What is Psychrometric Chart? Introduction to Reading the Psychrometric Properties of Air

What is Psychrometric Chart? Introduction to Reading the Psychrometric Properties of Air
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The series of articles on properties of air discussed important properties of air like relative humidity, dry bulb temperature, wet bulb temperature, dew point temperature, sensible heat and latent heat. We shall now see how the air behaves when it is subjected to changes in temperature and humidity to suit the various applications for which the air conditioning is meant. The behavior of the air can be studied very conveniently and accurately by using a psychrometric chart.

What is Psychrometric Chart?

Psychrometric charts are graphic representations of the psychrometric properties of air. By using psychrometric charts HVAC engineers can graphically analyze different types of psychrometric processes and find solution to many practical problems without having to carry out long and tedious mathematical calculations.

The psychrometric chart looks complicated with vast numbers of lines and curves in it, but is very easy to understand if you know the basic properties of air. You will also understand its worth when you actually use it considering the fact that you won’t have to use any formulae to find the properties of air in different conditions, all you will have to know is two parameters of air and the rest are easily found on the chart.

Various Lines and Curves in the Psychrometric Chart

All the properties of air indicated in the psychrometric chart are calculated at the standard atmospheric pressure. For other pressures relevant corrections have to be applied. The psychrometric chart looks like a shoe. The various lines shown in the chart are as follows (please refer the figs below):

Psychrometric Chart 2

Psychrometric Chart 3

  1. Dry Bulb (DB) Temperature Lines:

The dry bulb temperature scale is shown along the base of the shoe shaped psychrometric 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.

  1. Moisture Content:

Moisture content is the water vapor present in the air and is measured in gram per kg of dry air (gm/kg 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.

  1. Wet Bulb (WB) Temperature Lines:

The outermost curve along the left side indicates the Wet Bulb (WB) temperature scale. The constant WB temperature lines are the diagonal lines extending from WB temperature curved scale downwards towards the right hand side of the chart. All the points located along the constant WB temperature line have the same temperature.

  1. Dew Point (DP) Temperature Lines:

Since the dew point temperature of the air depends on the moisture content of the air, constant moisture lines are also constant DP temperature lines. The scale of the DP and WB temperature is the same, however, while the constant WB temperature lines are diagonal lines extending downwards, the constant DP temperature lines are horizontal lines. Thus the constant DP and WB temperature lines are different.


  1. Book: Principles of Refrigeration by Roy J. Dossat

  2. Book: Basic Refrigeration and Air Conditioning by P N Ananthanarayan

Image Courtesy

  1. https://www.acr-news.com/news/news.asp?id=633
  1. https://www.sp.uconn.edu/~mdarre/NE-127/NewFiles/psychrometric_inset.html

This post is part of the series: Psychrometric Chart

This is the series of articles that describes psychrometric chart and various psychrometric processes like sensible heating, sensible cooling, humidification, de-dumidification, evaporative cooling etc.

  1. What is a Psychrometric Chart? Overview of Components
  2. How to Use a Psychrometric Chart
  3. Psychrometric Processes: Sensible Cooling and Sensible Heating of Air
  4. Psychrometric Processes: Cooling & Humidification and Heating & Humidification
  5. Psychrometric Processes: Cooling & Dehumidification and Heating & Dehumidification