Confined space gas detectors all have the ability to perform an auto zero function on their sensors. What does an auto zero do exactly? Is an auto zero actually needed? Should you auto zero your sensors before each use?
Auto Zero - Answers
Two things are in play with sensors and an auto zero (also known as a fresh air calibration). The first thing is the readout on the display. I'm referring here to the actual digital readouts displayed on a confined space monitor: 20.9% O2, 0% LEL, 0 PPM CO, 0 PPM H2S.
The second thing in play is the actual electrical signal output of the sensors. This might come as a shock, but the electrical output of the sensors and the LCD display readout are two different things all together. Importantly, however, an auto zero (or calibration for that matter) can properly correlate these two things when properly performed on a "warmed up" monitor in fresh air.
With this in mind we can now understand what an auto zero does and why it is important. First, as we've already stated, an auto zero resets the readings shown on the display. In other words, if the CO sensor shows 2 PPM in a fresh air environment, it sets the display reading back to "0" (and "20.9" for O2).
The second thing an auto zero does is this (from Bob Henderson): "...you are telling the instrument that at this moment, the instrument and sensors are located in fresh air that contains 20.9% O2 ["0" CO, "0" H2S, and "0" LEL], without any detectable contaminants and that THIS is the electrical signal value to use as the reference point for fresh air." In other words, the current voltage on the sensors in the fresh air environment in which the auto zero is performed, now becomes the reference point by which the sensor calculates "0" (or 20.9 for O2).
For this reason, it is crucial that an auto zero (fresh air calibration) is ONLY performed in a fresh air environment. It is never to be performed in an environment that may be contaminated. To do so will mistakenly "tell" the gas monitor that the electrical signal output that correctly correlates to 3% LEL, as an example, is to be treated as if it is 0% LEL. This is obviously a problem.
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