What's the Difference Between PSI and PSIG?

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.

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.

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

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

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

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

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.

Finally someone who actually answered the question.

Thank you Guest,

But most of comments rated with Good Answers, except me

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

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

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.

Thanks to all.

I see that here a lot of good answers.

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

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

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