What is Psychometric Chart ? How to Read Psychometric Chart ( With PDF Downloads)

Psychometric is the science dealing with the physical laws of air-water vapour mixtures.It contains Relation between-Properties of air and moisture.It is an important tool for HVAC engineers to carry out heat load or cooling load calculations and find solutions to various air condition related problems. Psychometric processes include physical and thermodynamic .

The properties of Psychometric chart are as follows :-

  1. Dry Bulb Temperature
  2. Wet Bulb Temperature
  3. Dew Point Temperature
  4. Relative Humidity
  5. Humidity Ratio.

Dry Bulb Temperature :-

•It is the temperature of air measured by thermometer.
The dry bulb temperature scale is shown along the base of the shoe shaped psychometric chart forming the sole. The DB temperature increases from the left to the right. The vertical lines shown in the chart are the constant DB temperature lines and all the points located along a particular vertical line have same DB temperature.

•Wet Bulb Temperature:-

It is the temperature of the air measured by a thermometer whose bulb is covered with a wet wick or cloth. The wet-bulb temperature of the air is recorded after rapidly moving the thermometer in the air and allowing the temperature to stabiles. It is called the wet-bulb temperature because the bulb of the thermometer is wet when the temperature reading is taken.

Dew Point Temperature:

It is the temperature of the air when it is fully saturated and the moisture condenses on a surface. It is equivalent to a wet bulb temperature at 100% relative humidity. At this combination of temperature and humidity, further removal of heat results in water vapor condensing into liquid.

Relative Humidity:-

It is the ratio of the actual moisture present in the air to the maximum moisture the air can hold. RH is dimensionless, and is usually expressed as a percentage. 100% RH indicates the air is saturated and cannot hold any more moisture. Preferred values of comfort for people are between 35% and 60%.

Lines of constant relative humidity are shown as exponential lines on the psychrometric chart. The line at 100% is referred to as the saturation line.

Humidity ratio :-

Humidity ratio describes the actual weight of water in an air-water vapor mixture and is usually designated as W. Also, Humidity ratio can be expressed as grains of moisture per pound of dry air. The moisture present within the air is indicated by the vertical scale located towards the extreme right. The horizontal lines starting from this vertical scale are constant moisture lines.


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