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

 

[msquared48]

The elevation of my beers change real fast!

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

 

[Compositepro]

Barometric pressure changes constantly. A barometer is simply an absolute pressure gauge. An altimeter is also an absolute pressure gauge, but it must be adjusted to current local barometric pressure in order to read correctly.

 

[zdas04]

I've done projects where current barometric pressure was material to the answer, but they have been exceedingly rare. The spread between very high barometric pressure and very low barometric pressure is around 4 mmHg (0.0773 psi) and mostly an analysis works out just fine in the tenths of psi.

I've seen people try to use an atmospheric pressure to 4 decimal places in gas measurement, but they don't get materially different answers if they stop at one decimal place. The precision of the calculation just doesn't support the extra digits. I've never seen anyone try to adjust atmospheric pressure in gas measurement to account for changes in barometric pressure or atmospheric temperature, and I would probably object to someone trying. For most industrial applications that I'm familiar with adjusting atmospheric pressure hour to hour would lead to more law suits than improved analysis.

David

 

[Compositepro]

"World records vary from a high pressure of 32.0 inHg in Siberia to 25.7 inHg during a typhoon (both readings are off the scale of most barometers). For the US, extreme levels can be considered as 30.5 inHg and 28.5 inHg."

 

[waross]

If a refrigeration man living and working at an elevation of 2000 feet does not offset his compound gauge to compensate for the elevation, he may easily be over one degree F off when setting his superheat, and some may consider a one degree error on a ten degree setting to be excessive. On the other hand I have seen 20+ degree F errors in superheat setting so some may not care.

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

 

[BigInch]

A bar is a bar like a psi is a psi. The difference is noted between 1 atm = 14.7 lbf/in2 = 29.92 in Hg = 1.01 Bar

David, what I want to know is why do you spell insid like "inside", but outside like "out side" and would you write "into" or "in to"?

This is where it really makes a difference.

http://academy.ivao.aero/node/339

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[zdas04]

I don't think there is any plan to my spelling.

David

 

[JStephen]

You know, I don't ever recall hearing of a bar as a pressure unit until a few years ago. In school, everything was in SI units or US standard units, so it was either Pascals or multiples thereof or PSI. And the few exceptions I've seen at work have been kgf/cm^2, not bar.

 

[SomptingGuy]

"bar" is hugely common in the world of IC engines and associated fuel equipment (at least outside of USA). Cylinder pressures, BMEPs, injection pressures are usually described using bar.

- Steve

 

[zdas04]

30 years ago "bar" was pretty well limited to scientific applications (used as a more reasonable surrogate for "atm" because it had a precise definition and atm seemed to depend on local atmospheric pressure). In industrial applications that were eventually forced into non-U.S. units (I hesitate to say "SI" because nobody like Pa and kPa or MPa aren't much better) it seems to be limited particular industries (like IC engines, gas compression, and Oil & Gas).

For the last 20 years or so I've seen an increased use of "bar(g)", but no consistency about how to convert to bar(a) (that is why I started this thread if you'll recall). I still see a bunch of pressure gauges with a dual scale in psi and kPa (and every time I work with someone using that dual scale they talk in psi).

kg/cm^2 makes me laugh very hard. I've gotten grief from metrified people forever for the "slug" (which no one likes) or "g_c". SI purists point to g_c as the most evil of concepts. Well ... to get from kg to kgf, you need a g_c of your own. It equals 9.81 kgf*m/s^2/kg. A whole bunch of equations are going to need that particular conversion factor and my bet is that the metric guys will have exactly the same difficulty using it as the FPS guys have. Welcome to my world.

David

 

[BigInch]

No 1 (Std) Atm = 14.7 psia ... not a Bar, but 1.01 Bars

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[zdas04]

I have three references that say "atm" is equal to local atmospheric pressure. They all agree that a standard "atm" has a definition (most of them claim it is 14.696 psia, but one said 14.73 psia).

My college Chemistry teacher (in Fayetteville, Arkansas, elevation 1,400 ft, atmospheric pressure 14.0 psia) made a big deal about this in the 1970's. He said that if you did an experiment at Fayetteville and calculated "atm" as 14.696 psia, and did it again in LA with the same "atm" you would get different answers (and not JUST because all things are strange in California). I never tried it, I try to avoid equations with "atm", but some Chemistry stuff seems to require them.

They also say that the "atm" used in equations is physical, not "standard". If an equation uses "atm" it should be assumed to be a multiple of local atmospheric pressure and that if a Bourdon tube reads zero then the pressure for calculations is 1.0 atm. Hence the confusion.

David

 

[BigInch]

What's your Uconner reference say?

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[zdas04]

It has the standard value for atm. So?

David

 

[BigInch]

14.7


Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[zdas04]

Yeah, so?

 

[BigInch]

I give.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[zdas04]

Regardless of its magnitude, it is a confusing and stupid unit of measurement that is improperly used much more often than it is properly used.

David