In Part One of our discussion on the psychrometric chart we discussed three sets of lines; the Constant Dry Bulb Temperature Lines that run from the bottom of the chart to the curved top, the Constant Wet Bulb Temperature Lines that run from an angle of approximately 30 degrees from the curved line to the right, and the Constant Relative Humidity Lines that follow the pattern of the curve line.

In Figure One below, we’re showing a fourth set of lines, known as the Constant Dew Point Lines.

Figure One

Figure One

The Dew Point Temperature numbers are the same ones used for the wet bulb temperature scale, but the lines coming from the numbers run directly horizontal rather than at a diagonal.

The Dew Point Temperature lines also correspond directly to another listing to the right of the chart which expresses moisture level on the very fine scale of grains per pound. This scale is also known as the Specific Humidity Scale.

 The fifth set of lines on the psychrometric chart, shown in Figure Two, run down from the curved line at an angle of about 60 degrees, and they are the Constant Specific Volume Lines.

Figure Two

Figure Two

These lines represent the idea that air has a certain density that changes as the temperature and water vapor level changes.

This scale is built on the fact that 1 lb. of air at a saturation temperature of 65-degrees F has a specific volume of 13.50/ft3lb (13.50 cubic feet per pound).

The sixth set of lines on the chart that run from the curved line to points on a numbered scale above the chart, are shown in Figure Three Below, and they are known as the Constant Enthalpy Lines.

Figure Three

Figure Three

The term Enthalpy means “total heat content”, and these lines are simply extensions of the dew point temperature lines.

The scale shows the total heat content measured in BTU/lb (BTU, British Thermal Unit, which is the common measurement of heat content used in the HVACR industry, per pound). One BTU represents the amount of heat it takes to raise the temperature of one pound of water, one degree Fahrenheit.

The two kinds of heat that make up total heat are Sensible Heat, which is heat that can be measured, and Latent Heat, which is also known as “hidden heat” and is defined as heat that brings about a change in state, but not a change in temperature.

And, when we bring together all the lines we’ve discussed so far, the complete psychrometric chart appears as shown in Figure Four below.


Figure Four

Figure Four


In Part Three of this series, we’ll look at how the psychrometric chart is used to provide the HVAC technician with information they need to know about the properties of air as it is being handled throughout the conditioned space in a comfort cooling application.

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