An accurate experimental research to check the psychrometer coefficient A has been made in the BICL. Three ventilated psychrometers were compared with the two-pressure humidity generator and with the precise dew point hygrometer. The results for dry bulb temperature from 12 to 30°C and the relative humidity in the range 20 to 70%rh (or the wet bulb temperature from 4 to 26°C) give an average value of A near 6.28×10{sup}(-4)K{sup}(-1) with the uncertainty of ±1.5%. The result confirms the psychrometer coefficient A given by Dr. Wylie and Lalas, which in the meteorological range varies little from 6.21×10{sup}(-4)K{sup}(-1), and supports the conclusion of Wylie that the true values of A for well-designed ventilated psychrometer approximate this value rather than the value of 6.62×10{sup}(-4)K{sup}(-1) which are in general use. This difference of value A creates a 1.2%rh error at 20°C and 50%rh. The differences between Dr. Wylie and Dr. Sonntag for the Assman and other type psychrometers are discussed.
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