How to Read a Psychrometric Chart

The psychrometric chart is a handy tool that enables the reader to determine and calculate dry bulb, wet bulb, relative humidity, dew point temperature as well as other gas-vapor relationships with only limited information. This chart helps people, especially involved in heating, ventilating and air-conditioning systems, to determine human comfort levels in relationship to changes in temperature and humidity. With limited information, the reader can determine how changes in one aspect of temperature, humidity or pressure will effect the others.

The psychrometric chart is necessary when designing HVAC systems.
  1. Determine the dry bulb temperature at your location. This can be done by reading a simple thermometer at your location.

  2. Determine the relative humidity at your location. This can be done by reading a hygrometer at your location, or by obtaining the information from a local weather station.

  3. Locate the temperature value measured in Step 1 on the bottom (X) axis of the psychrometric chart. Follow the line indicated by this value vertically until you reach the intersecting line of the value you obtained from your hygrometer. The lines indicating relative humidity start at the bottom left of the chart and radiate toward the upper right of the chart increasing in value from the bottom of the chart to the top.

  4. Follow the diagonal line from where the dry-bulb and relative humidity lines intersected to the upper left. Where this line meets the left side of the chart indicates the wet-bulb temperature. This value is can also be obtained through measurement using a sling psychrometer.

  5. Follow the line where the relative humidity and dry bulb temperature meet to the left hand edge of the chart along the horizontal line to obtain the dew point temperature at your location as measured above. With this data you can determine how humidity, dew point temperature and wet-bulb temperature will be effected by changes in dry bulb temperature and vice versa.

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