THE PTB COULOMETRIC HUMIDITY GENERATOR AS PRIMARY STANDARD FOR TRACE HUMIDITY MEASUREMENT IN GASES

摘要

A coulometric trace humidity generator has been reconstructed at the PTB to improve the capability of calibration and testing of hygrometers in the range from 500 μl/l to 0,1 μl/l volume fraction of water vapour, resp. from -25 to -90°C frost-point temperature. The generator operates on the principle of FARADAYs law of electrolysis and produces a gas stream of a given moisture concentration in terms of mixing ratio. The expanded uncertainty of the generated humidity concentration is estimated not to exceed 1% of the volume fraction of water vapour resp. 0,1 K frost point temperature even at 0,1 μl/l volume fraction resp. -90°C frost point temperature. A comparison with the PTB two-pressure humidity generator by means of a chilled mirror hygrometer MBW DP3-D has established the accuracy of the trace humidity generator within an uncertainty of 0,1 K frost point temperature.