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The psychrometric chart 2

In my first post, we became familiar with the psychrometric chart and took a brief tour around the various scales and axis. In this post I will look at the different information that can be determined from a single point on the chart.

The point I will look at is air with a temperature of 21.0°C db (dry bulb) with a humidity of 50% RH (Relative Humidity).

To plot this point on the chart, first locate 21.0°C on the X axis at the bottom of the chart and project a line upward. Next find the curved line marked 50% RH. Where the two cross is our point.


Having plotted our point on the chart, let’s see what other information we can obtain.

Firstly projecting a line horizontally to the right from our point, we can read off from the Y axis that the air has a moisture content of 0.0078 kg/kg i.e. there is 0.0078 kg of water vapour in every 1.0 kg of dry air.

Drawing a line through our point parallel to the lines of specific volume allows us to determine that the specific volume of the air is 0.844 m3/kg (nearly half way between 0.84 and 0.85). This tells us that every kg of air has a volume of 0.844 m3. Specific volume is 1/density so the density of the air can be found by taking the reciprocal of the specific volume.

Density = 1/specific volume,

In our example: Density = 1/0.844 = 1.184 kg/ m3.

Drawing a line parallel to the wet bulb lines through our point will give the wet bulb (wb) temperature. In our example this is between the 14.0°C and 15.0°C lines and is approximately 14.7°C wb.

The last bit of information that can be gleaned from the chart is the specific enthalpy. Draw a line between the enthalpy scales that go through our point, being sure that the line starts and ends on the same number. This reveals that the specific enthalpy for our point is 41.0 kJ/kg.

So, in summary, air with a temperature of 21.0 C db and a humidity of 50% RH, has the following properties.

  • Temperature db = 21.0 C
  • Humidity = 50% RH
  • Moisture content = 0.0078 kg/kg
  • Specific volume = 0.844 m3/kg
  • Temperature wb = 14.7 C
  • Specific enthalpy = 41.0 kJ/kg

In my next post we'll look at what happens when the air is heated.

Steve Rix is your contact for all humidification enquiries - call him on 07966423165 or email

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