We describe a technique for measuring the trace humidity level delivered by a permeationtube type moisture generator (PTMG). Amount fractions in the range 10 nmol/mol to 100 nmol/mol of H2O in N2 carrier gas were considered. The measurements are referenced to the NIST low frostpoint generator (LFPG). A quartz crystal microbalance (QCM) hygrometer is used as a comparator to measure the concentration difference between the respective outputs of the PTMG and LFPG. The measurement procedure follows a standard substitution method, in which the water vapor amount fraction produced by the permeationtube moisture generator is inferred by interpolation and scaled by the reference concentrations of the low frostpoint generator. Differences in the water vapor amount fraction of 1 nmol/mol are observable. Two permeationtube moisture generators were independently tested by this technique. Systematic differences with respect to the LFPG output of 3.8 % 0.9 % and 2.9 % 0.7 %, were measured for the respective PTMG devices.
Pub Type: Journals
humidity, humidity generator, humidity standards, moisture