Psig or Psi

By ctacker (****)

Date 01-03-2008 05:51

Psig (pound-force per square inch gauge) is a unit of pressure relative to atmospheric pressure at sea level. By contrast, psi measures pressure relative to a vacuum (such as that in space). Most pressure gauges, such as tire gauges, are calibrated to read zero at sea level, because most applications require the difference of pressure.

By G.S.Crisi (****)

Date 01-03-2008 15:24

ctacker's explanation is excellent, but I'll repeat it with different words.
psi (pounds per square inch) really doesn't mean anything, although is widely used in current technical conversation, including by me.
What do have meaning are the following expressions:

psig (pounds per square inch gage) is the pressure measured by a pressure gage. Imagine that you've got a pressure gage in your hands, telling you that pressure is zero. Is it true? No, because inside the gage there's the atmospheric pressure, approximately 14,7 pounds per square inch, which is neglected by the gage.

psia (pounds per square inch absolute) on the other hand, is the pressure read by the pressure gage PLUS the atmospheric one.

Example: when the pressure gage reads a pressure of 10 psig, the actual pressure inside the gage will be 24,7 psia, i.e., the psig reading plus atmospheric.

Giovanni S. Crisi
Sao Paulo - Brazil

By Kix

Date 01-03-2008 16:18 Edited 01-03-2008 16:31

Where the psi and psig numbers on the tank the same or was one higher then the other? They should have the same number I think because psi is just the lazy man's way of saying psig. I wonder if you use a guage that was calibrated at sea level in Denver how much it will actually throw off your reading. Denver Colorado psia is 12.2 so technically it could be off by 2.5 psi, but for some reason I doubt it would be exactly that.
This brings up a question I asked myself awhile back wondering if steam driven turbines would be more efficient up in the mountains. I ask this because water boils at a lower temp the higher you go up because of pressure drop. So a bit less heat to make steam might be a bit more cost effective, but I guess it's not that much of a gain so they just build them where they can get to em I guess.lol

By ssbn727 (*****)

Date 01-03-2008 16:34

Hi Kix!

The abbreviation "psi" should be: "psia" if "psig" is also listed, and as both ctacker and Giovanni did a superb job of explaining the differences between the two, the only person being "lazy" was the one that forgot to include the letter "a" after psi. ;)

Respectfully,
Henry

By Kix

Date 01-03-2008 16:57

You're absolutely coorect because why would they put the same number on there twice.lol I just meant that when we refer to 30psi in our tires it should really be 30psig, but we're being lazy and dropping off the g right? I'm starting to scratch my head.lol

By 803056

Date 01-03-2008 19:19

The power extracted by a steam turbine is temperature in versus temperature out, so the ambient pressure has little to do with the efficiency of the system. That being said, there is a relationship between pressure and temperature when dealing with steam (or any other vapor). There are published steam tables available that provide that type of information. It all goes back to Thermal-God-Damics!

Best regards - Al

By Kix

Date 01-04-2008 01:42

I was thinking more along the lines of less energy to make the steam to then turn the turbine, but I see what your saying. "Thermal-God-Damics" Good one Al! I like it....