Psychrometers

PSYCHROMETERS (from Greek psychros—cold and metron—measure), instruments for measuring the absolute and relative humidity of the air (see). Psychrometers are built on the principle of determining the intensity of water evaporation by the degree of lowering of the temperature of the evaporating surface. In sanitary practice, psychrometers are most often used to determine the relative humidity of the air. The simplest of all psychrometers is the August psychrometer. It consists of two thermometers mounted on a stand and a small graduated beaker for water (Fig. 1). A cap made of thin paper material (cambric) is placed on the mercury bulb of one of the thermometers, the end of which, in the form of a small wick, is lowered into the beaker with water (wet-bulb thermometer); the other thermometer remains free (dry-bulb thermometer). The first thermometer, when the cambric cap is moistened, will show a lower temperature than the other, due to the fact that a certain amount of heat is expended on the evaporation of water from its surface. At the same time, the less the air is saturated with water vapor, the more intensely the evaporation of water from the wet bulb proceeds, and the greater the difference between the readings of the dry and wet thermometers. The absolute humidity is calculated using the August formula: A = F_t - K(t - t₁)H (1), where A is absolute humidity, F_t is the maximum tension of water vapor corresponding to the temperature of the wet-bulb thermometer, expressed in millimeters of mercury, t is the temperature according to the dry-bulb thermometer, t₁ is the temperature according to the wet-bulb thermometer, H is the barometric pressure, and K is a constant, the so-called psychrometric coefficient. Knowing the absolute humidity, one finds the relative humidity using the following formula: R = A/F * 100 (2), where R is relative humidity expressed as a percentage, A is absolute humidity, and F is the maximum tension of water vapor corresponding to the temperature of the dry-bulb thermometer (maximum humidity). The values of F and F_t in formulas (1) and (2) are found from tables of water vapor elasticity. The value of the psychrometric coefficient K changes depending on the movement of the air; at different air movement speeds, the following values of K are obtained (according to Zvorykin): [Table data omitted]. To simplify the work when calculating relative humidity, ready-made psychrometric tables (with corresponding values for K) are used, where the corresponding relative humidity value is found from the readings of the dry and wet thermometers. Working with the August psychrometer is simple: the instrument is hung in the appropriate place, the bulb of the wet thermometer is moistened, and after 10 minutes, readings of both thermometers are taken. However, despite its low cost and simplicity of handling, this instrument suffers from a very significant drawback, which is that its readings are influenced by the movement of the air. This drawback makes it poorly suited for more or less precise research in sanitary practice, especially in the conditions of factory and plant premises. This drawback is eliminated in so-called aspiration psychrometers, the most popular representative of which is the Assmann aspiration psychrometer. The determination of humidity with the Assmann psychrometer, as with the August psychrometer, is performed by the difference between the readings of the dry and wet thermometers. A significant difference of the Assmann psychrometer is that it has a constant speed of air movement in the space surrounding the mercury bulbs of the thermometers. The Assmann psychrometer is constructed as follows (Fig. 2). Both of its thermometers are immersed with their lower ends into metal tubes, open at the bottom, and connecting at the top into one central cylindrical tube. The tube contains an inner tube in which the mercury reservoir of the thermometer is located. Such a double shell has the task of isolating the thermometers from the influence of possible thermal radiation. The end of the common tube is connected to a small fan driven by a clockwork spring. When the fan is operating, a certain

Figure 2.

constant air movement of about 2.4 m per second. The calculation of absolute and relative humidity is performed using the Sprung formula: A = F₁ - 0.5(t - t₁) * B/755 (3); R = A/F * 100, where A is absolute humidity, F₁ is the maximum tension of water vapor at the temperature of the wet-bulb thermometer, F is the same at the temperature of the dry-bulb thermometer, t and t₁ are, respectively, the readings of the dry and wet thermometers, B is the barometric pressure, and R is relative humidity. The Sprung formula does not differ significantly from the August formula; the only difference is that the former already has a constant psychrometric coefficient corresponding to the constant movement of air in the instrument. For the Assmann psychrometer, there are also special psychrometric tables in which the relative humidity value is found immediately from the readings of the dry and wet thermometers. Measurements using the Assmann psychrometer are conducted as follows: using a special pipette, the wet-bulb thermometer is moistened, then the fan is wound up with a key, and after 3-5 minutes, a reading is taken. The Assmann psychrometer is the most convenient and accurate instrument for measuring humidity in the practice of sanitary research. In addition to the August and Assmann psychrometers, a so-called sling psychrometer is sometimes used (Fig. 3). During the study, the thermometers fixed in a frame are set into a rotational movement around a rod. The calculation of humidity can be performed using the formula and tables compiled for the Assmann psychrometer. Bibliography: Kolobkov N., Meteorological Instruments, Their Construction and Application, M., 1932; Marshak M., Meteorological Factor and Labor Hygiene, pp. 72-78, M.-L., 1931 (Works and Materials of the State Scientific Institute for Labor Protection, No. 13, Vol. V, Issue 2); Okhlyabin S., Meteorological Instruments, Production and Processing of Observations, P., 1915; Pakhomychev A., Methodology for Investigating Relative Air Humidity and Comparative Evaluation of Hygrometric Instrumentation (Improvement of Labor and Revolution of Everyday Life, Issue 7, pp. 173-200, M., 1925, lit.); same author, Methods for Determining Relative Air Humidity in Work and Residential Premises, Gig. truda, 1925, No. 10, pp. 14-40; Rozanov S., Methods for Determining Air Humidity, diss., M., 1899; Bongards H., Feuchtigkeitsmessung, Munchen-B., 1926.

A. Letavet.