Psychrometric Basics

In this psychrometric basics podcast, Bryan and Kaleb talk about the properties of air. They also discuss dry-bulb, wet-bulb, dew point, and relative humidity. Psychrometrics is the study of the relationship between air and its properties.

The psychrometric chart can be a bit intimidating, but you can use it in a variety of ways. A technician should care about this chart because it helps with whole-home diagnosis. You can’t see the whole picture of someone’s comfort unless you know the properties of the air. The left side of the chart is centered on wet-bulb and enthalpy, and the right side is centered on the absolute moisture content; the chart provides a comprehensive comfort profile if you use it correctly.

Dry-bulb temperature is the basic sensible temperature of the air and gives you a one-dimensional heat measurement. Wet-bulb temperature directly relates to the evaporative properties of water in the air; the wet-bulb temperature changes based on the moisture content even if the sensible heat stays the same. So, wet-bulb temperature gives us a better picture of the enthalpy, which is the total heat content (latent AND sensible). The wet-bulb temperature will usually be lower than the dry-bulb temperature, and the difference is called wet-bulb depression.

The only time when wet-bulb and dry-bulb temperatures will be the same is at 100% relative humidity, also called the dew point. At the dew point, the air can no longer hold any more moisture, so any additional water vapor in the air has no choice but to condense.

Bryan and Kaleb also discuss:

• Radiant gains and dry-bulb measurements
• “Cold air is dry air”
• Relative vs. absolute humidity
• What really is temperature?
• Sling psychrometers vs. digital probes
• Load calculations
• Supply air and relative humidity
• Insulation and humidity