A method for calibrating a dual-beam NDIR gas sensor for detecting water vapor by obtaining a variant sensor components domain (“G0”) when there is no water vapor gas in the sample chamber, obtaining a physics measurement domain (“G”) for a set of known concentrations of the water vapor gas in the sample chamber, the known concentrations being measurable quantities exceeding zero, using G to calculate a G0i from a known concentration of water vapor in the sample chamber for each of multiple master calibration curves, and then selecting the master calibration curve with the lowest difference between its G0i and the G0 of the NDIR gas sensor as the calibration curve. G and the G0i are obtained, preferably, at substantially the same temperature, G is a signal channel output (“VS”) divided by a reference channel output (“VR”), and the master calibration curves are obtained from a set of multiple calibration curves obtained from a large number of NDIR gas sensors, each of the calibration curves being represented by an equation of P=F(G/G0) where P is equal to a gas pressure and F is a function.