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# What's the Difference Between PSI and PSIG?

01/16/2009 5:31 AM

Hallo to all,

What is the difference between PSI and PSIG?

I know, that PSI - pressure unit = 1Pound / 1sq.inch.

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#1

### Re: What is the difference between PSI and PSIG?

01/16/2009 7:28 AM

Andre,

In dat sheets either psig or psia is used to denote the pressure.Mere psi is not sufficient to understand the correct value unless a suffix is added as indicated.

PSIG refers to psi gauge:normal pressure gauges start with 0 -ignoring the atmospheric pressure.

The other unit is psia - psi absolute.This can be stated as :

PSIA = PSIG + 14.6959

You have ,of course,figured out what psi means!

Trust this helps you!

Chuck Cowlagi

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#3

### Re: What is the difference between PSI and PSIG?

01/16/2009 8:22 AM

chuck - i am watching you.

too many GAs - why don't you join up ?

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#14

### Re: What is the difference between PSI and PSIG?

01/19/2009 9:35 AM

Careful: 14.6959 is the correct value at sea level. Abolute pressure is different at different altitudes (more in Death Valley and less on Mt Everest. It can be considered as the "weight of the local atmosphere" which needs to adjust the local gauge reading to get psia.

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#2

### Re: What is the difference between PSI and PSIG?

01/16/2009 8:16 AM

Hi Andrei,

PSIG = "Pounds per Square Inch, Gauge" = pressure relative to the surrounding atmosphere.

How to interpret "PSI" (absolute or relative pressure), without the "G" or another modifier at the end, is a function of the application you're working with (for example, tire pressure, blood pressure, pressure vessel pressure, etc.). Vacuum is also a reference point for pressure.

Textbook I was trained on at Clarkson in the early 1990's was written by Frank M. White (University of Rhode Island). You could try and get this textbook to go deeper on this. MIT also has free courseware related to Mechanical Engineering on their web site.

Much luck.

- Larry

http://en.wikipedia.org/wiki/Pounds_per_square_inch

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#4

### Re: What's the Difference Between PSI and PSIG?

01/16/2009 11:46 AM

PSI (Gage) a gage can be compensated or calibrated for standard berometric pressure. 29.92 inches of mercury. 29.92 can also be considered 1 atmosphere.

Think as though there's really no such such thing as vacuum, but pressures less than 1 atmosphere. Or absolute zero PSI being a perfect vacuum, which isn't actually achieveable.

This is extremely important stuff if you go to fill a pressure tank and you think it's "empty" because the valve is open. If you fill it with anything other than air you are dilluting the gas you intended to store. Could be a dangerous mixure. That's why they purge pressure tanks, before filling. Probably way too much info. but if it saves a BOOM so much the better.

Anonymous Poster
#5

### Re: What's the Difference Between PSI and PSIG?

01/16/2009 2:19 PM

PSIG = Pondal square inch gauge, PSIA = Pondal Square inch absolute.

A Roman

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#6

### Re: What's the Difference Between PSI and PSIG?

01/16/2009 11:09 PM

Guest -- You got an off topic that I think was unjustified. Maybe a bit more explanatory test is needed for the language challenged fellow with an "off topic" finger on a hair trigger. Specifically about the early origins of the abbreviations.

Ed Weldon

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#7

### Re: What's the Difference Between PSI and PSIG?

01/16/2009 11:49 PM

It's important to note that only at the so called "standard" atmospheric pressure or 1 bar or 14.6959 psi or 29.92" Hg is PSIA = PSIG + 14.6959 psi. Change the barometric pressure due to weather variations, change altitude where the pressure gauge is located or place the gauge in some environment isolated from atmospheric pressure and the 14.6959 constant is no longer valid.

Most common pressure measuring devices measure the difference between the local atmospheric pressure and a fluid that exists at a different pressure that you are interested in. This is typical of common bourdon tube gauges and strain gauge transducers that measure the deflection of a membrane of sorts. It's pretty important to keep this in mind particularly in calculations involving physical chemistry or chemical reactions of materials. It's all to easy to bury this 14.6959 constant in a formula where it's not very visible such as a computer program or spreadsheet and have your calculations end up wrong for your particular conditions.

Also worth noting is that there are bourdon tube gauges that are made with double connections to measure pressure difference between two separate fluid bodies. These guys are not very common and are easy to spot. Not so easy to spot are some similar pressure transducers.

Ed Weldon

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#8

### Re: What's the Difference Between PSI and PSIG?

01/17/2009 12:04 AM

PSIG or psig above atmospheric (gauge) pressure in pounds per square inch ,where as ,

PSI stands for pounds per square inch (= 6.894 kPa)

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#9

### Re: What's the Difference Between PSI and PSIG?

01/17/2009 1:44 AM

Hi,

PSI is a measureof pressure in imperial units i.e pounds/per square inch.

In engineering we always measure the actual pressure , in pipework for example, using a pressure gage, hence PSIG is pounds per square inch gage . It is the actual pressure of the contents of the pipework, e.g. water.

You will come across another term which is PSIA which is pounds per inch absolute. The A stands for absolute. It is merely the presuure read off the gage plus atmospheric at sea level which is 14.7 PSI

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#10

### Re: What's the Difference Between PSI and PSIG?

01/17/2009 2:53 AM

There are a lot of good answers, but I'd like to add that we used to use the abbreviation of gauge pressure as PSI, which have to be written as PSIG. But for absolute pressure, the abbreviation must be PSIA. So, when you see PSI or PSIG, the both mean Gauge Pressure.

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Anonymous Poster
#15

### Re: What's the Difference Between PSI and PSIG?

09/11/2009 9:59 PM

Finally someone who actually answered the question.

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#16

### Re: What's the Difference Between PSI and PSIG?

09/12/2009 4:02 AM

Thank you Guest,

................................................................... just joking.

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#17

### Re: What's the Difference Between PSI and PSIG?

12/02/2011 11:05 AM

Dear Mr. Abdel Halim Galala,

Referring to your post ( No.11) here, I would like to put as follows.

During our college study days, we have been taught that unless otherwise stated, psi refers to psia i.e ABSOLUTE PRESSURE, and for guauge pressure it is denoted as psig.

psi or psia= psig+14.696 in BRITISH UNITS and KSCM OR KSCMA= KSCMg + 1.03 in Metric units. Here 14.696 psi is atmospheric pressure in British Units, at Mean Sea Level and 1.03 KSCM is Atmospheric Pressure in Metric Unit.

You have referred psi or psig is gauge pressure. I would like to gt clarified on this from you.

Thanks,

DHAYANANDHAN.S

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#18

### Re: What's the Difference Between PSI and PSIG?

12/05/2011 6:12 PM

1) A delayed GA to Mr. Galala (not sure I was on this forum back then)

2) Conventions are unpublished standards.

Perhaps India uses a different convention, but in the US, there are 3 engineering units for pressure related to our historical English units measurement system:

PSIA (absolute pressure)

PSID (differential pressure)

PSIG (gauge pressure)

If the 4th character is omitted, as in PSI, US convention defines PSI as PSIG, gauge pressure, not absolute pressure.

Absolute pressure must have the 4th character A, as in PSIA.

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#11

### Re: What's the Difference Between PSI and PSIG?

01/17/2009 3:51 AM

Thanks to all.

I see that here a lot of good answers.

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Anonymous Poster
#12

### Re: What's the Difference Between PSI and PSIG?

01/17/2009 9:16 PM

Andrei,

PSI stands for pounds per square inch while PSIG refers to pounds for square inch guage.

The latter is the pressure seen on the guage of a cylinder with gas contents.

PSI is sort of nebulous in its meaning. It could refer to total pressure as explained below.

PSIA is pounds per square inch absolute. One adds atmospheric pressure to PSIA to arrive at the total pounds per square inch.

Regards,

Stan Alekman

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#13

### Re: What's the Difference Between PSI and PSIG?

01/19/2009 1:43 AM

When U know PSI,nothing to worry about PSIG. It signifies Gauge pressure.

You know wel that gauge pressure (Pr. gauge reading) = absolute pressure (force generating pr.) - atm. pressure.

Regards

sanjib

ss_sanjibdas@rediffmail.com

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#19

### Re: What's the Difference Between PSI and PSIG?

05/06/2013 5:01 AM

Dear Mr.Andrei,

This has been covered about 8 months back and pl. open the below mentioned link and you will find a lot of response from CR4 members.

http://cr4.globalspec.com/search/sitesearch?do=show&sort=textmatchrank&srch=Difference%20between%20PSI%20and%20PSIG&order=asc

DHAYANANDHAN.S

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