measurement in psig vs psia
measurement in psig vs psia
(OP)
Hello Everyone,
There is a debate at work over whether we should measure the pressure(inside a pipe) on a piece of test equipment using psia or psig. A coworker and myself say psig because we can zero the equipment before each use and then work from there. The boss and another engineer say that we should measure it in psia because it takes into account the changing atmospheric pressure. If we zero the equipment each time and measuring in gage pressure aren't we essentially accounting for the changing atmospheric pressure. Our current test equipment uses psig and we want the new equipment to match the old. The software for the new equipment reads and records all data in psia but it doesn't have a sub program that stores the atmospheric pressure and subtracts it so that we now have gage. Basically what I see wrong with using only psia is that if one day the atmospheric pressure is 14.7 and the other day it is 14.8 then if we use psia as our analysis data then we will get different results on the same part if the atmospheric pressure changes. I'm really long winded. :) Please come in and let me know what everyone thinks.
Thanks,
TheBigTomHanks
There is a debate at work over whether we should measure the pressure(inside a pipe) on a piece of test equipment using psia or psig. A coworker and myself say psig because we can zero the equipment before each use and then work from there. The boss and another engineer say that we should measure it in psia because it takes into account the changing atmospheric pressure. If we zero the equipment each time and measuring in gage pressure aren't we essentially accounting for the changing atmospheric pressure. Our current test equipment uses psig and we want the new equipment to match the old. The software for the new equipment reads and records all data in psia but it doesn't have a sub program that stores the atmospheric pressure and subtracts it so that we now have gage. Basically what I see wrong with using only psia is that if one day the atmospheric pressure is 14.7 and the other day it is 14.8 then if we use psia as our analysis data then we will get different results on the same part if the atmospheric pressure changes. I'm really long winded. :) Please come in and let me know what everyone thinks.
Thanks,
TheBigTomHanks





RE: measurement in psig vs psia
RE: measurement in psig vs psia
RE: measurement in psig vs psia
If you are doing a hydrotest or something of that nature, all you need is the gauge pressure, and there's no reason to measure or calculate the absolute pressure.
If you're doing any kind of chemical or physical analysis or metering of the pipe contents, then you may need the absolute pressure for that.
How are you measuring the atmospheric pressure, by the way?
Seems like I remember hearing way back in college that the pressure given on the weather reports is always corrected for sea level, not necessarily the actual pressure at your altitude...
RE: measurement in psig vs psia
Barometric pressure can be approximated by the following forumla P(inches Hg) = 29.92-0.001H where H is height of your place in feet above mean sea level(MSL). If you require 1 psi accuracy then you can use the same equipment with in 2000 ft altitude variation. (13.7*29.92/14.7 = 27.88 inches of Hg, 27.88 = 29.92-0.001H => H = (29.92-27.88)/0.001 = 2040ft)
RE: measurement in psig vs psia
One of the things I verify on a static test is that the chart starts at zero, moves during the test, and goes back to zero after the test. You'd be amazed how often they don't go back to zero.
Gas measurement equipment is required to be calibrated to psig. Those guys have more financial interest in pressure measurement than anyone else. Were I you, I'd go with the experts.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The Plural of "anecdote" is not "data"
RE: measurement in psig vs psia
Again what level of accuracy do you require. 1 or 2 psi. in a 100 gallon vessel (carbon steel) is not even calculated in most NIST labs since you will not see a change more than .01% Ex: A prover is calibrated to zero pressure correction @ 100 PSI. @ 50 psi. you'll have a nominal correction of -.05% or @ 150 psi a correction of app. + .05%. Example is for 100 gallon vessel.
RE: measurement in psig vs psia
Yes we are measuring a gas in this situation. Compressed air actually. Do you know why gas measurement equipment is required to be calibrated to psig? Is it because of money matters or some physical explaination or both?
RE: measurement in psig vs psia
David
RE: measurement in psig vs psia
zdas04 advises that he would not accept a gauge or instrument "calibrated" to psia as they are just zero shifted. This is not true for absolute pressure transmitters. BTW, pressure differental transmitters or the better differental pressure gauges could provide the atmospheric correction if required.
John
RE: measurement in psig vs psia
RE: measurement in psig vs psia
In a more controlled test environment I would ask myself the question why not measure in gage pressures and measure the atmospheric pressure at all times also?
Thanks.
Scalleke
RE: measurement in psig vs psia
My experience has been that every gauge or transducer that I've ever seen read out in psia has simply been zero shifted. This is thousands of instruments over the last 32 years. There may be some way to do it differently, but I haven't seen it yet (and I did check with Rosemount, when you cut through the slick literature you get a zero-shifted instrument).
David
RE: measurement in psig vs psia
John
RE: measurement in psig vs psia
All others can be in psig since the athmospheric pressure is present ultimately at both the suction & discharge end unless you are deadheading the pump or compressor, without a relief valve or diaphram or fluid surface that seas the atmospheric pressure.
RE: measurement in psig vs psia
A sealed gaged transducer measures in PSIS. You are never sure what you are getting with a PSIS unit unless you ask the vendor. Like the PSIA unit, a PSIS unit usually has a sealed reference chamber. However these are offset similar to the way zdas04 was mentioning. The output is usually set to zero somewhere around 14.7PSIA and the external pressure is measured relative to this reference.
The PSIS are usually used by people who really want to use a PSIG unit, but don't want to expose the internal reference side of the transducer to corrosive ambient conditions and don't want the added expensive or size of a wet/wet dfferential style unit.
It's also worth noting that some manufacture's will ignore the distinction on some higher range pressure units. That is: no matter what you order, PSIA PSIS or PSIG, you would probably get something between a PSIS and a PSIG unit depending on the housing. For some manufacturers and products this may start as low as 100PSI.