A Differential Psychrometer for Continuous Measurements of Transpiration

Abstract

The instrument described provides a simple yet accurate means of continuously measuring transpiration of plants or parts of plants to enclosures. A thermostated equilibration water bath is used so that nominally similar dry bulb temperatures exist to the air samples drawn from the main airstream before and after it passes through the enclosure. Each airstream is passed through a tube of the psychrometer and differential wet bulb temperature is measured. Maintenance of common dry bulb temperature to the instrument makes the measurments very tolerant of fluctuating environmental temperature and pressure. Also the use of small psychrometer tube dimensions and turbulence inducing entry orifices enable maximum wet bulb depression to be achieved at apparent aspiration speeds of <20 cm/second. Several procedures for calculating transpiration rate are given.