Pressure gauge readings at different elevations 1

[BronYrAur]

I know the answer to this question might depend on the type of gauge, but let's just assume I am talking about an "off the shelf," "cheap-charlie," dial-type pressure gauge that would be on a low pressure HVAC steam system.

Is such a gauge calibrated to read 0 psi only at one elevation (e.g. sea level), or does the 0 psi remain regardless of elevation? In other words, if I take a 10 psig reading at sea level, the steam is at 24.7 psia. But if I take that same gauge to Denver and read 10 psig, do I still have 24.7 psia or do I only have 10 psi plus Denver's atmospheric pressure (12.0 psi), or 22.0 psia?

Thanks for your help.

 

[Latexman]

This . . . "10 psi plus Denver's atmospheric pressure ".

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

To expand, all Bourdon Tube Gauges (i.e., Cheap Charlie pressure gauges) indicate a differential pressure relative to current local atmospheric pressure. If the gauge at rest reads zero in LA, it will still be zero in Denver if the gauge didn't get damaged in shipping. Your 24.7 psia in LA becomes 22.0 psia in Denver.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[BronYrAur]

Latexman and zdas04,

You both confidently answered my question, and my first instinct was the same. However, what would cause the gauge to "automatically' re-zero itself at different altitudes? From what I can tell from this video and others, external pressure doesn't affect the gauge. Only the pressure applied at the inlet to the gauge affects motion of the inner tube.

If that is true, wouldn't a gauge that is reading 0 psig at sea level actually be reading in a slight vacuum in Denver due to the reduced atmospheric pressure acting on the inlet. The needle stop prevents a vacuum from showing, but wouldn't it take some artificial pressure at the inlet to restore the gauge to its actual zero psig point? Then if that is true, the actual pressure in the pipe is less than the reading on the gauge.

https://www.youtube.com/watch?v=Ja_XCJAg_l8

 

[zdas04]

A Bourdon tube gauge simply shows the dP between imposed pressure and local atmospheric pressure. That is it. If local atmospheric pressure is 14.5 or 11.0 psia then if the end of the tube is open to atmosphere then it should read zero.

If you calibrated a gauge in LA (0 psig = 14.7 psia) and then sealed the end of the tube with it reading zero, when it got to Denver (0 psig =12 psia), it would read 2.7 psig. If you unsealed the tube in Denver and then resealed it and shipped it to LA it would read -2.7 psig out of the box.

Nothing in the video contradicts this.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

From what I can tell from this video and others, external pressure doesn't affect the gauge. Only the pressure applied at the inlet to the gauge affects motion of the inner tube.

This is NOT TRUE for a simple bourdon pressure guage. As Zdas04 and Latexman state the pressure you measure is, as it says, "guage" pressure". This means pressure relative to the local air pressure at the guage. So it doesn't "re-zero", it simply stays at zero as therie is no differential pressure if the inlet is open to the air.

The ONLY pressure transmitter which seals in a near vacuum and uses that for measurement are pressure transmitters which measure absolute pressure. In that instance the pressure measured at sealevel would be 14.7 psia and in Denver 12.0 psia.

Think of this like an empty plastic bottle on an airplane. Seal it at take off and then open it at crusing altitude and you get a hiss as the excess air pressure escapes (sealing your guage inlet) Leave it open and nothing happens - the same volume of air, but a slightly lower pressure/mass. Ditto reverse seal it when the plane gets to cruising and when you land your bottle will be partly crushed, but leave it open and nothing happens.

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