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Pressures

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Pressures

Unread postAuthor: Spitfire » Thu Mar 20, 2008 3:57 pm

Please help me i am admitidly a noob when it comes to foreign pressure systems. My question follows.

What is the difference between psi and psig?
or is there any difference?

I am from South Africa and we mostly just work with Bar, kPa.
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Unread postAuthor: Rokmonkey » Thu Mar 20, 2008 4:01 pm

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Unread postAuthor: Hotwired » Thu Mar 20, 2008 4:12 pm

PSI is a general unit of pressure

PSIG is what a gauge reads because when it reads 0 PSI it is ignoring atmospheric pressure which is aroundabout 15psi.

So pumping a tyre up to 15psig, it will be 15psi compared to atmospheric pressure but compared to a vacuum it's 30psi.
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Unread postAuthor: DYI » Thu Mar 20, 2008 4:16 pm

psi - pounds per square inch
psig - number of psi above whatever ambient pressure the gauge was calibrated for, generally 14.7 psia, as this is ambient pressure at sea level
psia - absolute pressure. the number of psi above zero pressure
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Unread postAuthor: Antonio » Thu Mar 20, 2008 7:09 pm

why dont we all use kiloPascal?? doesnt it make it so much easier

>>1bar~atm~1kPa
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Last edited by Antonio on Fri Mar 21, 2008 3:40 am, edited 2 times in total.
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Unread postAuthor: TurboSuper » Thu Mar 20, 2008 7:19 pm

Same reason we still use imperial measurment.

Besides, you'd still have kPag and kPaa...although I dont know if that's the correct notation for it.
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Unread postAuthor: pizlo » Thu Mar 20, 2008 8:47 pm

Wow I didn't know this either! good to know.
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Re: Pressures

Unread postAuthor: Ragnarok » Thu Mar 20, 2008 9:06 pm

Spitfire wrote:What is the difference between psi and psig?
or is there any difference?

Pretty much as has been said before, but I'm expanding on it slightly.

Psig is Gauge pressure. i.e. pressure above atmospheric - total vacuum is approximately -14.7 psig.
Psia is Absolute pressure - total vacuum is exactly 0 psia.

DYI is slightly out on one point, pressure gauges are not calibrated with a zero point. What actually happens is that these gauges tell you the difference between the atmospheric air pressure and the pressure in the system - this applies to both pressure and vacuum gauges.

If you were to have a system with a pressure of 44.7 psia, and atmospheric pressure was 14.7 psia, a pressure gauge would read 30 psig, but then if the ambient air pressure were to rise 1 psi to 15.7 psia, the gauge would now only read 29 psig - even though the absolute pressure in the system was unchanged.

Normally, when people say psi, they tend to mean psig - but... might be psia, usually it's obvious from the context when it is.

As I did above, I tend to use psig or psia when confusion might arise.
I prefer to use metric myself, but as the forum tends to rather use Imperial measurements, I'll convert most of my figures between the systems when I actually post them.
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Unread postAuthor: fatcat » Thu Mar 20, 2008 9:27 pm

I just learned that mmHg, torr and atm are also used as pressure measurements from my chemistry book. There was no mention of psi. I don't see why we need 5 different pressure measurements. it just makes me confused.
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Unread postAuthor: Ragnarok » Thu Mar 20, 2008 10:21 pm

@fatcat: Well Torr and mmHg were the same unit, until Torr was redefined in 1954 as 1/760th of an atmosphere, although there is no real difference even now.

For most purposes, 1 Torr = ~1 mmHg, 1 bar = ~1 atmosphere, 1 bar = ~14.5 psi.
Then of course, there's kgf/cm<sup>2</sup>, which is 0.98 of a bar.

Just for starters: Torr/mmHg, Bar, Pascals, Atmospheres, mmH<sub>2</sub>O, inHg, psi, ksi, kgf/cm<sup>2</sup>, and so on and so forth.

Pressure is a bad one for a huge number of units.
But normally all you need to worry about are psi, bar and perhaps atmospheres (which you can treat as equal to 1 bar for most calculations). Most of the other units are used for atmospheric pressure measurements.

EDIT: Spelking Erorrs.
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Unread postAuthor: Spitfire » Fri Mar 21, 2008 2:37 pm

Thanks for the info.

It all helps. But im thinking that it would be mutch easyer with a universal measurement with pressure, or all measurement in that case.
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Unread postAuthor: Spitfire » Fri Mar 21, 2008 2:41 pm

Thanks for the info.

It all helps. But im thinking that it would be mutch easyer with a universal measurement with pressure, or all measurements in that case.
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Unread postAuthor: D_Hall » Fri Mar 21, 2008 2:47 pm

ant wrote:why dont we all use kiloPascal?? doesnt it make it so much easier

>>1bar~atm~1kPa

Me thinks you might want to check your math on that last one. ;)
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Unread postAuthor: D_Hall » Fri Mar 21, 2008 2:50 pm

Spitfire wrote:It all helps. But im thinking that it would be mutch easyer with a universal measurement with pressure, or all measurement in that case.

It all depends on what you're trying to do.

How many inches is it from here to Pluto? Can you see how inches may not be the most appropriate unit of measure to measure that distance?

The same is true in the world of pressure. Different units are useful for different scenarios/industries.
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Unread postAuthor: Spitfire » Fri Mar 21, 2008 3:02 pm

D_Hall wrote:How many inches is it from here to Pluto? Can you see how inches may not be the most appropriate unit of measure to measure that distance?

The same is true in the world of pressure. Different units are useful for different scenarios/industries.


Also true, (um how many inches in a cm:?: :lol:) :wink:

Oops we double posted
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