measurement in psig vs psia

To put some perspective on your question, what level of accuracy do you require? What is the pressure range you are recording?

I would say that +/- 1 psi would be acceptable. Especially because the sensors are electronic and they vary by about 0.3 psi at any given time. The range is from 100-150 psi.

Why are you measuring the pressure?

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...

Generally day to day atmospheric pressure changes are not substantial and these need no correction in a measuring paramenter. If you are using your test equipment at various altitudes then you require atmospheric correction.

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)

I never accept a gauge or instrument "calibrated" to psia. What has been done is the "zero" has been supposedly shifted to local atmospheric. At least half the time I've checked, I've found that the "local atmospheric" used was 14.7 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
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

Tom
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.

David,

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?

It is because zero psig is so easy to verify. Take the applied pressure off of an instrument and if it doesn't go to zero then it is wrong. I live in the mountains and it is really hard to see if the "zero" is 11.6 like it should be at a particular station or 14.7 which it shouldn't be, but it can make a difference in the volume calculated (especially if the operating pressure is under 30 psig and the instrument is 0-500, which is still common in spite of all my complaining).

David

If measuring pressure around 150 psig then an atmospheric gauge pressure instrument is fine. If the pressure were much lower or included vacuum then absolute pressure is the way to go. Check with your Rosemount salesman
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

Use psig because reference pressure on pressure instruments is typically current athmospheric pressure. Even compound gauges have athmospheric pressure as reference. For a gauge to trully read psia, it must have absolute vacuum for reference.

Is it not so that in gas applications one wants to work with gage pressure as that is the difference as opposed to the base level pressure dominating on site i.e. atmospheric pressure? e.g. if you want to fill a vessel you will always have to start from atmospheric and all pressuree vessels must be calculated relative to the atmospheric pressure, hence in gage pressure.

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

We have two different questions here: first was the question about gauges being "calibrated" to pisa, and second was should you do arithmetic in psia or psig. The second question is silly -- except for some empirical equations you do arithmetic in psia. I get psia by measuring psig and adding the local atmospheric pressure. I know that is the way my grandfather did it, but that don't make it wrong.

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

zdas04 may be correct for atmospheric gauges. This is not how absolute pressure transmitters are factory calibrated. With the intellegent transmitters you rarely ever calibrate but only check the calibration or perhaps re-range if required. Absolute pressure transmitters are ABSOLUTE pressure transmitters based upon a vaccum not zero shifted.

John

It is the available NPSH that would be critical and is based on: the absolute pressure; the vapor pressure of the fluid pumped at the fluid temperature at suction; static head at pump suction and suction pipe losses including entrance loss and friction loss through pipe, valves & fittings.

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.

As jsummerfield noted, an absolute pressure transducer most commanly has a sealed referenced chamber that makes for a true PSIA reading. The reference chamber is most often a vacuum on the back side of the measuring diaphragm and should not be affected by ambient conditions within the accuracy of the unit.

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.