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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%.
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)
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
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 tw using the equation (BS 4833)
where e' is the saturation vapour pressure of water at temperature tw, P is the total barometric pressure, and A is the psychrometer coefficient, given for values of tw 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.
BS 1339:1965 (Confirmed 1981), Definitions, formulae and constants
relating to the humidity of the air.
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).
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