A Mixing Ratio Generator for H2O Laser Calibration: Another Fine Mess Bert Tanner Got Us Into.

Routine field calibration of water vapor analyzers is a challenging problem, particularly for isotope analyzers.  Unlike CO2 and other trace gases, water vapor standards cannot be stored in pressurized tanks, so they must be generated de novo at each calibration time.   Also, for long-term, unattended measurement  (e.g. at FLUXNET sites) it is necessary to have a calibration system that  can be controlled by a data logger or computer, without human intervention.  After experimenting with numerous designs,  we built such a system by coupling a small, Peltier-controlled dew point generator with a digitally controlled back-pressure regulator.  The system allows precise generation of a broad range of mixing ratios, and can easily be filled and emptied, which facilitates its use for Rayleigh distillation tests with H2O isotope lasers.