To find the spot on the chart for the current air conditions, begin again at the bottom axis and find the 90º Dry Bulb Temperature. For this example, let’s assume the current air conditions have a Dry Bulb Temperature of 90✯ and a Wet Bulb Temperature of 75✯. That means if the outdoor air cools from 75º to 60✯, it will become saturated and dew will form on outdoor surfaces.Ī common use of a psychrometric chart for sizing an air cooling system is to determine the Enthalpy value of the air that needs to be cooled. To determine the Dew Point from these air conditions, move along a horizontal line to the curved left axis. This is the point of the current air conditions. Next travel up the line at 75º until it intersects with the curved line for 60% Relative Humidity. What if we want to know the Dew Point? To find the spot for current air conditions, start at the bottom axis and find the 75º Dry Bulb Temperature. For example, the weather person says that the current air conditions are 75✯ and 60% relative humidity. The most important principal of a psychrometric chart is that if you know any two values of the air conditions, you can determine all other parameters. Wet Bulb Temperature: the lines that slant down from left to right.Relative Humidity: the lines that curve up from left to right.Saturation Temperature or Dew Point: the curved left axis.Humidity Ratio and Vapor Pressure: the right axis.Dry Bulb Temperature: the axis along the bottom.We will start our exploration by first locating on the graph all of the terms that we learned from my previous article:
Psychrometric ChartĪ psychrometric chart packs a tremendous amount of information in an odd looking graph.
In the second part of this series, we will use these terms and learn how to read a psychrometric chart. In Part 1: Demistifying Psychrometrics, I provided the scientific definition and a little explanation for the terms used in psychrometrics.