Psychrometric charts

Thermodynamic properties of dry and moist air have been tabu­lated by a number of authorities including CIBSE (The Char­tered Institute of Building Services Engineers) in the United Kingdom. However, a chart presentation of the data is prefera­ble, especially where this can encompass all the values of hu­midity from completely dry air, through fractional humidities, to completely saturated air.

Such charts can be drawn for a number of temperature ranges but that for normal atmospheric air is shown in Figure 2.2. To quote CIBSE Guide C:

The chart has been constructed using two fundamental proper­ties specific enthalpy and moisture content as basic, linear co-ordinates. Other physical properties are not then shown as linear scales. The 30 °C dry bulb line has been constructed at right angles to lines of constant moisture content and the scale of specific enthalpy inclined obliquely to the vertical scale of moisture content. In this way lines of constant dry bulb tempera­ture are approximately vertical, diverging slightly each side of

30 °C and the traditional appearance of the chart preserved.

The wet bulb values plotted are those read from a sling, or ven­tilated, psychrometer but lines of percentage saturation are plotted instead of relative humidity. Within the comfort zone there is little practical difference between percentage satura­tion and relative humidity and, of course, the difference dimin­ishes as the saturated or dry states are approached.

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