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Thermodynamic Quantities

Humidity Measurements

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Technical Contacts:
Peter Huang
(On extended leave - please contact Christopher Meyer)

Christopher Meyer
Tel: 301-975-4825
E-mail: cmeyer@nist.gov   

Gregory E. Scace
Tel: 301-975-2626
E-mail: gregory.scace@nist.gov

Please contact the technical staff before shipping instruments or standards to the address listed below.

National Institute of Standards and Technology
100 Bureau Dr., Stop 8363
Gaithersburg, MD 20899-8363
Fax: 301-548-0206

Service ID Number	Description of Services	Fee ($)
36010C	Dew-Point Hygrometers (+25 ºC to -15 ºC)	2226
36020C	Dew-Point Hygrometers (-70 ºC to -15 ºC)	5216
36030C	Electric Hygrometers	At Cost
36040C	Electrolytic Hygrometers	At Cost
36050C	Aspirated Hygrometers	At Cost
36060C	Pneumatic Bridge Hygrometers	At Cost
36070S	Special Tests of Humidity	At Cost

Fees are subject to change without notice.

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Hygrometers (36010C-36060C)

NIST provides calibration services for a wide variety of humidity-measuring instruments. Calibrations are performed by subjecting the instrument under test to atmospheres of known moisture content produced by the NIST two-pressure humidity generator. The instruments and ranges of calibration are listed below:

A. Dew-Point Hygrometers calibrated over the dew/frost range of -70 °C to +60 °C.

B. Electric Hygrometers classified under this category are sensors which sorb water vapor as a function of relative humidity; associated with this sorption is a corresponding change in an electric parameter (that is, resistance, capacitance). The range of calibration is 3 % to 98 % relative humidity over the temperature range -55 °C to +40 °C.

C. Psychrometers are wet-dry bulb hygrometers (aspirated psychrometers). The contact person should be consulted for the special features of the psychrometer which are necessary in order that the instrument can be calibrated at NIST.

D. Electrolytic Hygrometers are devices which electrolyze water into gaseous oxygen and hydrogen by the application of a voltage in excess of the thermodynamic decomposition voltage for water, and then measure the electrolysis current. The range of calibration in volume ratio is 10^-6 to 3 x 10^-2.

E. Pneumatic Bridge Hygrometers are instruments that measure the variation of the pressure drop across two combinations of nozzles, operating at critical flow, with a desiccant between one pair of nozzles. The range of calibration in mixing ratio (grams of water vapor/grams of dry air) is 0.0005 to 0.015. Table 6.5 illustrates typical NIST uncertainties for measurement of humidity standards with atmospheric air at atmospheric pressures.

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Special Tests of Humidity (36070S)

Tests for response time, hysteresis, and stability can be provided upon request.

Table 6.5. NIST Two-Pressure Humidity Generator, Mark 2, Range and Uncertainty

Humidity Parameter	Range	Expanded Uncertainty
Mixing ratio, rw (g water vapor/kg dry air)	0.0015< rw < 0.005	1.5 % of value
	0.005 < rw < 0.1	1.0 % of value
	0.1 < rw < 0.3	0.5 % of value
	0.3 < rw < 515	0.3 % of value
Volume ratio, V (X 10-6 )	3 < V < 10	1.5 % of value
	10 < V < 170	1.0 % of value
	170 < V < 500	0.5 % of value
	500 < V < 820 000	0.3 % of value
Dew point temperature, Td ( °C)	-70 < Td < -35	0.1 °C
	-35 < Td < +40	0.04 °C
Relative humidity, RH (%) at test chamber temperature, Tc ( °C) of:	-55 < Tc < -40	3-98 1.5 %
	-40 < Tc < -20	3-98 0.8 %
	-20 < Tc < 0	3-98 0.2 %
	0 < Tc < +40	3-98 0.2 %

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References-Humidity Measurements

The New NIST Low Frost-Point Humidity Generator, G. E. Scace, P. H. Huang, J. T. Hodges, D. A. Olson and J. R. Whetstone, presented at the 1997 NCSL Workshop and Symp., Atlanta, GA (1997).

Thermodynamic Properties of Moist Air Containing 1000 to 5000 PPMv of Water Vapor, P. H. Huang, NISTIR 5241, 43-51 (Apr. 1993).

NIST Calibration Services for Humidity Measurement, P. H. Huang, NISTIR 4677-A (Superseding NISTIR 4677, Oct. 1991).

National Basis of Accuracy in Humidity Measurements, S. Hasegawa, ISA Trans. 25 (3), 15-24, 1986.

The NBS Two-Pressure Humidity Generator, Mark 2, S. Hasegawa and J. W. Little, J. Res. Nat. Bur. Stand. (U.S.), 81A (1), 81-88 (Jan.-Feb. 1977).

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Program questions: Calibrations

Phone: 301-975-2200, Fax: 301-975-2950

NIST, 100 Bureau Drive , Stop 8363, Gaithersburg , MD 20899-8363