An Exchange System for Precise Measurements of Temperature and Humidity Gradients in the Air Near the Ground

Abstract

Small differences can be very accurately measured with two sensors if precautions are taken to periodically interchange the sensors between observations. The Bowen ratio model of evapotranspiration requires measurements of air temperature and humidity gradients near the evaporating surface. The gradients are in the order of only 0.1 °C/m or 10 Pa/m. Precision in excess of 0.01 °C/m or 1 Pa/m can be obtained only through laborious calibration and replication of instruments, or through periodic interchange. The design of a simple system for interchanging psychrometers is described. The system will exchange sensors between two levels one meter apart at selected time intervals. The vertical exchange path of this design maintains a constant orientation of the sensors and has important advantages over rotating systems used elsewhere. The principles apply to a variety of measurement problems.