×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# formula for pressure to volume

## formula for pressure to volume

(OP)
I'm trying to find a formula that will determine what the volume of air is at varying pressures.
example:  1 cubic ft. of air at 100 psi
What would the volume be at 75 psi
at 50 psi etc.
Is there a formula that would make it easier to determine this??
Replies continue below

### RE: formula for pressure to volume

For air at ambient temperatures, you can assume it to be an ideal gas (don't go over a couple hundred psi if the results are critical with this assumption or go to very low temperatures).

P1/P2 = V2/V1 where P1 and P2 are the ABSOLUTE pressures and V1 and V2 are the volumes.  To convert from psig to absolute pressure, add the atmospheric pressure (14.7 psiA at sea level) to the gauge pressure.  For example, 100 psig would become 114.7 psiA at sea level.

Doubling the absolute pressure will half the volume and so on.

### RE: formula for pressure to volume

Here's a table out of Spirax Sarco's "Compressed Air - Practical Study". For example, if you had a compressor delivering 200 cubic feet per minute of free air, then at the compressor discharge pressure of (for example) 100 PSIG, this would be 200 / 7.8 (from the table below) = 25.64 cubic feet. The ratios are determined using absolute pressures. At 100 PSIG, or 114.69 / 14.69 = 7.8.

PSIG  Ration of Compression

10    1.68
20    2.36
30    3.04
40    3.72
50    4.40
60    5.08
70    5.76
80    6.44
90    7.12
100   7.8
110   8.48
120   9.16
130   9.84
140   10.52
150   11.2
200   14.6

### RE: formula for pressure to volume

(OP)
Thank you very much, your help is greatly appreciated.

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!