pressure - Pa vs. Psig vs psia

Question

pressure - Pa vs. Psig vs psia

cougarfan (Mechanical)

(OP)

11 Jun 02 16:10

I have a question and need some help on this - I have a vessel and is rises to 25 psig - when I find the vapor pressures for the particular chemical - do I add 14.7 first then multiply by 6894 to get Pa so I can read my correspnding temperature..... I am confused!!!!!!!!!

grt12 (Chemical) · Answer 1 · 2002-06-11 19:13:33

You should get the same answer either way as the 6894 applies to the 14.7 as much as the 25. According to my unit converter, 25 psig = 273721.8 Pa (absolute)172390 Pa (gauge) - a lot depends on your pressure base as to the exact answer to the above. Suffice to say, converting either way you need to convert both the gauge pressure and the atmospheric pressure with the same constant.

cougarfan (Mechanical) · Answer 2 · 2002-06-12 08:03:08

Okay, let me ask again.... I have a pressure vessel which has some chemical in it... I want to find the corresponding temperature - so I go to the chart of vapor pressure (Pa)vs. temperature. Now, I want to find the corresponding Temperature for say 25 psig - I first neeed to convert this to Pa - do I add 14.7 first and then multiply by the factor to get Pa or just multifply by the factor - I will get different answers...... HELP

jproj (Chemical) · Answer 3 · 2002-06-12 11:23:51

dideyjohn:

25 psig = 172368.932 Pa(gauge) = 273721.865 Pa (absolute)

25 psig = 39.7 psia = 273721.865 Pa (absolute)

To make my point, you can either convert psig to Pa (g)(multiply by 6894.76) then convert Pa(g) to Pa (a) (by adding 101352.9) -or- you can convert psig to psia (by adding 14.7) then convert psia to Pa (a) (by multiplying by 6894.76). Both give the same result.

(Note: 14.7*6894.76 = 101352.93)

Good Luck

jproj

P.S. Keep those units straight.

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pressure - Pa vs. Psig vs psia