The total heat content of air is the sum of two parts: (1) sensible heat and (2) latent heat. SENSIBLE HEAT is the amount of heat which, when added to or removed from air, changes the temperature of the air. Temperature changes caused by sensible heat can be measured by the common (dry-bulb) thermometer.

Air always contains some water vapor. Any water vapor in the air contains LATENT HEAT OF VAPORIZATION, which means the heat necessary to cause the water to change state from a liquid to a gas (heat added). The opposite case is called LATENT HEAT OF CONDENSATION, which refers to the heat that must be removed to cause the water vapor to change state from a gas back into a liquid. (The amount of latent heat present has no effect on temperature, and it cannot be measured with a dry-bulb thermometer.)

Any mixture of air and water vapor contains both sensible heat and latent heat. The sum of the sensible heat and the latent heat in any sample of air is called the TOTAL HEAT of the air.

TEMPERATURES

To test the effectiveness of air-conditioning equipment and to check the humidity of a space, we must consider two different temperatures. These are the dry-bulb and wet-bulb temperatures.

Measurement of Temperatures

The DRY-BULB TEMPERATURE is the temperature of the sensible heat of the air, as measured by an ordinary liquid-in-glass thermometer. Such a thermometer in air condi-tioning is referred to as a dry-bulb thermometer because its sensing bulb is dry, in contrast with the wet-bulb type described next.

The WET-BULB TEMPERATURE is best explained by a description of a wet-bulb thermometer. (Refer to fig. 17-1, view A.) It is also a liquid-in-glass thermometer with a loosely woven cloth sleeve or wick placed around its bulb, which is then wet with distilled water. The water in the sleeve or wick is forced to evaporate by a

Figure 17-1.-Temperature monitoring devices.

current of air (see next paragraph) at high velocity. This evaporation withdraws heat from the thermometer bulb, lowering the temperature by several degrees. The difference between the dry-bulb and the wet-bulb temperatures is called the WET-BULB DEPRESSION. When the wet-bulb temperature is the same as the dry-bulb temperature, the air is saturated (that is, evaporation cannot take place). This condition of saturation is unusual, however, and some value of wet-bulb depression is normally expected. The wet-bulb and dry-bulb thermometers are usually mounted side by side on a frame that has a handle and a short chain attached. This allows the thermometers to be whirled in the air, thus providing a high-velocity air current to promote evaporation. This device, known as a SLING PSYCHROMETER, is shown in figure 17-1, view B. When using the sling psychrometer, whirl it rapidly-at least four times per second. Observe the wet-bulb temperature at intervals. The point at which there is no further drop in temperature is the wet-bulb temperature for that space. Another variation of this device, known as a MOTORIZED PSYCHROMETER, is shown in figure 17-2. Motorized psychrometers are provided with a small motor-driven fan and dry-cell batteries. Motorized psychrometers are generally preferred and are gradually replacing sling psychrometers.