Saturation deficit is obtained by subtracting the actual vapor pressure from the maximum possible vapor pressure at the given temperature. It is expressed in millimeters of mercury. A relative humidity of 75 per cent at 15°C. corresponds to a vapor pressure of 12.73 mm. by 0.75, or 9.56 mm. The saturation deficit is the difference between the highest possible vapor pressure (12.73 mm. at 15°C.) and the actual vapor pressure (75 per cent), that is, 12.73 — 9.56 = 3.17 mm. As Bolas (1926) pointed out, the saturation deficit may vary greatly even when relative humidity remains constant, for it rises with the temperature (Fig. 71). (Braun-Blanquet 1932:134-5)
Since all types of porous porcelain atmometers will absorb water from rain or mist, returning it to the reservoir and thereby falsifying the reading, it is necessary to equip the mountings intended for field work with some type of rain-proofing valve. The valve most widely in use is one devised by Livingston and Thone (1920) (Fig. 76 A, c). This consists of a column of mercury, about 5 mm. high, resting on a plug of wool, absorbent cotton, or other porous material e. The mercury acts as a highly flexible yet perfectly sealed check valve, permitting upward movement of water but preventing a reversal. A second plug of porous material f higher in the tube prevents loss of mercury when the apparatus is being handled. Other successful rain-proofing valves for atmometers, devised by various workers, are described by Thone (1924). (Braun-Blanquet 1932:139)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.