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Water Pressure vs air pressure

Water Pressure vs air pressure
by Charles S. Fox on 03/01/02 at 07:48:32

I work in the plumbing industry.   Oftentimes contractors test piping systems with air instead of water.  An air test of 100 psig will not hold while a water test of 100 psig will hold.  Can anyone tell me why?  I have been told that there is a 3:1 ratio of air pressure to water pressure, in other words a 100 psig air test would be equivalent to a 300 psig water test?  I need to know where I can find information to substantiate this.

Many thanks,
Charles S. Fox, CIPE

Re: Water Pressure vs air pressure
by Robert Fogt on 03/03/02 at 02:18:32

Possibly it is because air molecules are smaller than water molecules? ???  Your guess is as good as mine. :)

There is a message forum I use when I need information like this. The plumbing forum is here:

You should be able to receive an answer to your question there. It is an often used forum.

Re: Water Pressure vs air pressure
by coldfuse on 03/10/02 at 21:12:18

May I suggest the website "Understanding Pressure Testing"?  It is at:


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