HVAC technicians in training are often intimidated when they see a psychrometric chart for the first time since there are six separate sets of lines that are used to plot conditions once temperatures in a given situation are established. However, the chart can be demystified by reviewing the sets of lines individually in skeletal illustrations.

In Figure One below, are the dry bulb temperature lines of a psychrometric chart.


Figure One

Figure One

These lines are properly referred to as Constant Dry Bulb Temperature Lines because any point on any given line has the temperature listed on the scale.

The lines shown below in Figure Two, are also constant temperature lines, and they are identified as Wet Bulb Lines.


Figure Two

Figure Two


These lines run down from the temperature scale on the curved section of the chart at an angle of approximately 30-degrees off of horizontal.

The next set of lines shown in Figure Three are the Constant Relative Humidity Lines.


Figure Three

Figure Three


The top curved line on the chart shown as the 100% relative humidity line is also known as the saturation line, meaning that we no longer have water vapor in the air, but liquid condensing from it. In most situations, many people are comfortable when the humidity level is around 50%. It is usually recommended that the humidity level in a building remain between 40% and 60%.

These first three sets of lines are the basic ones that allow a technician to take both a wet bulb and dry bulb measurement in a given area, and then plot the relative humidity conditions in the space.

In Part Two, we’ll look at three more sets of lines on the psychrometric chart.

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