Unlike other toxic gases, carbon dioxide (CO₂) is all around us, albeit at levels too low to cause health issues under normal circumstances. It raises the question, how do you zero a CO₂ gas detector in an atmosphere where CO₂ is present?

An increasing problem?
Levels of atmospheric CO₂ have risen from around 280 part per million (ppm) to just over 400ppm or 0.04% today. While it is widely believed this is causing issues of climate change, it is still too low to have any ill effects directly on your health. Toxic effects are associated with levels of 2,000ppm and over.

“Zeroing” for CO₂
Manufacturers of gas detectors build in an allowance to take the atmospheric CO₂ levels into account. Your detector expects to detect 400ppm of CO₂. By zeroing your gas detector, you are setting it to use the current environmental level as its 400ppm baseline. This works well as long as the environment you are in has 400ppm of CO₂, but all too often this is not the case.

Elevated baselines
CO₂ levels can typically range from 400 to 1,000ppm, even with good air exchange. In such an environment, what a newly zeroed CO₂ detector has set as its 400ppm CO₂ baseline could be as high as 1,000ppm. This detector’s readings would be 600ppm below the actual level; potentially dangerously misleading. This is 12% of 5000pmm – a common default setting for alarm level 1 on CO₂. Also, if exposed to genuine fresh air, this detector may go into fault mode, as it “thinks” it is experiencing -200ppm of gas.

Another source of an elevated baseline can be the user. When you zero your gas detector, are you looking at it? More importantly, are you breathing on it? If you are, you will be breathing CO₂ onto the detector. Pressing the zero button while breathing directly on a detector is likely to result in a similar problem to that caused by elevated environment CO₂ level.

In the clear
The upshot of all this – think carefully before you press the button to zero a CO₂ detector. If you’re not sure whether you are in an environment with elevated CO₂ levels, don’t do it. If you must do it, go outside, well clear of any potential sources of CO₂, hold the detector well away from yourself, and make sure you don’t breathe on it.