Relative humidity errors when measuring dry and wet bulb temperatures

Abstract

Relative humidity ( h r ) errors, implied in the psychrometric method of measurement, were calculated as a function of the accuracies in temperature sensors, regardless of their origin. An error propagation formula was used which included partial derivatives of h r with respect to dry ( T a ) and wet ( T wb ) bulb temperatures. Relative humidity errors ( E hr ) were predicted for T a between 5 and 35 °C, percentage h r from 30 to 99% and atmospheric pressures from 0.6 x 10 5 to 1 × 10 5 Pa. Sensor accuracies from 0.1 to 1 °C were considered. The latter value, when occurring simultaneously in both sensors, led to an E hr value as large as 9% (in percentage relative humidity units). For dry bulb temperatures varying from 15 to 25 °C and percentage h r from 50 to 80%, a sensor accuracy of 0.2 °C in dry and wet bulb temperature sensors led to E hr at or below 2%. Inexpensive temperature sensors with accuracies higher than 0.5 °C lead to unacceptably high errors in relative humidity. Values of E hr increase linearly with the increase in relative humidity and decrease exponentially with the increase in dry bulb temperature. The reduction of atmospheric pressure causes a very slight linear decrease in the relative humidity errors.