altered monitors lately and now seem to see them all the time. This is because some companies think that adverse measurements merely confuse users and mask them from their tools.
All negative measurements will be shown as null. This is a practice that can actually mask you from seeing and recognizing the existing hazards. If an H2S sensor has an offset of -10 PPM due to drift or a fake zero operation which the manufacturer has masked.
Exposure at a real concentration of + 10 PPM would still result in zero reading and a concentration of + 20 PPM would only be shown as 10. It would be simpler to acknowledge this scenario if the adverse reading were usually first presented.
Thus, while negative measurements to most gas monitor customers are puzzling and disturbing, they are not always a bad thing.