Abstract
A new humidity generator that produces gas streams of known moisture content at temperatures from 85 °C to 200 °C, absolute pressures from 0.2 MPa to 1.6 MPa, and relative humidities from 10 % to 90 % has been developed. The generator produces a moist gas stream by injecting fixed-rate streams of dry gas and liquid water into a vaporizer, where the water evaporates into the gas. The gas stream passes into a re-entrant radio-frequency (RF) cavity, which serves as our reference hygrometer, and then a test chamber. The present standard uncertainty of the RF hygrometer is 0.6 %, limited by the uncertainty of literature values for the polarizability of water. Dry nitrogen gas purging the pressure transducer line also combines with the moist gas stream downstream of the test chamber and flows through one of a set of capillaries. Modulation of gas flow through the fixed flow impedance of the capillary gives a simple method for controlling pressure. Individual insulated, temperature-controlled aluminum ovens enclose each major component. A larger oven encloses these ovens and their connecting tubing. To minimize corrosion, critical components are constructed of high-nickel alloys. The small total volume (<1 L) and small flow rate (<0.5 L·min−1) reduce operational hazards from steam scalding or from gas explosion.
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Abbreviations
- A n :
-
Molar polarizability of nitrogen, cm3 · mol−1
- A w, B w :
-
Debye constants describing the molar polarizability of water, A w + B w/T, cm3 · mol−1
- B nn :
-
Nitrogen second virial coefficient, cm3 · mol−1
- B ww :
-
Water second virial coefficient, cm3 · mol−1
- B nw :
-
Nitrogen–water second virial coefficient, cm3 · mol−1
- B mix :
-
Second virial coefficient of the gas mixture, cm3 · mol−1
- \({\varepsilon ={\varepsilon}^{\prime}-{i}{\varepsilon}^{\prime\prime}}\) :
-
Dielectric constant
- f :
-
Resonance frequency of the RF resonator hygrometer, Hz
- L :
-
Length of capillary, m
- P :
-
Pressure, Pa
- P 1 :
-
Pressure at the capillary entrance, Pa
- P 0 :
-
Pressure at the capillary exit, Pa
- Q m :
-
Mass flow rate, kg · s−1
- r :
-
Inner radius of capillary tube, m
- R :
-
Gas constant, J · mol−1 · K−1
- s :
-
Quantity \({{({{\varepsilon}^{\prime}-1})}/{( {{\varepsilon }^{\prime}+2})}}\)
- T :
-
Temperature, K
- T c :
-
Temperature of capillary, K
- x w :
-
Mole fraction of water
- η :
-
Viscosity, Pa · s
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Vega-Maza, D., Miller, W.W., Ripple, D.C. et al. A Humidity Generator for Temperatures up to 200 °C and Pressures up to 1.6 MPa. Int J Thermophys 33, 1477–1487 (2012). https://doi.org/10.1007/s10765-010-0838-1
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DOI: https://doi.org/10.1007/s10765-010-0838-1