Reducing the Uncertainty of Industrial Trace Humidity Generators through NIST Permeation-Tube Calibration |
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Authors: | G E Scace W W Miller |
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Affiliation: | (1) Process Measurements Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA |
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Abstract: | Permeation-tube moisture generators (PTGs) are commonly used by the semiconductor industry as transfer standards for the calibration
of hygrometer systems measuring trace amounts of water vapor in gases (water vapor mole fractions typically below 1 × 10−6). They are relatively simple devices that generate a steady stream of humidified gas by diluting water vapor delivered at
a constant rate from a permeable capsule with precisely metered purified gas, usually nitrogen. Here a new calibration service
enabling the measurement of PTG permeation rates directly in terms of NIST primary standards of trace humidity generation
is described. Rather than using commonly employed gravimetric methods for permeation-tube calibration, the method applied
here links the permeation rate of the permeation tube to the thermodynamic properties of ice. Using a hygrometer based on
cavity ringdown spectroscopy, we compare the water vapor concentrations produced by the NIST low frost-point generator (LFPG)
and a specially constructed PTG containing the permeation tube undergoing calibration. A least squares fit of the data determines
the permeation rate of the tube under test. We describe the calibration system, experimental procedure and present sample
calibration data. The expanded relative uncertainty of NIST permeation-tube calibrations is 1.8% with a coverage factor k = 2, dominated by the Type A uncertainties. |
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Keywords: | Cavity ringdown Frost point Generator Humidity Permeation tube |
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