## Pressure: bar, bara, barg : Where can be found a clarification

## Pressure: bar, bara, barg : Where can be found a clarification

(OP)

Hi all,

Where can I find a standard clarification between :

bar ?

bara (Absolute Pressure) ?

barg (Gauge Pressure) ?

The Standard being either : ASTM, ASME, British Standard, Euro-Norm, or ...

Sincerely,

A_Tsampalas

Where can I find a standard clarification between :

bar ?

bara (Absolute Pressure) ?

barg (Gauge Pressure) ?

The Standard being either : ASTM, ASME, British Standard, Euro-Norm, or ...

Sincerely,

A_Tsampalas

## RE: Pressure: bar, bara, barg : Where can be found a clarification

You can start reading this http://en.citizendium.org/wiki/Bar_(unit)and then continue with these thread378-33451: Psia vs. Psig and thread794-33449: Atm vs. Gauge Pressure.

## RE: Pressure: bar, bara, barg : Where can be found a clarification

## RE: Pressure: bar, bara, barg : Where can be found a clarification

Thanks for your answer.

Maybe my question was not clear enough. Let me put it otherwise :

i) ASTM E 380-93 (in my possession): Standard Practice for use of the International system of the modernized metric system : does not address the subject ;

ii) NIST 811-2008 (in my possession): Guide for the use of the International system of Units (SI) : does not address the subject ;

My question is : In which Standard document (ASTM, ASME, EURO-NORM, ISO, ...) can I find the interpretation between bar, bara and barg ?

Sincerely,

A_Tsampalas

## RE: Pressure: bar, bara, barg : Where can be found a clarification

## RE: Pressure: bar, bara, barg : Where can be found a clarification

I have a similar problem with expressing pressure in kg/cm^2, but that is a different post.

David

## RE: Pressure: bar, bara, barg : Where can be found a clarification

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

Chinese prisoner wins Nobel Peace Prize

## RE: Pressure: bar, bara, barg : Where can be found a clarification

I can at least answer your question. Where you live 0 psig is 0 bar(g) and 0.827 bar(a). Where I live 0 psi(g) is 0 bar(g), too (of course). But actual we have 1.027 bar(a) here.

bar and kg/cm² are (more or less) the same. To be precise 1 bar is 1.019 kg/cm².

## RE: Pressure: bar, bara, barg : Where can be found a clarification

^{2}, which makes the standard atmosphere = 1.01325 bars.A bar is a unit of pressure, which is a force/area, not mass/area You could possibly get by with bar = kgf/cm^2, but that would still not be a canonical form, and the SI definition of pressure has units of kg*m

^{-1}*s^{-2}http://physics.nist.gov/cuu/Units/outside.html

htt

http://physics.nist.gov/cuu/Units/units.html

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

Chinese prisoner wins Nobel Peace Prize

## RE: Pressure: bar, bara, barg : Where can be found a clarification

Micalbrch,

So If a gauge reads 2bar(g) at my home it is 2.817 bar(a)? I don't know of anyone who is doing that (seemingly proper) conversion. Mostly I see people say 2 bar(g)=3 bar(a). I also didn't say that was right, but it is what I see.

David

## RE: Pressure: bar, bara, barg : Where can be found a clarification

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

Chinese prisoner wins Nobel Peace Prize

## RE: Pressure: bar, bara, barg : Where can be found a clarification

The question remains: Where is a (printed) reference that describes the relation of bar/bara/barg?

Google of course produces a reference to this discussion, but it also eventually came up with a document from the British Compressed Air Society that explains it in layman's terms:

www.

It is of course wrong in the sense that David notes, but it may be close enough for, er, government work.

Mike Halloran

Pembroke Pines, FL, USA

## RE: Pressure: bar, bara, barg : Where can be found a clarification

ASME B40.100-2005 (Revision of ASME B40.100-1998)

Pressure Gauges and Gauge Attachments

## RE: Pressure: bar, bara, barg : Where can be found a clarification

## RE: Pressure: bar, bara, barg : Where can be found a clarification

## RE: Pressure: bar, bara, barg : Where can be found a clarification

I don't think that is right. "Bar" has a precise definition (100 kPa, or 14.5 psi) I think you are refereing to "atm" which is a function of height above sea level.

David

## RE: Pressure: bar, bara, barg : Where can be found a clarification

I'm with you 100% when you say 1 bar = 100 kPa = 14.5 psi without any added specification ("gauge" or "absolute"). And how could I disagree?

But the way I've always understood it is that the term "gauge" refers to the pressure of any given system relative to atmospheric pressure. The absolute pressure of any system is consequently the gauge pressure of the system plus the atmospheric pressure. The atmospheric pressure is not a universal constant since, as you have pointed out, it is affected by altitude and temperature and so my point was there is not a universally valid conversion from pressure(gauge) to pressure(absolute).

## RE: Pressure: bar, bara, barg : Where can be found a clarification

## RE: Pressure: bar, bara, barg : Where can be found a clarification

g has no fixed definition as ione pointed out. You will have to see the definition of NASA Standard Atmosphere for "standard" conversions at different elevation, which also varies with the weather, 29.92 in Hg being the average reference.

The only thing that's absolute is absolute.

## RE: Pressure: bar, bara, barg : Where can be found a clarification

Gauge pressure is used to measure the pressure difference between two mediums. This is very important for hydraulic equations, where you are concerned with pressure differences. barg, is used to reflect the pressure difference between atmosphere and the medium being measured. As pointed out, atmospheric pressure changes depending on location. This means for a closed box, the gauge pressure reading would change depending on location.

Absolute pressure is based on a scale from 0 upwards. 0 represents a perfect vacuum (ie pure space). Absolute pressure is used in many thermal calculations where gas laws are required to be relative to the vacuum. An absolute pressure gauge will give a constant reading for a closed box no matter the location.

Bar is just a measure of pressure. Others have listed the conversion factors to the Pa, which is an SI unit.

## RE: Pressure: bar, bara, barg : Where can be found a clarification

Example. A 3000 psig gas tank half-full is 1500 psig, and it won't really matter whether you are measuring that gas at sea level, 1500 meters, or 300 feet. The 14.7 psia (1 bara) of atmospheric pressure is irrelevent. Same case for a 1200 psig steam system.

The little difference of atmospheric pressure on the steam pressure gage as a cold front moves through (or if the steam generator could be moved from sea level to 1400 meters) is irrelevant, but the "value" of that steam going into a -2 psig condenser, a 0.0 psig condenser, or a +12 psig condenser is considerable!

If a pump has a minimum suction pressure of .75 meter, and you only provide 25 cm of water, the "fractions of a bar" you are missing will destroy the pump impeller.

Thermodynamically, you have to use the proper unit(s) for the energies (gas states, vapor pressures, thermal energies, combustion energies) you are concerned about.

## RE: Pressure: bar, bara, barg : Where can be found a clarification