Atmospheric Pressure - Short 114
HVAC School - For Techs, By Techs
Bryan Orr
4.9 • 1K Ratings
🗓️ 23 March 2021
⏱️ 13 minutes
🧾️ Download transcript
Summary
In this short podcast episode, Bryan explains what atmospheric pressure really is, pressure units and conversions, and why those are matter.
Atmospheric pressure is the weight of the air around us pushing down on us. We normally see that value expressed as 14.7 PSI (or 0 PSIG). Before we dive too deep into atmospheric pressure, we should understand some basic pressure units.
We may see pressure expressed in microns when we're pulling a vacuum; we are trying to pull the atmosphere out of the system, so our goal is to get as close to 0 as possible. Whenever we pull a vacuum, we get liquid water to boil off and remove molecules inside the system. The industry standard is 500 microns. 14.7 PSI(A) is equivalent to about 760,000 microns, so the micron is an extremely small pressure measurement. You may also see the bar scale, which is equivalent to 1 atmosphere (atm). One bar equals just over 14.5 PSIA.
You may also encounter the Pascal unit, which is common on the building science side of our industry. One PSI is equal to 6,894.76 Pascals. When we look at small pressures, such as static pressure or gas pressure, we may use the inch of water column ("wc). One inch of water column is equal to 248.84 Pascals. We also have inches of mercury ("Hg) and the torr (mmHg), which are related to the micron. All units are interrelated, but they have their appropriate applications.
Atmospheric pressure matters when altitude enters the equation. When the pressure changes at a higher altitude, the air density also changes. The air is less dense, so you have less oxygen in the air. When you have less oxygen in the air, combustion is more likely to be incomplete. So, we may need to derate furnaces. We also need to take altitude into account when we calibrate gauges at significant altitudes compared to sea level.
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Transcript
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| 0:00.0 | Hey, this is Brian with the HVAC School Podcast. In this episode we're going to talk about |
| 0:08.8 | atmospheric pressure, but we're also going to talk about some of the common scales that are used to measure pressure and just some good general information, just some basics, some things as you might have forgotten along the way, as well as some things you might have forgotten to know in the first place. |
| 0:23.2 | But before we do that, we want to thank the companies that support this podcast. |
| 0:28.4 | carrier and carrier.com, Navac and Navac Global.com. |
| 0:33.0 | Refrigeration Technologies at Refriges Tech.com. |
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| 0:44.4 | Find out more by going to a Fridgetech.com. Mitsubishi Electric. |
| 0:48.3 | We have been Mitsubishi Diamond Dealers, Diamond Contractors for years and years at Kailos, and Mitsubishi is coming out with more and more innovative products to serve both commercial and residential, as well as high efficiency, high performance homes. |
| 1:03.0 | If you haven't looked at Mitsubishi in a while, |
| 1:05.0 | now is the time to do so. |
| 1:07.0 | Find out more at Mitsubishi Comfort.com. |
| 1:11.0 | All right, so let's talk about atmospheric pressure. So what is atmospheric pressure? |
| 1:17.0 | Atmospheric pressure is the pressure exerted on us from all angles, the weight of the atmosphere above us, and around us that pushes down from all directions. |
| 1:25.2 | So inside of our bodies we have an equal pressure that pushes out against what's pushing in so that |
| 1:30.5 | way we don't collapse. |
| 1:31.5 | It's 14.7 pounds per square inch is what pushes |
| 1:35.2 | down on us in all directions and so that's what we also call zero PSIG. So pounds |
| 1:40.1 | per square inch gauge, the gauge is zeroed out to whatever the current |
| 1:44.3 | atmospheric pressure is and it's kind of a trick because if you were to go to the |
| 1:49.0 | mountains for example and you were to zero out your gauge, well then your PSIG scale is going to be different than it is down at sea level. |
| 1:57.0 | So as far as PSIA, which is our absolute scale, that always takes into account whatever the current real atmospheric pressure is. |
| 2:05.1 | Because when we say atmospheric pressure is 14.7, we're talking specifically about sea level, |
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