The lithium chloride Dewcel covered under the Foxboro Company patent number 2,359,278 has been modified slightly and used to measure small dew‐point or vapor‐pressure gradients above a crop. The advantage of the Dewcel is that the relative error of the measurement is the same at all dew points and that Dewcel data are conveniently reduced to dewpoint and vapor‐pressure data. The disadvantage of the Dewcel is its large absolute error which often may be as large as the dew‐point gradient measured. The Dewcel modification and the equipment for excluding the absolute error of the Dewcels from the dew‐point gradient measurement are reported. In order to operate the Dewcel suitably, several of the Dewcel characteristics have been determined. The effect of ventilation and changes in air temperatures upon the Dewcel temperature and the slope of the dew‐point — Dewcel‐temperature curve at various Dewcel temperatures above ambient are given. Two methods for Dewcel calibration are reported. A scheme for generating constant dew‐point air at rates of 20 liters per minute is given.
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Tanner et al. (1956) studied this question.