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Pressure: bar, bara, barg : Where can be found a clarification Helpful Member!(6) 

ATsampalas (Materials) (OP)
25 Feb 11 10:29
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 ...
Helpful Member!(2)  21121956 (Mechanical)
25 Feb 11 11:35
Hello everybody:

You can start reading this then continue with these thread378-33451: Psia vs. Psig and thread794-33449: Atm vs. Gauge Pressure.
Helpful Member!  micalbrch (Mechanical)
26 Feb 11 5:39
What clarification do you mean? Bar gauge (barg) is the pressure gauge reading. Bar absolute (bara) is barg + atmospheric pressure (in most cases 1 bar higher than barg). Bar itself is an old pressure unit but still very common. The official ISO unit is Pa, kPa and MPa. 100000 Pa are around 1 bar.  
ATsampalas (Materials) (OP)
28 Feb 11 3:54
Hello micalbrch,
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 ?
micalbrch (Mechanical)
28 Feb 11 5:14
I do not know any valid standard which uses bar officially as the unit for the pressure. I know that the CGPM (Conférence Générale des Poids et Mesures) declared that bar can still be used but the official unit is Pa.
Helpful Member!  zdas04 (Mechanical)
28 Feb 11 9:00
Micalbrch's response is my understanding as well.  Until fairly recently (last 5 years or so), I never saw any reference to "bar" that was qualified (i.e., a "bar" was what is now called "bar(a)") and I don't think there is an "official" sanction for the distinction between "bar(a)" and "bar(g)".  It is messy.  I live where normal atmospheric pressure is around 12 psia.  A bar is 14.5 psia.  So, is 0 psig equal to 0 bar(g) and 1 bar(a)?  Or is 0 psig 0.827 bar(a) and 0.17 bar(g)?  I don't know the answer to that.  20 years ago, 0 psig at 5688 ft ASL would have clearly been 0.827 bar, but today I just don't know.

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

Helpful Member!  IRstuff (Aerospace)
28 Feb 11 10:19
Per Bureau International des Poids et Mesures (BIPM) "The International System of Units" Table 8, the bar is defined as 0.1 MPa.  While not an SI unit, it's considered to allowed for use by "special interest groups"


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micalbrch (Mechanical)
28 Feb 11 10:29

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.
IRstuff (Aerospace)
28 Feb 11 13:58
The SI definition of a standard atmosphere is 101325 newton/m2, 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|search_for=atm


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zdas04 (Mechanical)
28 Feb 11 16:52
I didn't say that kg/cm^2 made sense, I said that it is appearing on more and more gauges that I see in SI countries.  What it means is that the "metric" folks will have to deal with the silly "g(c)" that we started using when lbm replaced the slug.

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.

IRstuff (Aerospace)
28 Feb 11 17:16
If the NIST publication doesn't float your boat, IEEE/ASTM SI 10-2002, paragraph defines bar as described about.  Likewise, an atm = 101.325 kPa from Table 8 of same.


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MikeHalloran (Mechanical)
28 Feb 11 18:15
I guess I've been lucky in recent years, trying to understand and debug relatively simple systems designed by others, equipped with (if any at all) physically small gauges that couldn't resolve the difference between 2.8 bar and 3 bar even if they were located where you could see them.

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:

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

Helpful Member!  danw2 (Industrial)
28 Feb 11 22:30
To amplify IRStuff's explanation, ASTM B40.100 acknowleges reference to CIPM:
ASME B40.100-2005 (Revision of ASME B40.100-1998)
Pressure Gauges and Gauge Attachments

danw2 (Industrial)
28 Feb 11 22:31
typo in first line above, should be ASME, not ASTM
ione (Mechanical)
1 Mar 11 3:22
Actually there is no proper (exact) conversion from bar(g) to bar(a) as atmospheric pressure is not a constant but it changes (even if in a quite small range). There's anyway an accepted conversion from an engineering point of view 0 bar(g) = 1.01325 bar(a) at sea level.
zdas04 (Mechanical)
1 Mar 11 8:36
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.

ione (Mechanical)
1 Mar 11 8:55

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).
stevenal (Electrical)
1 Mar 11 11:10
Please substitute "ambient" for "atmospheric" above. Although it has been awhile now, my journeys have occasionally taken me below water, where the gauge on my air cylinder measures the difference between the tank air pressure and the surrounding water.  
BigInch (Petroleum)
1 Mar 11 11:40
zdas  14.5 is a conversion barg to psig, not a definition of either a bar or gage.  

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.
rgvrider (Mechanical)
1 Mar 11 18:51
First of all, you have to understand what you require a measurement for.

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.
racookpe1978 (Nuclear)
1 Mar 11 21:10
Well, a lot of the concern relates to what you are measuring, WHERE you are measuring it, and what the actuall (differential) pressure.  (Regardless of whether your units are Imperial =psix - or metric = barx)

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.    
BigInch (Petroleum)
2 Mar 11 2:43
It starts making a big difference when you're laying pipe, or driving an SSBN at the bottom of the ocean.

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