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2.1.4 Hygrometry

Relative humidity

The relative humidity is the ratio (expressed as a percentage) of the partial pressure of the water vapour actually present to the partial pressure of water present at saturation in air at the same temperature (BS 1339). The partial pressure at saturation in air is greater than the vapour pressure over pure water or ice (tabulated in section 3.4.1); for example, at room temperature and normal atmospheric pressures the increase is about 0.5%.

Dew point

The dew point (or frost point) of a gas is the temperature at which condensation occurs. (For most practical purposes this is equivalent to the saturation temperature.) Tables of saturation vapour pressure e are shown in section 3.4.1 for pure water vapour. Actual vapour pressure  in air is given by (Sonntag, 1990)

= ef

where the approximate value of f, the water vapour enhancement factor, is given for barometric pressure P in the range 30 mbar to 1100 mbar, and temperatures from −50°C to +60 °C, by

  f = 1.0016 + 3.15 × 10⁻⁶ P − 0.074/P.

Absolute humidity, or vapour concentration

The values below show the mass of water (in grams) contained in a cubic metre (m³) of saturated air at a total pressure of 101 325 Pa (1013.25 mbar).

Wet- and dry-bulb hygrometer (psychrometer)

For this instrument, the water-vapour pressure in air is found from the actual or dry-bulb temperature t and the wet-bulb temperature t_w using the equation (BS 4833)

= e' − AP(t − tw)

where e' is the saturation vapour pressure of water at temperature t_w, P is the total barometric pressure, and A is the psychrometer coefficient, given for values of t_w above 0 °C by

   A = 6.66 × 10⁻⁴ °C⁻¹ for moving air as in the Assmann ventilated psychrometer (BS 5248)

   A = 8.0 × 10⁻⁴ °C⁻¹ in a Stevenson screen as used by the Meteorological Office (Handbook, 1981).

Values of A other than those above may apply to certain psychrometers. The use of a particular value of A is only valid if the correct conditions of air flow are provided for the instrument in question.

In practice, results obtained from the above equation are sometimes in error by as much as several percent relative humidity, because the performance of psychrometers varies according to design and circumstances of use. For accurate measurements, the psychrometer should be checked by comparison against a hygrometer that has been calibrated with traceability to a recognised humidity standard. It should be noted that operator error can also lead to inaccuracies of several percent (Meteorological Office Handbook, 1981).

Wet- and dry-bulb humidity tables

The tables below are for use with forced-ventilated instruments. (For more complete tables see BS 4833).

Humidities over saturated salt solutions

Saturated solutions of various salts in water can be used to maintain particular values of relative humidity inside sealed enclosures at constant and uniform temperature (Greenspan 1976, Wexler 1954, Young 1967, O’Brien 1948).

In practice, the humidity generated by a given salt may differ by several percent from the value in the table, due to influences such as slow equilibration, temperature variations, impurities and incomplete saturation (BS 3718). Stability is improved by the use of a slurry containing excess solid. For accurate results the value of humidity should be verified using a calibrated hygrometer.

References

BS 1339:1965 (Confirmed 1981), Definitions, formulae and constants relating to the humidity of the air.
BS 3718:1964 (Confirmed 1984), Specification for laboratory humidity ovens (non-injection type).
BS 4833:1986, Hygrometric tables for use in the testing and operation of environmental enclosures.
BS 5248:1973, Specification for aspirated hygrometer.
L. Greenspan (1976) Humidity fixed points of binary saturated solutions, J. Res. Nat. Bur. Stand. 81A(1), 89–96.
Meteorological Office (1981) Handbook of meteorological instruments, Volume 3, Measurement of humidity, 2nd edn,    HMSO, London.
F. E. M. O'Brien (1948) The control of humidity by saturated salt solutions. J. Sci. Instr. 25, 73–76.
D. Sonntag (1990) Important new values of the physical constants of 1986, vapour pressure formulations based on the ITS-90, and psychrometer formulae, Z. Meteorol. 40(5), 340–344.
A. Wexler and S. Hasegawa (1954) Relative humidity–temperature relationships of some saturated salt solutions in the    temperature range 0°C to 50 °C, J. Res. Nat. Bur. Stand. 53(1), 19–26.
J. F. Young (1967) Humidity control in the laboratory using salt solutions—a review, J. Appl. Chem. 17, 241–245.

It should be noted that references dated before 1990 contain data relative to temperature scales other than ITS-90. For best accuracy these data values must be corrected for the difference between ITS-90 and the temperature scale in question (IPTS-68, IPTS-48).

S.A.Bell

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