Where the horizontal line from (P) intersects the saturation curve a similar condition occurs, whereby the air cannot absorb any additional moisture (5). The pressure of the saturated air can now be read. An extension of the horizontal line through this point of intersection to the left intersects the partial pressure scale at point (4). it cannot absorb any further moisture without a dense mist forming. To obtain the saturation pressure (4) the isotherm from P must be extended until it intersects the 100% relative humidity line. If this horizontal is extended to the left it will intersect the scale for partial water vapour pressure pD (3). The absolute humidity (2) is found by drawing a horizontal line through the point P and extending it until it meets the ordinate on the right. The point of intersection P between the 20☌ isotherm from the dry bulb temperature (1) and the line of 50% constant relative humidity (6) clearly defines the position of the required condition. The point on the chart is defined by the temperature and the relative humidity given above. These are typical values to provide comfort in an office.Įxample: Find the missing values for the following

Note that the chart indicates a condition of 21 ☌ Dry bulb temperature and 48% Relative humidity.

Allowances may be made for changes in barometric pressure by using correction factors. Since the properties and behaviour of moist air depend on barometric pressure, a psychrometric chart can only be drawn for a specific barometric pressure. The procedure can be time consuming.īy presenting the interrelated factors on a psychrometric chart, an immediate decision can be made regarding the feasibility of controlling an airĬonditioning system and the means required to carry this out.įor a given air sample nine different parameters are shown on the psychrometric chart.Ī position on the chart can be established at the intersection of two ordinates for known conditions and the others obtained. This may be achieved by a good knowledge of physics, with theoretical calculations using complicated formulae and tables. In air conditioning technology, it is necessary to define thermodynamic processes and the properties of moist air.

How to draw a psychrometric chart excel