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Calibration of Water Permeation Tubes



Gregory E. Scace, W. Wyatt Miller


NIST provides traceability of permeation tube humidity generators to national humidity standards by measuring the water permeation rate of permeation tubes submitted for calibration. Permeation tubes undergoing calibration are installed into a specially built permeation tube humidity generator (PTG). The water vapor concentration of the nitrogen stream produced by the PTG is compared to a similar gas stream produced by the NIST Low Frost-Point Generator, using cavity ringdown spectroscopy (CRDS) as the sensing technique. Comparisons are performed at several points spanning the humidity range produced by the PTG / permeation tube combination. A least-squares fit of the data determines the permeation rate of the tube under test. The expanded relative uncertainty of NIST permeation tube calibrations is 1.8 % with coverage factor k=2, dominated by the Type A uncertainties.
Special Publication (NIST SP) - 250-82
Report Number


calibrations, permeation tube, trace humidity


Scace, G. and Miller, W. (2008), Calibration of Water Permeation Tubes, Special Publication (NIST SP), National Institute of Standards and Technology, Gaithersburg, MD (Accessed April 23, 2024)
Created October 16, 2008