A NDIR system "counts" the molecules of CO2 in the gas path....the greater the number of CO2 molecules the less signal to the detector....the number of molecules in the path is due to a combination of proportion of CO2 and pressure. Regardless, you cannot tolerate but so high a percentage of CO2 molecules in the gas mix reported as if it was at the surface.

Therefore it does not care about pressure or concentration. It just tells you the absolute amount of CO2 in the IR path.

Just like oxygen....pure oxygen at the surface is not a real problem to breath....but take that pure oxygen to 100 feet and we have a problem...houston control

We certainly do not want to breath CO2 levels of 5% at the surface or at 130 feet breath 1% CO2. Fortunatly properly working scrubbers keep the CO2 at lower and lower percentage as depth increases.

Hope this helps,
Tom