Standard psychrometers: a matter of (p)references

In hygrometry the dew-point meter and the psychrometer are considered as secondary references. The value of the psychrometer coefficient of reference psychrometers has been subject to much discussion during the past decade. The present work gives the results of comparisons between a standard mirror dew-point meter and several psychrometers of different types, amongst which the WMO reference and the Assmann psychrometer. Both instruments have been accepted by the World Meteorological Organisation as references, both with a defined psychrometer coefficient. The results found here demonstrate an inconsistency in the values of the accepted psychrometer coefficients. A recapitulation of the most important parameters affecting the good performance of a psychrometer is given.