PSIA vs PSIG

[miller863]

Please help me out here.... I have a pressure controller and vacuum pump and I can pull 0.008 PSIA on my system. However, when I switch the display to read PSIG, it reads -14.0796 PSIG. If -14.7 PSIG is an absolute vacuum, why wouldn't the display (In PSIG mode) read -14.7 - 0.008 = -14.692 PSIG?

I'm pulling my hair out here trying to understand the logic. The controller shown is a Mensor CPC6050, but I have confirmed this same thing occurs on a Fluke 6270A.

 

[JStephen]

The absolute air pressure at standard conditions at sea level is 14.7 psi, and the -14.7 comes from measuring from this reference point.
If you're not at sea level, air pressure is different, usually less.
If you are at sea level, there will be some variation depending on weather conditions, etc.- what a barometer is measuring.
I would assume those are the source of the difference.

 

[rb1957]

absolute pressure is, as it says, absolute ... measured from a datum of 0psi.

gauge pressure is measured with respect to a datum, either 1 standard atmosphere (14.7psi) or local outside atmosphere pressure.
I suspect it is the former, the latter sounds like "differential pressure"

maybe this datum is shown by the "setpoint" ?

so -14.08psig is (I'd've thought) 0.62 psia. So the two screen shots are not the same test case ?

another day in paradise, or is paradise one day closer ?

 

[BrianE22]

Do you know what altitude you are at? I found the following:

https://www.mide.com/air-pressure-at-altitude-calculator

You'd also want to look at the temperature dependence.

 

[JR97]

miller863,

the other posts are correct. psig is the absolute pressure offset by the atmospheric pressure which is 14.7 psi at sea level and decreases with increasing altitude. I imagine somewhere in the instrument menu structure there will be an input for atmospheric pressure (it is currently set at 14.087). You can find maps/formulas of local pressure at various places on line. Depending how accurate you need to get you may have to factor in barometric pressure based ont he current weather as well.

JR97

 

[LittleInch]

So your display thinks "normal" 0.00 psig is 14.0876 psia.

Now you need to find out why it thinks that?

Time to read the manual...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

What did the instrument measure at atmospheric pressure?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[miller863]

Thanks for your answers.

To address rb1957's question ("So the two screen shots are not the same test case?"): Actually, the two screenshots are exactly the same test case. I just pressed the "Pressure Type button" to toggle between absolute and gauge pressure modes on the pressure controller, with the same setpoint. I started at PSIA, with a setpoint of 0 PSIA. The 0 PSIA setpoint was replaced by the -14.6959 PSIG setpoint, populated automatically by the controller, which is exactly equal to negative 1 atmosphere. Both screen shots are taken within moments of each other, with no change to the setpoint or unit under test.

I didn't grab the barometric pressure at the moment I took the screenshots. Roughly an hour later it is 29.74 inHg or 14.6069 PSI as reported by localconditions.com. Turns out it is already accounted for (see last paragraph).

Most of you already know the following, but I'll state it anyhow: PSI Gauge is a relative measurement. The gauge applied pressure value (ie. the vacuum by way of the pressure controller in this case) is referenced to the existing room pressure at that moment in time. The vacuum pump has the capability to pull a certain amount of negative pressure (vacuum) relative to room pressure (it's reference). Absolute pressure on the other hand is referenced to an absolute vacuum. Nobody can pull an absolute/perfect vacuum, so the reference sensors that are used in these types of controllers are adjusted by interpolation to the amount of vacuum that can be achieved by their equipment. My particular vacuum pump (Leybold) can pull to <0.002 mbar (<0.0290 PSI).

Having stated the obvious, here's my confusion: If my pump can pull within 0.029 PSI of an ideal vacuum, since I started in absolute mode, I would have expected that, in a system with zero leaks, I could pull to 0.029 PSIA. Actually the pump performed even better and pulled to 0.008 PSIA. When switching to gauge pressure, I would have expected that I would see -14.6959 + 0.029 = -14.667 PSIG with respect to room pressure.

So I DID find out since my original post that the controller was in "gauge emulation mode" (indicated by the lighter background behind "gauge"). The manual states: "In the gauge emulation mode the atmospheric pressure reading from the barometric reference transducer is subtracted from the absolute pressure reading of the channel to emulate a gauge pressure". That calculation is then 0.080-14.6069 = -14.5269. But instead, it displays -14.0796 per the screenshot, not even close. The logic behind this still eludes me.

 

[Compositepro]

You have two sensors. Each sensor has a rated accuracy and precision. In your case both sensors are absolute pressure sensors and one of them used to measure barometric pressure. Both sensors are designed to measure a full range of one bar, and the accuracy is a percentage of full range. Your gauge emulation subtracts one sensor measurement from the other sensor measurement, so the absolute error is doubled (not percent error, which is based on full range).

You are concerned about a discrepancy of one part in 1400 and say it is not even close. If a fan turns on in the room, the pressure change will be ten times that.

 

[rb1957]

I agree, it looks "odd" ...

it seems to be saying 0.08 psia, local atmosphere 14.6959psi, and -14.0796 psig ... which would be equal to 0.6163psia ??


another day in paradise, or is paradise one day closer ?