Pressure in the Bar

[zdas04]

I had a discussion yesterday that makes me scratch my head.

When I was in university, my Chemistry book (copyright 1971) had an introductory section on units that clearly said that the pressure unit "bar" could only start at absolute zero. That a gauge labeled in "bar" should be at rest somewhere in the general area of "1.0", but the actual value would vary with elevation.

The guy I was talking to yesterday said that 145 psig, was 10 bar(g) and 11 bar(a) regardless of elevation.

I live in the Rockies and my atmospheric pressure is 12 psia. I know that:
145 psig = 999.7 kPa(g)

And I think I know
157 psia = 1082.5 kPa = 10.82 bar(a) (at least that is what Uconeer tells me)

But this guy was contending that it would be "1 bar higher than gauge pressure or 11 bar(a)". So I asked him how many compression ratios would it be to go from 0 bar(g) to 10 bar(g) at my current elevation. He said "11" without hesitation. The correct answer is 13.08 in psi, kPa, or the "bar" in my old Chemistry Book.

My question is "how could something physical get this screwed up in only 40 years"?

David

 

[MintJulep]

My recollection agrees with yours that bar should only be absolute.

Some months ago I was reviewing a submittal that used bar as gauge pressure.

I was going to slam the vendor for this, but I couldn't find any substantiation for my recollection in any of the texts that I have on hand.

 

[stevenal]

http://www.malcleanse.co.uk/lg_images/Pressure_Gauge_BE400_GG2SB400.jpg

A gage can only measure a pressure difference, so unless this is installed in a perfect vacuum chamber it is using ambient atmospheric pressure as a reference. Looks like bar(g) to me.

 

[zdas04]

I can put what ever label I want to on a gauge face. A Bourdon tube is at rest when the pressure inside of it is equalized with the pressure out side of it. Now, I can take a gauge at rest and have the needle point to whatever value that I want on the gauge face. That can be 1 Bar, it can be 0.8 bar, it can be zero bar (and I can call that barg if I want).

The confusion comes from being able to predict what a gauge calibrated in deep space to read zero bar would read in LA, CA; Bogota, Columbia; or Farmington, NM. Physically it would read 1.02 bar, 0.7 bar, and 0.8 bar respectively, but do the people using the unit know that? The guy I talked to yesterday doesn't. How common is stupid?

David

 

[IRstuff]

see:

http://en.wikipedia.org/wiki/Bar_(unit)

The usage of bara and barg is now deprecated, with qualification of the physical property being preferred, e.g., "The gauge pressure is 2.3 bar; the absolute pressure is 3.3 bar".[2]

...

2. British Standard BS 350:2004 Conversion Factors for Units

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MintJulep]

Perhaps more the one or both of us would care to admit.

I've seen you use "compression ratios" in previous posts. I've never used it calculations in my years of work with compressed air, preferring to work things out with absolute pressures with actual units.

Of course I thought I knew what "compression ratio" is, and confirmed by my circa 1939 copy of "Compressed air data" it is the "Ratio of absolute final pressure to absolute initial pressure".

So I asked him how many compression ratios would it be to go from 0 bar(g) to 10 bar(g) at my current elevation. He said "11" without hesitation. The correct answer is 13.08 in psi, kPa, or the "bar" in my old Chemistry Book

For convince, at STP:

0 bar(g) = 0 + 14.7 = 14.7 psia
10 bar(g) = 145 + 14.7 = 159.7 psia.

159.7/14.7 = 10.86 on my calculator. (pretty close to 11)

How does your old chemistry book arrive at 13.08?

 

[waross]

I remember seeing a spec for a DP cell with one side sealed so as to be suitable for absolute pressure measurements.
I don't follow the "ratio" argument. You are about 2.7 PSI below standard atmospheric pressure. I would expect most pressure gauges to read 2.7 PSI high.
Rather than basing your ratio on your atmospheric pressure of 12 PSIA I am more comfortable basing the ratio on standard atmospheric pressure and then using an offset of the difference between standard atmospheric pressure and your pressure. Then the ratio does not change as the measured pressure changes.
You don't need an absolute vacuum to measure absolute pressure. You just need a DP cell with one side sealed from the atmosphere.
Think Barometer.
I used to use compound refrigeration gauges. (Both pressure and vacuum).
They had an adjustment screw so that you could offset the zero to compensate for your elevation, and if you were real fussy the barometric pressure that day.
But I agree with you that atmospheric pressure must be considered when using converting from "absolute" units to "gauge" units.
By the way, are you using actual average atmospheric pressure at sea level or defined standard atmospheric pressure at an elevation of about 100 meters? grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

I think that defining bar as only an absolute pressure is pointless. In that case the following statement is incorrect. The difference between 10 bar and 12 bar is 2 bar.

Every pressure measurement is relative to a reference. It should always be made clear what that reference is. Using "a" and "g" or writing it out is unambiguous.

 

[zdas04]

MJ,
1 bar does not equal 14.7 psia anywhere. 1 bar is 14.5038 psi. "1 atm" is defined by some as "14.6959 psia" (see Uconeer), but a bar is 14.5 psia.

It is not my old Chemistry book, it is physical. When I said "the bar in my old Chemistry book" I meant "with 0 bar(g) = 0.8 bar(a)" at my elevation.

145 psig = 157 psia (at 12 psia atmospheric pressure)
0 psig = 12 psia (at my elevation)

157/12=13.08333333333333333

If I was at sea level then
159.7/14.7= 10.8

Fundamentally every calculation I do with regard to compression has a compression-ratio term in it somewhere. If I'm compressing natural gas (k-1.31) from zero psig and 80F to 145 psig in a single step (say I'm using a water cooled cylinder on a recip) then at sea level the discharge temp would be 488F and at 5400 ft elevation it would be 532F--if the piping was rated for 500F that would be material.

I initially found it amazing that you had spent a whole career that included air-compressors and you never had to do any calculations that involve compression ratios. Then I realized that those calculations have a rigidly fixed suction pressure (minus inlet losses) and that industry has developed a whole lexicon of equations, graphs, and nomographs that assume a fixed (and known) suction pressure and a single "k" factor. In natural gas compression neither of those things are even close.

Waross,
When you take a gauge out of the box at sea level or at 12,000 ft elevation, it reads zero. It is the "at rest" reading. That is what "gauge pressure" is. That is why in any non-empirical equation you can't do calculations with psig, kPa(g), or bar(g).

When you are doing calculations with pressures you must reference them back to a theoretical absolute zero. So if I do compressor calculations based on some mythical "standard" then I will get wrong answers. I hate predicting that a compressor discharge will be under 500F and finding it over 530F. That is what happens when you try to superimpose an arbitrary "standard" on physics.

"Standard" is a whole can of worms all by itself. There is no such thing as a standard. NIST would have you believe it is 14.696 psia. DOE would say 14.73 psia. The state of New Mexico (and Louisiana) would say 15.025 psia. Each contract will say whatever it says (I've seen numbers between 14.5 and 15.7 psia).

Compositepro,
If every calculation we do with pressure could be done on an abacus, then you would be absolutely right. The difference between 12.8 bara and 10.8 bara is exactly the same number as the difference between 12 bar(g) and 10 bar(g). But if I divide 10 bar(g) by 500K I get a different number than if I call the starting point 11 bara or 10.8 bara.

I couldn't agree more that specifying psi, kPa, or (now) bar is lazy, sloppy, and ambiguous. When I was in school "bar" (by definition) could only be bar(a). Today that is not true.

How often is this material? For pressures under 50 psia it is always material. For pressures over 150 psia it is rarely material. The vast majority of my work is below 50 psia.

Guys,
My reason for starting this thread was to discuss where the heck "zero" should be on a "bar" scale. I know that I get the right answer to engineering calculations if it is relative to local atmospheric pressure (i.e., 0 bar(g) = 0.8 bar(a) at my elevation). If it is relative to the "bar" scale (i.e., a gauge at rest would read 0.2 bar(g) at my elevation) then all of the arithmetic is wrong. Bar(a) does not equal bar(a)+ 1bar except at those limited locations where atmospheric pressure is 14.5038 psia.

David

 

[MintJulep]

David.

1 bar does not equal 14.7 psia anywhere. 1 bar is 14.5038 psi

Absolutely correct. Did I write something different?

An increment of 1 bar = an increment of 14.7 psi.

But 0 bar(g) = 0 psig

So imagining a gage (referencing atmosphere) with dual scales (bar and psi):

0 is 0

1 Bar = 14.5038 psi (if you can read the graduations that closely)

10 Bar = 145.038 psi

100 Bar = 1450.38 psi

So converting to psia:

0 Bar(g) = 0 psig, add 14.7 psia (STP sea level atmospheric pressure)to convert psig to psia gets us to 14.7 psia.

If we want to go back to Bar(a) then 14.7/14.5083 = 1.01 Bar(a)

Right?

 

[msquared48]

David:

Ask the bartender what he thinks.

As a point of discussion, in your initial definition of "bar", the reference was made to absolute zero. I assume this means
-273K?

And Mintjulip referred to the same definition using STP, which would be totally different.

Seems like people are talking apples and oranges here, but what do I know.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[zdas04]

No, by "absolute zero" I was refering to a pressure of zero psia. A purist would tell you you cannot actually evacuate a gauge that far (there will always be a few molecules that don't want to leave). So it is pretty theoretical. I find that if I include elevation and ignore variations in barametric pressure I get answers that are better than the precision of the equations I'm using.

MJ,
I'm not sure how you get to an increment of 1 bar being 14.7 psia. If I use 0=0, then 1 bar(g)=14.5 psig, 1 bar(g)=29 psig, etc. The increment is 14.5 psi not 14.7 psi. The rest of your post used 1 bar=14.5 psi.

I was with you till the STP discussion. If I add 14.7 to a gauge pressure I get a wrong answer by 2.7 psi. For most of what I do that is not a small error (I have a number of compressors that have a suction pressure of 2-3 psia, calling that 4.7-5.7 would get a hp calc to be low by a factor of about two (a job that really requires 300 hp would look like it only needs 150 hp).

David

 

[MintJulep]

I'm not sure how you get to an increment of 1 bar being 14.7 psia

I got there by typo in my earlier post.

Ok, increment of 1 bar = increment of 14.5 psi.

Now, to clarify the STP discussion.

To convert from gage pressure to absolute pressure: Take the gage pressure and add the local atmospheric pressure.

At STP this would be 14.7 psi. At altitude it will be a smaller number.

And now I think we are on the same page.

 

[IRstuff]

"As a point of discussion, in your initial definition of "bar", the reference was made to absolute zero. I assume this means
-273K?"

No, absolute zero is, by definition, 0 kelvins. It might be -273.15 ºC.

In any case, it's also a bit theoretical, although people have gotten down arount 0.1 nK.


TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[stevenal]

"The difference between 12.8 bara and 10.8 bara is exactly the same number as the difference between 12 bar(g) and 10 bar(g)"

Exactly the point Compositepro was making. The answer in both cases is 2 bar with no a or g. To insist that bar can only refer to quantities referenced to absolute zero would make the unit meaningless if used for a delta quantity.


"To convert from gage pressure to absolute pressure: Take the gage pressure and add the local atmospheric pressure."

The above is valid if your gage is surrounded by atmosphere. As a sometimes diver, I know a more universal statement would be to add the local ambient pressure.

 

[waross]

Hi David. Now I understand the significance of the pressure ratio in your field. Thanks.
Re the gauge, After taking a manifold and gauge set out of the box but before using it, the needle may be adjusted off of zero to compensate for the elevation if it will be used above sea level.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[msquared48]

You're right IR. Thanks. Been a long time...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Compositepro]

The problem is you would have to adjust the zero every hour. I've seen things like this done, and it is simply a mis-calibration of an instrument so that it is no longer acurately reading anything. A pressure gauge does not measure absolute pressure (unless specifically designed to).

 

[Ron]

My head hurts. Is there a shear or moment in there anywhere? Who buys this round? I'll take a nice cold lager.

 

[waross]

The elevation never changed that fast.

Bill
--------------------
"Why not the best?"
Jimmy Carter