Book - School of Science and Technology

Application 15a Stevenson screen. The louvres are arranged so as to allow a natural circulation of airaround the thermometers but, of course, they cannot exclude the effect of radiant heatcompletely, with the result that what are known as screen wet bulb temperatures are alwaysabout 0.5 K higher than those read from the alternative instruments used by engineers.The more common of these is the sling psychrometer which consists of two thermometersmounted side by side in a frame which is fitted with a handle such that it may bewhirled by hand, as shown in Figure 1.3. One thermometer is fitted with a wick which isfed with water from a small reservoir in the frame and both the water, preferably distilled,and the wick must be clean. Readings are taken after the instrument has been whirledvigorously to simulate an air speed above about 3 m/s. An alternative instrument, theAssmann type, consists of a miniature air duct within which the two thermometers aremounted, air being drawn over them by a small fan, clockwork or electrically driven, at aknown velocity.Air velocityRequirements for reading air velocity fall into two very distinctly different categories. Onerelates to measurements made for the purpose of determining air volume in a duct or atdelivery to, or extract from, a space, and the other to the identification of movement of airwithin a room. In the former case, velocities are not likely to be less than 4 m/s and (unlessthings have gone badly astray) the direction of flow should be easily identifiable. For thispurpose the traditional field instruments have been vane anemometers of one type oranother as will be noted in more detail in a later chapter (p. 471).Within a room, however, circumstances are different and it is generally accepted that atolerable level of air velocity at the back of the neck is not much in excess of 0.1 m/s whenthe dry resultant temperature is 20 C, although, in extreme summer conditions, withoutcooling, a transient increase to 0.3 m/s may be acceptable. For measurement at these lowlevels, use must be made of either an instrument known as the Kata thermometer or of oneof the more sophisticated anemometers such as a hot-wire type.The Kata instrument is made in four patterns, and that most used for measurement oflow air velocities has a very large silvered bulb filled with coloured alcohol. In order totake a reading, the bulb is first warmed in hot water, causing the alcohol level to rise, andthe instrument is then located in the position at which an observation is required. Thetime taken for the level of the spirit to fall between two marks on the scale, as a result ofcooling due to the air movement, is noted and this, together with a reading from a drybulb thermometer, is applied to a nomogram calibrated for the particular instrument toprovide a record of velocity.As may be appreciated, the Kata thermometer is best suited to laboratory researchwork and is tedious to use elsewhere. The hot-wire instrument is much more convenient touse than the Kata but neither provides any absolute indication of the direction in whichthe air is moving. It is thus necessary to rely upon a cold smoke, produced chemically, tosupply a visual appreciation but this is not wholly satisfactory either, since the smoketends to diffuse rather too rapidly.ApplicationFrom what has been said in preceding paragraphs it will be clear that, while it is possibleto define the various factors which relate to comfort and to list the methods used to