Water vapour sensing using polarization selection of a Zeeman-split argon discharge lamp emission line

Water vapour concentrations are measured using the optical absorption of a Zeeman-split emission line of an argon discharge lamp. When a longitudinal magnetic field of the proper magnitude is used to split the emission line, two components of opposite circular polarization are produced: one coincident, and one off-resonance with respect to a water vapour absorption line at 10 687.36 cm−1. Differential absorption is directly measured by alternately selecting the polarization of the light which is allowed to propagate down the optical path into a multipass absorption cell. Water vapour concentration measurements were obtained over a range of over three orders of magnitude (∼0.002–3.0%). The monitor was also shown to act directly as a mixing ratio (mole fraction) sensor (within ±10%) over a pressure range of 0.3–1.2 atm.