Psychrometrics and the Magic Line - Short #265
HVAC School - For Techs, By Techs
Bryan Orr
4.9 • 1K Ratings
🗓️ 11 November 2025
⏱️ 13 minutes
🧾️ Download transcript
Summary
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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Transcript
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| 0:00.0 | Indeed, indeed. This is the HVAC school podcast. I am Brian J. Or Esquire, the third. And this is the podcast where I just keep talking about history and weird nerd stuff. But that's what we're going to do again today, talking about psychometrics, not psychometrics, psychometrics, and the magic line. |
| 0:23.5 | But before we do that, we want to thank our great sponsors. |
| 0:27.8 | Refrigeration technologies, and specifically viper wipes, the all-purpose wipe that is really |
| 0:35.0 | amazing what it can cut through. Everything from mastic to oil grease tar, grime, and it's nice on the hands. |
| 0:42.5 | Make them nice and soft when you're done. |
| 0:44.3 | So that way, you can go home and head out a dancing when you get back to the house. |
| 0:50.6 | Find out more at refrigetect.com slash viper dash wipes. |
| 0:55.3 | Refraggetech.com slash viper dash wipes. |
| 1:00.2 | Carrier and carrier.com. |
| 1:02.7 | Carrier has been a long-term sponsor of the podcast. |
| 1:05.4 | They've made it possible very early on for us to do what we do and they are the products |
| 1:09.8 | that we sell day in and day out, |
| 1:11.3 | everything from single stage equipment all the way up to the green speed extreme, one of the |
| 1:17.0 | most efficient products on the market today. Find out more about what Carrier has to offer |
| 1:21.9 | and about becoming a carrier dealer by going to Carrier.com. Navak at Navakglobal.com and their A2L compatible refrigerant recovery units, specifically |
| 1:32.8 | the NR7. |
| 1:34.8 | This thing is just 20 pounds, which is really light for a recovery unit in case you have |
| 1:39.3 | never weighed them. |
| 1:40.8 | The NR7 is built with efficiency in mind. |
| 1:43.2 | It has an oil-free compressor and a large condenser inside to dissipate heat quickly as you recover, which makes it go faster. |
| 1:51.5 | Has twin cylinders with a brushless DC motor, a large fan with plenty of room for airflow within the body of the recovery machine. |
| 2:00.1 | It has a CSA safety certification with full |
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