The Effect of and Correction for Different Wet-Bulb and Dry-Bulb Response in Thermocouple Psychrometry

Abstract

Fast-responding thermocouple psychrometers are often used in atmospheric boundary-layer turbulence measurements for the computation of heat and moisture fluxes. Small size, low cost, ease of interchange-ability and the use of the familiar psychrometric equations make this an ideal sensor for many applications at temperatures above freezing. However, a feature of these instruments that is frequently disregarded is that, due to wicking, the wet-bulb sensor has a frequency response that is an order of magnitude slower than the dry-bulb sensor. This difference in response time between the wet and dry sensors causes errors in the variances of humidity in one set of data as large as a factor of 5 and major errors in the shape of the humidity spectrum at high frequencies. We present a known but infrequently applied solution to this problem of sensor response differences in the hope that its simplicity, together with the reminder that a problem exists, will serve to encourage its use in the computation of humidity from fast-response psychrometric sensors.