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#electrical meter

Tech Tips:

If you have a multimeter for HVAC work, you most likely have a meter with a CAT III rating on it. Some might also be rated for CAT II at higher voltages or CAT IV at lower ones—or both, like this model of the UEi DL599 from our Multimeter 101 for HVAC 3D video. In […]

Tool Selection: Electrical Meters

I'd like to give a special thanks to Tony Gonzalez from Fieldpiece for providing technical guidance about the SC680's power measurement capabilities, as well as VAC and AAC measurements on ECMs. You can learn more about the SC680 and other electrical meters on Fieldpiece's website. We’ve talked through specifications, applications, and features for a few […]

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How to Evacuate an AC system, Full Vacuum Procedure

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In this short podcast episode, Bryan talks about psychrometrics and the magic line inside a messy-looking chart. The psychrometric chart is a key tool for understanding the relationship between air and water.

“Psychrometric” comes from the Greek roots for “cold” and “measurement.” As such, a tool called the psychrometer (with paired wet-bulb and dry-bulb thermometers) could measure two different temperatures, and we could use these to determine how much water vapor the air can hold (even though the air doesn’t “hold” water vapor; it’s all about vapor pressures).

The psychrometric chart was developed with the contributions of James Apjohn and William Ferrell in the 1800s, but Willis Carrier was the one who drafted the paper titled “Rational Psychrometric Formulae” in 1911. He created a chart that plotted temperature, humidity, and enthalpy on a single piece of paper.

The horizontal axis represents the dry-bulb temperatures, the curved lines represent relative humidity, the left axis represents humidity ratios, the diagonal lines represent wet-bulb temperatures, and the other angled lines represent enthalpy. Every possible air condition can be plotted on the chart. Moving horizontally changes sensible heat (temperature), and moving vertically changes latent heat (moisture content); both are critical to human comfort.

The “magic line” is the dew point. When this line is crossed, that’s when surfaces like ducts or HVAC coils begin to sweat, as air can’t hold any more moisture. You can use this information and the psychrometric chart to help you make sense of customer complaints (e.g., “clammy”) and predict how air and moisture will behave if you adjust airflow or add insulation.

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Psychrometrics and the Magic Line – Short #265

11/11/2025

Bryan Orr

Learning Hydronics, Boilers and Steam w/ Ray Wohlfarth

11/06/2025

Bryan Orr