Determination of the Psychrometer Coefficient A of the WMO Reference Psychrometer by Comparison with a Standard Gravimetric Hygrometer

Abstract

An accurate determination has been made of the psychrometer coefficient A of the reference psychrometer defined in a specification adopted by the World Meteorological Organization (WMO) in 1977. A psychrometer constructed in accordance with the specification by the Academy of Meteorological Science of China was compared with a gravimetric hygrometer constructed as a temporary primary standard by the National Institute of Metrology (NIM). The results, for a temperature of 23°C and relative humidities in the range of 40 to 50%, give a value of 6.23 ? 10?4 K?1 with an estimated absolute uncertainty of 1.2% at the 99% confidence level. This value is within 0.4% of that derivable for these conditions from the tabulation given for the reference psychrometer by Wylie and Lalas. The difference corresponds to less than 0.2% in the mixing ratio and less than 0.1% relative humidity. The work confirms the value of A determined by Wylie and Lalas, which in the meteorological range varies little from 6.21 ? 10? K?1, and supports the conclusion of Wylie that the true values of A for well-designed ventilated psychrometers approximate this value rather than the values about 8% higher which are in general use.