unit for partial pressure 1

[Chumpes]

Hello
This is quite nothing, but people around me keep on using the unit "bara" for describing partial pressures.

Even if partial pressure of A can be seen as (mol fraction of A) x (total absolute pressure), i find that expressing partial pressure in bara is not relevant because partial pressure must be expressed in bar. For example, nobody would have the idea of expressing partial pressure in barg !!
In quite the same way, pressure drops have to be expressed in bar, not bara or barg.

Do you find relevant expressing partial pressures in "bara" unit ? isn't the unit "bar" sufficient for expression of partial pressures ? why would people add an "a" to bar to point out that absolute pressure SHOULD be used, when absolute pressure MUST always be used for partial pressures ?

remark : bara, barg and bar can be replaced by psia, psig and psi in the previous text.

Regards

 

[LittleInch]

I'll bet the people around you are flow assurance Engineers. They are the only ones who habitually use bara round me.

However with gas, if you have a total pressure greater than atmospheric, partial pressure could be higher than 1 bara, I think.

Anyway, it's not a bad thing to get it right as "bar" is habitually thought of as barg so if you mean bara then just say it.

You are correct though in differential pressure - it should always be simply bar, not barg or bara.

IMHO

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

I hate bara, barg, and bar as I prefer psia, psig, and psi.

 

[georgeverghese]

Partial pressure units should be in absolute values not guage.

 

[Chumpes]

thank you LittleInch for your answer, i had known barg only few time ago when i had to deal with mechanics and construction of pressure vessels. from my side i usualy see "bar" as being "bara" and not "barg" =), maybe because thermodynamic, chemistry and science in general (and quality assurance =) ? exception made for mechanics) does not deal with gage pressure.

@georgeverghese : your point is understood, but what is your practice for unit of partial pressures ? bar or bara ?

another analogy is that composition pourcentage for condensed phases is (always?) expressed in %w, thus % only is sufficient for expressing weight concentrations in condensed phases instead of %w.
in the same way for gases, composition is (always?) given in %mol (or %vol), thus % only is sufficient for expressing concentrations in gases instead of %...

@Chicopee : your preference shows you're probably american, do you express partial pressure in psi or psia (please don't use psig for that =) ?

bye

 

[Compositepro]

In any communication it is always good to be as unambiguous as possible. Always use bara or barg except for differential pressure. It is sloppy practice to say lbs. when you mean psi, or degrees without specifying F, C, K, or R. Same for bar. Do not make your audience guess what you mean. You will also find that your thinking process gets more precise as you learn to speak and write more precisely.

 

[georgeverghese]

Partial pressures can only be expressed in abs pressure units, bara or psia or kPaa etc.

 

[Chumpes]

thanks for your answers

Always use bara or barg except for differential pressure. [...] You will also find that your thinking process gets more precise as you learn to speak and write more precisely

I am glad that using Paa unit for partial pressure helped Compositepro having more precise thinking processes, but i will still never use gauge pressure for partial pressures
Expressing Pi . V = ni . R . T with Pi the partial pressure of i in Paa unit still looks odd to me =) do you have any accademic reference where the Paa (or psia or bara) unit is used for partial pressure ? every valuable references i've consulted use SI units for pressures, i.e. Pa
thanks

 

[Compositepro]

Academic papers are required to use SI units, so you will see 6 mm used to describe 1/4" tubing. In industry you encounter traditional units, like barrels, which may have different definitions is different industries. What is your concern? You want to save typing an extra letter? Most sentences can still be understood if you leave out all the vowels or if you scramble the order of letters in words after the first letter.

 

[georgeverghese]

When pressure in in Pa, R is 8314J/kgmole/degK. When pressure is in kPa, R is 8.314kJ/kgmole/degK.