**Psychrometric
chart**

A Psychometric chart graphically represents the thermodynamic properties of moist air. Standard psychometric charts are bounded by the dry-bulb temperature line (abscissa) and the vapour pressure or humidity ratio (ordinate). The Left Hand Side of the psychometric chart is bounded by the saturation line. Figure 27.2 shows the schematic of a psychometric chart. Psychometric charts are readily available for standard barometric pressure of 101.325 kPa at sea level and for normal temperatures (0-500C). ASHRAE has also developed psychometric charts for other temperatures and barometric pressures (for low temperatures: - 40 to 100C, high temperatures 10 to 1200C and very high temperatures 100 to 1200C)

**Measurement
of psychrometric properties:**

Based on Gibbs' phase rule, the thermodynamic state of moist air is fixed if the barometric pressure and two other independent properties are known. This means that at a given barometric pressure, the state of moist air can be determined by measuring any two independent properties. One of them could be the dry-bulb temperature (DBT), as the measurement of this temperature is fairly simple and accurate. The accurate measurement of other independent parameters such as humidity ratio is very difficult in practice. Since measurement of temperatures is easier, it would be convenient if the other independent parameter is also a temperature. Of course, this could be the dew-point temperature (DPT), but it is observed that accurate measurement of dew-point temperature is difficult. In this context, a new independent temperature parameter called the wet-bulb temperature (WBT) is defined. Compared to DIYF, it is easier to measure the wet-bulb temperature of moist air. Thus knowing the dry-bulb and wet-bulb temperatures from measurements, it is possible to find the other properties of moist air.

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