Book - School of Science and Technology

14 Fundamentalsthermometer, used for measurement of low temperature radiation, consists of a hollowcopper sphere or cylinder about 150 mm in diameter, blackened on the outside and havinga simple mercury-in-glass instrument projecting into it such that the bulb is in the centre,as Figure 1.2(b). The mean radiant temperature (t ri ) at a single point in a room may, withdifficulty, be measured with a globe thermometer or may be calculated from the arithmeticmean of the surrounding areas, each multipled by the relevant surface temperature.HumidityThe instrument most commonly used to measure the moisture content of air in a room is apsychrometer which makes use of what is called wet bulb temperature. This is measured bymeans of a simple mercury-in-glass thermometer which has its bulb kept wet by means ofa water-soaked wick surrounding it. As the water evaporates, it will draw heat from themercury with the result that a lower temperature will be shown. The rate of evaporationfrom the wetted bulb depends upon the humidity of the air, i.e. very dry surrounding airwill cause a more rapid evaporation ± and a lower temperature in consequence ± than airwhich is more moist, although the temperature shown by an ordinary dry bulb thermometerwould be the same in each case. The difference between dry and wet bulbtemperatures may thus be used as a measure of humidity, individual values being knownas the wet bulb depression.The rate of evaporation, and hence the extent of the depression, depends also upon themanner in which the wetted bulb is exposed to the air. Records of external dry and wet bulbtemperatures are kept by meteorological authorities world-wide and for this purposemeasurement is made using instruments placed in the open air within a louvred box calledDry bulb thermometerSpindleWickWet bulbthermometerReservoirFigure 1.3 A sling psychrometer