Psychrometrics is the field of engineering that involves the properties of gas(air)-vapour(water) mixtures. A psychrometric chart is a popular way to include a lot of information of these gas-vapor mixtures in a concise way. Because of the amount of information included in these charts they can be a little complicated to read unless you are in an industry or class that uses them frequently – such as the HVAC industry or an HVAC class.
The psychrometric chart has information on 6 properties of air: air’s dry bulb temperature, wet bulb temperature, relative humidity, humidity ratio, specific volume, and enthalpy.
Dry bulb temperature
The dry bulb temperature is the temperature of the air without considering moisture. This is the temperature you would read on a typical thermometer.
Wet bulb temperature
The wet bulb temperature is the temperature read when using a wet-bulb thermometer. A wet-bulb thermometer is a essentially a thermometer that utilizes soaked fabric to mimic 100% relative humidity.
Relative Humidity
The relative humidity (specific humidity) is the ratio of the amount water vapor in the air to the amount of water vapor in the fully saturated air.
Humidity ratio
The humidity ratio is the ratio of the mass of water in the air to the mass of the air itself.
Specific Volume
The specific volume is the volume of the air per unit mass of the air. It is also the inverse of density.
Enthalpy
The enthalpy is the heat or energy content of the air. It is the equal to the internal energy in addition to the energy from pressure work (pressure x volume).
In order to read the psychrometric chart you need any 2 of these properties. If you have 2 of these properties, you can find the remaining 4. Below is a psychrometric chart along with a legend of how to find each of these values.
The dew point temperature can also be found from a psychrometric chart. The dew point temperature is temperature you would need cool the air to in order for the vapor in the air to condense from a vapor to a liquid.
In order to find the dew point temperature, you first must identify a point based off the 6 properties previously mentioned. To find the dew point temperature for air at these properties, travel horizontally to the left until you hit the edge of the curve. Whatever dry bulb temperature corresponds to this new point on the edge of the curve is the dew point. There is an example of this below.
*Note: You could also use the dew point temperature with 1 other property to find a point on the psychrometric chart. The process of finding the dew point temperature will just be reversed in order to find a specific point on the chart.
In this example we have a dry bulb temperature of 30°C and a wet bulb temperature of 21°C. Following a horizontal line left to the edge of curve gives us a dew point temperature of 17°C.
If you prefer an interactive online psychometric chart, we have found the following useful links
