Don’t Use Barometric Pressure For Calibration

Take a look at the screenshots above. The one on the left is for Death Valley at 282′ below sea level and the one on the right is Denver, CO at 5,280′ above sea level.

Notice the barometric pressure, they are almost the same

This means that barometric pressure is corrected or “normalized” to sea level so that the weather can easily be compared from one place to another.  In other words, barometric pressure is there for weather forecasting not for calibrating tools or for calculating air density. In fact, If you own a simple dial barometer and you change its elevation you would need to recalibrate it to get it to match the forecast. Take a look at the chart below, you will notice that pressure drops by about 1″ for every 1000′ of elevation change. This is the way altimeters in aircraft work, they really aren't measuring distance, they are measuring pressure and converting it into altitude.

If it wasn't for this barometric normalization barometers in Denver would hover around 24″hg rather than the 30″ you will find in the forecast.

Of course, you can correct for this by subtracting from the forecast barometer pressure based on the altitude.

For the Denver reading of 30.24 you would do it like this –

5,280 ÷ 1000 = 5.28″ of mercury pressure below sea level 

30.24 – 5.28 = 24.96″hg (inches of mercury)

If you want to covert that to PSIA you divide by 2.036 to get PSI

24.96 ÷ 2.036 = 12.26 PSIA

Obviously, none of this is a problem when using tools that you simply “zero” to the atmospheric pressure, but in some cases, an instrument or calculation may require you to enter the pressure directly. This is when using barometric pressure can be an issue… unless you live at sea level like I do.

— Bryan

Related Tech Tips

VRV / VRF Installation Tips
This tech tip is written by experienced tech and VRF / VRV specialist Ryan Findley. Thanks Ryan! (Note: Ryan refers to VRV rather than VRF because he specializes in Daikin and these articles are written from a Daikin VRV perspective) In this tech tip, I’ll be going over a few things related to the install […]
Read more
Why Selecting Equipment is Important (and Hard)
I get questions all the time about performing “load calculations” and “rules of thumb” as well as how to do it properly. This article isn't about load calculation but the only good answer is to find a quality ACCA approved Manual J software and get used to using it. You may have heard from others […]
Read more
Why is the Breaker Tripping?
Breakers are designed to trip anytime the circuit draws a current above the rating for a period of time. The time the breaker takes to trip is a function of how high the circuit amperage in comparison to the breaker rating. The higher the amperage above the rating the faster the breaker will trip Breakers […]
Read more

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.


To continue you need to agree to our terms.

en English