Reading a Psychrometric Chart1

A psychrometric chart is designed so that if any two properties of the air are known, values for the other five properties can be found on the chart. Different

1 An ASHRAE PSYCHROMETRIC CHART NO. 1 is used in this section. Obtain from American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc. 1791 Tullie Circle, NE, Atlanta, Georgia, 30329. (404-636-8400). If a different chart is used the values may be different.

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35 40 45 50 55 GO 65 70 75 80 65 90 95 100 105 110 115 120

Dry bulb temperature (°F)

35 40 45 50 55 GO 65 70 75 80 65 90 95 100 105 110 115 120

Dry bulb temperature (°F)

FIGURE 23.1. Dry bulb, wet bulb, and saturation temperature scales.

charts are used. It is common to see high temperature, normal temperature, and low temperature charts. In addition, charts are produced for special purposes. To complete the activities of this section you should obtain a normal temperature psychrometric chart. Note: the scales for each property of air are different and the number of graduations for each scale is not the same. The recommended practice is to interpolate only one decimal point beyond the scale.

A psychrometric chart has a scale and lines for each property. To find the values for the properties of air, you must be able to locate what is called the state point. The state point is the junction of any two property lines, Figure 23.1. The following section will explain how to locate the seven properties of air.

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