Psychrometers contain two identical thermometers, one to measure the dry bulb temperature and one to measure the wet bulb temperature. The sensing bulb of the wet bulb thermometer is covered with a wick, which is saturated with distilled water before taking a reading. Evaporation occurs at the wick of the wet bulb thermometer, giving it a lower temperature reading. The wet and dry bulb temperatures can be used to find the relative humidity of the measured air using either a built-in chart on the psychrometer or a separate psychrometric chart.

Sling psychrometers (Figure 1-6) are often used when servicing air conditioning equipment. One use is to measure the wet bulb temperature of the indoor air entering the indoor coil ( evaporator). Wet bulb temperatures take into account the latent heat load in the indoor air and give a better indication of the load on the indoor coil. To get an accurate wet bulb temperature measurement, the sling psychrometer must be spun rapidly in the air being tested.

Another use of the sling psychrometer is to measure dry and wet bulb temperatures in order to determine the room temperature and percent relative humidity (RH). Relative humidity is measured to determine the level of environmental comfort in the various rooms of a house or other conditioned space. Proper control of RH is also critical to many commercial and manufacturing processes. Most people feel comfortable when the indoor temperature and humidity conditions fall within certain ranges called comfort zones. For winter, temperatures between 6 7° F and 76° F and an RH of about 30% are considered comfortable. For summer, the comfort zone is between 72° F and 8 1° F with an RH of about 40%. Properly-controlled temperature and humidity conditions are important to insure comfort and health in a ll seasons. Proper control of these conditions also helps to reduce the load on cooling and heating equipment.

Squeeze-bulb and battery-operated aspirating psychrometers (Figure 1-7) are used in confined spaces where a sling psychrometer would be difficult to operate. The squeeze-bulb aspirating psychrometer works by rapidly squeezing the bulb to draw air over the thermometers. In the battery-operated version , a fan draws air over the thermometers.

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