The Bureau of Mines has developed a quantitative method to determine water vapor in helium in the range of 0.1 to 15 ppm by volume. This method uses the quantitative chemical reaction of h2o with cah2, which produces 1 mole of hydrogen for each mole of h2o in the sample. A gas chromatograph is then used to determine the concentration of hydrogen in the product. The precision of the h2o analysis is +/- 0.05 Ppm at a concentration of 1 ppm. This method is used to calibrate a commercial platinum-phosphorus pentoxide (pt- p2o5) electrolytic water vapor analyzer that has an improved inlet system and digital voltmeter readout. Research at the Tuscaloosa Research Center is carried out under a cooperative agreement between the Bureau of Mines, U.S. Department of the Interior, and the University of Alabama.