PRINCIPLES OF NAVAL ENGINEERING 



temperatures are usually considered. These are 

 the dry-bulb temperature and the wet-bulb tem- 

 perature. 



The dry-bulb temperature is the temperature 

 of the air as measured by an ordinary dry-bulb 

 thermometer. The dry-bulb temperature re- 

 flects the sensible heat of the air. 



The wet-bulb temperature is the temperature 

 of the air as measured by a wet-bulb thermom- 

 eter. A wet-bulb thermometer is an ordinary 

 thermometer with a loosely woven cloth sleeve 

 or wick placed around the bulb and then wet 

 with water. The water in the sleeve or wick is 

 made to evaporate by a current of air at high 

 velocity. The evaporation lowers the tempera- 

 ture of the wet-bulb thermometer. The differ- 

 ence between the dry-bulb temperature and the 

 wet-bulb temperature is called the wet-bulb 

 depression . When the air is saturated, so that 

 evaporation cannot take place, the dry-bulb 

 temperature is the same as the wet-bulb tem- 

 perature; the condition of saturation is unusual, 

 however, and a wet-bulb depression is normally 

 to be expected. 



The wet-bulb thermometer and the dry-bulb 

 thermometer are usually mounted side by side 

 on a frame. A handle or a short chain is attached 

 to the frame so that the thermometers may be 

 whirled in the air, thus providing an air current 

 of high velocity to facilitate evaporation. Such a 

 device is known as a sling psychrometer. (See 

 fig. 19-13.) Motorized psychrometeTi are 



provided with a small motor-driven fan and dry 

 cell batteries. Motorized psychrometers are 

 gradually replacing the sling psychrometers. 

 An exposed view of a hand electric psychrometer 

 is shown in figure 19-14. With either type of 

 psychrometer, the wet-bulb temperature must 

 be observed at intervals as the water is being 

 evaporated. The point at which there is no fur- 

 ther drop in temperature on the wet-bulb ther- 

 mometer is the wet-bulb temperature of the 

 space. 



As may be inferred from this discussion, 

 the wet-bulb depression is an indication that 

 latent heat of vaporization has been used to 

 vaporize the water in the sleeve or wick around 

 the wet-bulb thermometer. 



When the air contains some moisture but is 

 not saturated, the dew-point temperature is 

 lower than the dry-bulb temperature and the 

 wet-bulb temperature is between the dew-point 

 and the dry-bulb temperatures. As the amount 

 of moisture in the air increases, the difference 

 between the dry-bulb temperature and the wet- 

 bulb temperature becomes less and less. When 

 the air is saturated, the dew-point temperature, 

 the dry-bulb temperature, and the wet-bulb 

 temperature are identical. 



AIR MOTION.— In perfectly still air, a layer 

 of air adjacent to the body absorbs the sensible 

 heat given off by the body and increases in tem- 

 perature. This layer of air also takes up the 



GRIP 



5.65 



Figure 19-13.— A standard sling psychrometer, 

 486 



