Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations IFRs on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Pressure: bar, bara, barg : Where can be found a clarification 7

Status
Not open for further replies.

ATsampalas

Materials
Feb 26, 2004
43
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
 
Replies continue below

Recommended for you

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.
 
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 ?
Sincerely,
A_Tsampalas
 
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.
 
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.

David
 
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"

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize
 
David,

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.
 
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


TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize
 
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.

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
 
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
 
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

vngfpe.jpg


29209wx.jpg
 
typo in first line above, should be ASME, not ASTM
 
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.
 
ione,
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
 
David,

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).
 
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.
 
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.
 
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.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor