bar 2

[McCormick93]

A firm in the US is designing systems for a process plant in China. Over here, when we say "100 psi steam" it is understood that it means psig. Over there, they use "bar" for pressure. A 100 psig system would be 7 bar (gauge) or 8 bar (absolute). In places where "bar" is used, is it assumed gauge or absolute?

 

[katmar]

Never assume anything. Bounce the information back to the supplier/client with a request to specify gauge or absolute.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[zdas04]

If you go back to the original definition, a gauge calibrated in "bar" would show "1" at rest. I saw one the onther day that showed zero at rest. Katmar is right, never assume it is one or the other.

I mostly work in low pressures and designing a compressor to go from 1 bar to 7 bar is a very different machine depending on what "1 bar" means (if it absolute then you're doing 7 ratios and need a two-stage machine, if it is gauge then you're doing 4 ratios and it is a pretty easy single stage).

Don't guess, ask.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[JohnGP]

Working in bars or psi is no different, bars are not some magical beast. Where there could be confusion I would specify "barg" or "bar(abs)" if I had to use those units, just as I used to say "psig" or "psia" if it was not clear one way or the other.

If you are talking design pressures for equipment, then you can be reasonably certain that specified values are in gauge pressures, but if for process design then it could be absolute. I find that Process Engineers quite often like to work in absolute pressures, and as zdas04 points out absolute pressures are a must in compression calculations (although I don't agree that a gauge at rest should show 1 bar, if at atmospheric pressure). It's taken a bit to train some Process guys that I want design pressures for vessels specified in "gauge" pressure.

But as katmar says, never assume anything. Someone once told me that to "assume" something means that you will probably end up making an "ass" out of "u" and "me". On occasion, I have been able to demonstrate that this statement is correct.

 

[zdas04]

JohnGP,
Thank you for making me think about this in a different light. I always assumed that 1 bar(a) was local atmospheric pressure, but it isn't. 0 bar(a) is equal to 0 psia, so at my elevation (5300 ft, atmospheric pressure is 12.0 psia) 1 bar(a) is equal to 2.5 psig. The gauge that I saw that had been zeroed to 0 bar(g) at rest was actually out of calibration relative to 0 bar(g).

Calibrating a pressure gauge to bar is a really complex undertaking since local atmospheric pressure is 0.83 bar here, 100 psig is 7.7 bar(a), if you zero the gauge at atmospheric pressure then the reading on the gauge at 100 psig is 6.9 bar, adding 1 bar to get to absolute would make 100 psig 7.9 psia (a 2.5% error that would be signinficant in gas measurement applications). I think I'll start asking people who send me data in bar(g) what their gauge reads at local atmospheric conditions.

I've always thought of "bar" as "easy" since going from 1 bar(a) to 7 bar(a) is 7 compression ratios regardless of atmospheric pressure. Throw in bar(g) and it is even more complex than psi or kPa.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[McCormick93]

Thanks to all for your thoughtful replies. In the documents we put out to the client, the design engineer (not me, by the way) specified "bar (gauge)" leaving no room for interpretation.

zdas04, to make matters more confusing, I have a reference that shows 1 bar=0.9869 atm. As far as bar being related to atmospheric at sea level, that's a very interesting point.

And now a "teachable moment" for alexawy2611:
Out here in non-academia, I have never seen a correlation of gauge/absolute applied to incompressible/compressible fluids. However, I can offer the following advice:
My education was almost all done in SI units. My professional work is almost never done in SI units. It is very important to be comfortable with all unit systems, now that the oceans are "shrinking".
Also, to borrow from a fellow Eng-Tips user, "This is a professional forum, not a chat room." Words like "cuz" and "u" are not commonly recognized out here in non-academia. English slips are certainly forgivable for the multi-lingual, but it is advisable to practice your best writing.
Your enthusiasm is admirable, and this is indeed a fine place to learn. I wish you the best as you launch your career.

 

[quark]

1 bar is 1x10⁵Pa and atmospheric pressure at sea level is 101325 kPa and thus one atmosphere is 1.01325 bar. So, one bar is (at sea level) about 1/1.01325 = 0.9869 atm.

Adding 1 to 6.9bar is as bad as adding 14.69psi to 100 psi and the difficulty is there if we go with any unit of pressure.

As long as a gauge reads 0 at any elevation, it is ok and we have to carefully add the local atmospheric pressure or we can anyway go for an absolute gauge.

 

[zdas04]

Quark,
You are wrong on this one. Zero bar(a) is equal to zero psia. That is the only constant. Except at about 100 ft elevation, a gauge exposed to atmospheric pressure should never read zero bar(g). Adding local atmospheric pressure (0.828 bar at 5,300 ft) to a gauge that has been set to zero at rest would give you the wrong answer every time.

With a gauge calibrated in psi or kPa, you can adjust the zero as you change elevation without affecting the validity of the calibration. With a gauge calibrated in bar, that adjustment is not to zero but to some fraction of a bar away from zero.

David

 

[katmar]

For a question that started out very simply, this thread has introduced an enormous amount of confusion. I think we should try to tie the loose ends together rather than leave this thread dangling in space-time with so much uncertainty.

David, you commented that

With a gauge calibrated in psi or kPa, you can adjust the zero as you change elevation without affecting the validity of the calibration. With a gauge calibrated in bar, that adjustment is not to zero but to some fraction of a bar away from zero.

Why would the unit "bar" be treated any differently than "psi", "kPa" or any other unit? My understanding of gauge pressure (irrespective of the units used) is that it is the pressure difference from the local atmospheric pressure. In my understanding, a pressure gauge calibrated to gauge pressure would show zero at rest at whatever altitude it was calibrated for.

Quark stated that (btw - good to see you around again Quark - I haven't seen many posts from you recently)

So, one bar is (at sea level) about 1/1.01325 = 0.9869 atm.

I thought that 1 bar was (approximately) equal to 0.9869 atm everywhere, at sea level or on the moon. When I use the "atm" as a unit in this way it is (IMHO) a well defined unit of pressure and is not dependent on local atmospheric conditions.

The two comments by David and by Quark seem somehow linked and pointing to something that I have missed. If anyone can clarify this I would be grateful.

The comments by JohnGP and by alexawy2611 refer to conventions applied to the interpretation of pressure units. Conventions are helpful in that they make things easier for people within the same paradigm, but they can confuse people from the outside. In the USA it is conventional for vessel engineers to specify design pressures as gauge pressures, but this is not always true in Europe.

In the design of a vessel, the engineer is concerned with the pressure differential between the inside and outside of the vessel (i.e. across the pressure envelope). If the outside of the vessel is subject to atmospheric pressure, then defining the internal pressure in gauge terms gives you this differential. But if the outside of the vessel has a jacket that has a pressure other than local atmospheric it would lead to confusion.

The internal pressure and temperature conditions for the vessel would have been determined by a process engineer (hopefully in absolute terms!) and they would be the same whether the vessel was operated at sea level or at 15,000 ft - but the internal gauge pressure would be different for each location. So in strict terms, the design pressure in gauge terms varies with location - but in absolute terms it remains constant. But we have to stay practical and appreciate that when the inspector comes to do the in-situ pressure test he is going to have a gauge calibrated to the local conditions and in fact he can only measure the gauge pressure.

I believe it is always safer to use absolute pressures, but it is not too important as long as the units are fully specified as gauge or absolute and the engineer on the receiving end can interpret them according to his/her needs.


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[quark]

Katmar,

Yes, I should agree that at sea level is a redundant or rather faulty expression.

I see a point in David's argument that, for example, if I take a gauge - combination gauge (pressure as well as vacuum) and get the zero set at sea level, then when I go higher up, it should read below zero otherwise it will never work for a vacuum application at sea level. In other words, a gauge - pressure gauge bourdon set to zero at sea level experiences a shrinkage in its diameter when we go up and thus the pressures it measure are always faulty.

But like you said, this is true in all cases of pressure units. David, further, may be arguing about the dimensionless quantum of 14.6 against 1 where you have 14 set points in case of a psi gauge vs 1 set point in case of a bar gauge. But we end up with same problem if the least count of psi gauge is 10 psi(which may not be a likely case) instead of 1 psi.

I await David's response on my understanding.

Thanks for the warm welcome. My new job is keeping me busier than I expected and also I feel that I am at my wits end of late:-(

Best regards,

 

[zdas04]

If you zero a gauge calibrated in bar(g) in Farmington, NM, USA then zero bar(a) would be -2.5 psia which is an impossible concept.

Pressure increments are arbitrary, but we have two physical points on the pressure scale that have historically been satisfied--zero absolute pressure is the minimum pressure that is physically possible, and zero gauge pressure is a convenient "constant" local reference point. With other scales, both of these physical points are easily satisfied. The increment on the bar scale requires that (except at a very specific elevation) they can't both be satisfied. If I zero a bar(g) scale at local atmospheric pressure then I can't reach zero bar(a), if I calibrate the bar(g) scale so that 0 bar(a)=0 psia, then the needle will be somewhere other than zero on the scale at rest.

Since all of this is based on an arbitraty scale, I know that I can get to usable numbers by adding local atmospheric pressure to a gauge reading (in Farmington I would add 0.828 to the gauge reading to get to a universally usable absolute pressure), but looking at the corespondence I've gotten over the last couple of years, everyone who writes to ask me compressor questions just adds "1" to bar(g) to get to bar(a). Maybe this is just a problem in Oil & Gas. Maybe it is just a problem with us lazy compressor guys. Maybe it is a universal laziness in the world.

My guess is that there are a huge number of bar(g) pressure instruments that are zeroed at local atmospheric and that the conversion from bar(g) to bar(a) is done by adding "1" to the bar(g) reading. For most calculations, this is way the heck close enough. For compression calculations from vacuum to a significant positive prssure it is a real problem. For gas measurement calculations it would be a real problem.

I guess I spent too many years doing gas measurement to ever be cavilier about calibration and unit definitions. I know that someone designing a relief valve for 300 bar(g) doesn't care if that is 301.000 bar(a) or 300.828 bar(a), especially since relief valve calculations are done in gauge terms. The equations give you the same size orifice regardless.

If I'm designing a compressor to go from 103 torr (measured backward from zero gauge) to 3 bar (in Farmington compressing from 10 psia to 55.5 psia) if 0 bar(a)=0 psia then it is 5.5 compression ratios. If 0 bar(g) = 0 psig and I assume that bar(g)+1 bar(a) = bar(a) then it is 4.0 compression ratios. I would design the first as a two-stage recip (since I don't allow over 4 ratios per stage in the design step, you have to leave some slop for actual conditions), the second would appear to be an easy single-stage unit that would forever have problems with discharge temperature and/or rod load.

I'm probably just being anal here, but this thread has forever changed my thinking about "bar" as a unit of pressure measurement (but not as a place to drink).

David

 

[JohnGP]

David, I agree with pretty much all that you are saying, except when you talk about apparent differences in the scales when "zeroing" gauges in different units, i.e.-

"If you zero a gauge calibrated in bar(g) in Farmington, NM, USA then zero bar(a) would be -2.5 psia which is an impossible concept."

That statement only makes sense if you zero the "Bars" gauge at 1 bar(a), which you wouldn't do if the local atmospheric pressure was 12psia (=0.827bar(abs)). If you "zero" both gauges at 0 (psig and barg) then a full vacuum at that location would read -12psig and -0.827barg on compound gauges. At that location -12psig=0psia, and -0.827barg=0bar(abs).

Katmar, I take your point about gauge and absolute pressures for vessel design. I've not worked in the US, but managed to assume (there's that word again) a universality in the need to use gauge pressures when specifying design pressure - I guess I haven't done any jacket designs. In fact we nearly overdesigned a low pressure air receiver on my present project, where the specified design pressure was 250kPa(abs). Fortunately picked up the units in time (come to think of it - that was a European company specifying the design).

Cheers,John

 

[katmar]

John, I had the same misunderstanding of what David was saying about bars. I discussed it with him off-line and realised that what he was saying was not how bars should be calculated, but how they are calculated.

I had the same experience when working in Johannesburg, South Africa, where the atmospheric pressure is about 84 kPa(abs). People do just add or subract 1.0 to the value in bar, even though the factor should be 0.84

Glad you picked up the design pressure for your vessel before it was manufactured. It is when things like that happen that you realise the importance of engineering checking.

regards
Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JohnGP]

Harvey,
Yes, I should have digested the whole lot fully before jumping in. I've been guilty in former lives of adding 1 bar (or mostly 100kPa) in compression calculations, but then most of that work was at or near "sea level", so atmospheric pressure was not much different to 100kPa(abs).

David, pardon me for being anal this time, but in your example I see 4.6 compression ratios, rather than 4.0, in the "rounded up to 1 Bar atmospheric pressure" case ((3.0+1.0)/(-.137+1.0)), but I take your point. Obviously the lower the value of atmospheric pressure, then the greater the inaccuracy in taking the "sea level" value. I don't normally work in bars, but then that's not what they're meant for.

 

[rcooper]

This whole thread has confused me. I had always assumed the gauge pressure is the pressure you would read from a manometer with one end open to the local atmosphere and the other connected to your pressure source.

Absolute pressure is the pressure measured in a manometer when one end of the manometer is sealed and a vacuum exists beneath this seal, and the other end of the manometer is connected to the pressure source.

There seems to be some concern about the exact correlation between the two. Surely this depends upon the weather, as the local atmospheric pressure can change by significant amounts - 0.1 to 0.2 bar. (It is a change in pressure so can be either bar(a) or bar(g))

 

[katmar]

rcooper, your definitions of gauge and absolute pressures are correct. Fortunately we do not get anything like the swings in atmospheric pressure that you referred to.

If the atmospheric pressure did swing by 0.2 bar the boiling point of water would vary from 93 C to 105 C. This would play havoc with the process industry. I do not track the weather myself, but I would be surprised if the pressure at a specific location varies by more than 0.01 bar (10 millibar). Perhaps some geographers in the forum can correct this?


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rcooper]

Katmar,

According to the UK Met Office, typical UK pressures are 950mbar to 1040mbar, giving a difference of 90mbar. Record high and lows in the UK are 925mbar to 1054mbar giving a difference of 129mbar. World record high and lows are 1083mbar and 870mbar, giving a difference of 213mbar.

 

[unclesyd]

After a table of SAPs'
This "standard pressure" is a purely arbitrary representative value for pressure at sea level, and real atmospheric pressures vary from place to place and moment to moment everywhere in the world.

http://en.wikipedia.org/wiki/Barometric_pressure

Even the standard changes.
Standard Conditions for Gases
Temperature, 273.15 K (°C) and pressure of 10^5 pascals. IUPAC recommends that the former use of the pressure of 1 atm as standard pressure (equivalent to 1.01325 ' 10^5 Pa) should be discontinued.
See STP.
1990, 62, 2216

 

[vehazle]

So how does kg/cm2g (or kg/cm2abs) relate?

Sorry, I couldn't resist. I mostly work in either bar or psi but just about the time I forget the conversions for kg/cm2, a project from Asia (mostly Japan) comes along and throws me back a bit.

 

[JohnGP]

vehazle,

Numerically, kg/cm2 are close to bar, i.e. 1kg/cm2=0.980665bar approx.

The odd looking conversion comes about through the involvement of the gravitational constant in converting from kilograms force to Newtons, 1kgf=9.80665N (so 1kg/cm2=9.81N/cm2=98066.5N/m2=0.980665bar).

Cheers,
John