Pressure Gauges 1

[slickstyles5]

Hi,

Would anyone know a manufacturer of pressure gauges in psia (absolute)?

I know of Winter's. Anyone else?

Thanks

Gabriel

 

[budt]

Go to this site

http://www.hydraulicspneumatics.com/Directory/Default.aspx

and look under "Gauges, Pressure and Flow, Hydraulic"


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[slickstyles5]

Thank you!

 

[zdas04]

NO BODY MANUFACTURES PRESSURE GAUGES IN ABSOLUTE!! (that is the first time I've ever used an exclamanation point on eng-tips.com).

You can calibrate any gauge to indicate psia, but the gauge is still "reading" the difference between local atmospheric and your process stream. The problem is that a gauge calibrated to read out in psia better never leave the altitude where it was calibrated.

Sea level is somewhere close to 15 psia. My house is 12.5 psia. Some nearby well sites are 11 psia. If I take your sea level gauge to a compressor (which is actually seeing 6 psia or -5 psig suction and 105 psia or 94 psig discharge for 17.5 compression ratios) it will show that I'm going from 10 psia to 109 psia which is 10.9 ratios--the real case is right at the edge of the machine capabilities and the indicated case with the wrong zero is in the middle of its capabilities.

This is one of those things that frequently crop up to bite people in the mountains.

David

 

[hydtools]

See WIKA gauges.

http://wika.com/WIKAWeb/Product/pdf/WT 1500 Series 8.5 Absolute.pdf

Ted

 

[unclesyd]

Checkout the line of Heise gauges.

http://www.heise.com/products.cfm?doc_id=31

 

[zdas04]

OK, maybe my rant should have been "no body should manufacture a pressure gauge in absolute". It is just a calibration shift, but it is dependent on where the gauge is sitting when you calibrate it and far too many users don't know what they're seeing.

David

 

[btrueblood]

Dave, I don't think I'm following you:

"no body should manufacture a pressure gauge in absolute".

Are you thinking of compound gages? I can't imagine an absolute reading gage that is sensitive to atmospheric pressure, but maybe there are some out there?

If the gage is built to reference a vacuum, and designed to not respond to atmospheric pressure variations, e.g. the Wika gages that hydtools posted a link to...where's the problem? Ideally, one would perform a calibration (or have the manufacturer do so) at two different ambient pressures, to verify that the gage is accurate over some range of same. In the aero world, we routinely used absolute transducers, whose diaphragms had one side evacuated and hermetically seal-welded. We never bothered to repeat the mfgr's. calibration, just verified that the output went to zero (or 4 mA) when we pulled a hard vacuum on the measurement port. Any leak of the vacuum would be readily apparent as a shift in the calibration, and be cause for rejection of the transducer.

 

[zdas04]

No, compound gauges measure from atmospheric pressure in both directions, no problem there.

The reason I'm so exorcised is once I got a compressor that had all the gauges zero-shifted to read psia in Houston (zero psig is about 14.8 psia at their shop). The compressor was set at 7,000 ft (atmospheric pressure 11.3 psia). Our target was to run this machine just above zero psig because we hadn't done anything to prevent air incursions, but the gauge said 15 psia at zero psig so people kept adjusting the suction pressure based on this stupid gauge and their knowledge that atmospheric pressure was around 11.5 psia. When the field got shut down due to oxygen in the delivery gas I threw the psia gauge off a cliff.

I spend most of my life working at sub 2 bar(a) and in that range small differences in pressure readings can be a big deal. Gauges that read out in psia can be a really big deal when they are not properly zeroed for the local conditions. This zero shift has more potential to do harm than it could ever be worth.

David

 

[btrueblood]

Um, the gage was indicating pressure in psig or psia? Sounds like it was reading in psig, but calibrated to 14.8 psi absolute as "zero", resulting in a shift at altitude? Or do I have it backwards.

In any case, your point is taken - make SURE you know how the gage was calibrated, and better yet, know its design well enough to know, or at least be able to guess, how it might be affected by changes in its environment.

 

[hydtools]

Sounds like zdas04's gage was a common psig gage that was zero-shifted to add local atmospheric pressure in an attempt to make it read like an absolute pressure gage. When the gage was moved to high altitude, the reading was wrong for local atmospheric pressure.

In Denver, CO, I had to remember to correct for 12.2 atmospheric pressure to get absolute pressure.

Ted

 

[dcasto]

Zadas, remember M40 in a Barton meter. Compound, but not liear, had a booster spring when going vacuum.

 

[zdas04]

Any Bourdon Tube gauge is measuring the difference between atmospheric pressure and process pressure. Any "psia gauge" is simply zero shifted to look like something it isn't. You have to know what the local atmospheric pressure is in order to have the gauge read that value when gauge is equalized.

David

 

[Compositepro]

Zdas, I've never encountered gages as you describe but I'm sure someone has made them. It would lead only to confusion. There are absolute pressure gages made by many manufacturers and with different designs. They all (or, I guess, most)contain a sealed vacuum chamber that is used as the reference pressure, in contrast to most pressure gages that measure the pressure difference from ambient atmopheric pressure.

 

[zdas04]

I've never seen a gauge with a sealed vacuum section and I'm having a hard time picturing how that would work. Would a reference chamber that was evacuated to -14.7 psig at sea level always reference that pressure or would it somehow know that it had been moved to an altitude where -14.7 psia was impossible? The physics of this are confounding me.

All of the "absolute pressure" gauges I've ever seen (hundreds by dozens of manufacturers) have simply shifted the zero so that when the gauge is at rest the needle reads the value for the atmospheric pressure where it was calibrated.

David

 

[JLSeagull]

An altimeter is an absolute pressure gauge that is measuring the ambient pressure against the sealed chamber. Although I occasionally buy absolute pressure transmitters, I don't recall buying an absolute pressure gauge. However, I did a web search and found several.

 

[hydtools]

Here is WIKA's description of how their absolute pressure gauge functions. I could not find a picture of the inside mechanics which would show how the evacuated capsule is connected for zero reference pressure. Reguires no adjustment for various ambient pressure locations.

Operating Principle
? The aluminum case holds the system pressure. The
capsule element or Bourdon tube is permanently sealed
with the zero reference pressure.
? The specially shaped capsule fully collapses to provide
overpressure safety regardless of the scale range.
? Any pressure applied is compared to the sealed reference
chamber (capsule element or Bourdon tube) to get an
accurate measurement of absolute pressure.

Ted

 

[hydtools]

Here is one of WIKA's models data sheet with a gauge sectional.

http://www.wika.com/WIKAWeb/Product/pdf/532.5X.pdf

Ted

 

[btrueblood]

Thanks for the clarification, David, and I agree, I don't see how a single Bourdon tube would work as a true absolute gage, unless it had a sealed, evacuated case (but how to know the case didn't leak?). A pressure capsule, where the capsule is evacuated and the outside of the capsule pressurized, would work - but there is still a worry about leakage of pressure out of the sealed case.

But, you can have two identical Bourdon tubes, one acting as an aneroid (i.e. the tube is evacuated) and arranged mechanically in opposition to the pressure-sensing Bourdon tube. The outsides of both tubes would reference the same external, atmospheric pressure.

That is not how the Ashcroft gages work, that unclesyd posted, given that their spec sheet reads: "Ranges Available in Gauge, Compound, Vacuum and Absolute (requires manual barometric compensation)" (underline added by me)

It does seem to be how the higher-pressure absolute gages from Wika work, according to the spec sheet that hydtools posted: "Series 1500 Gauges pressure elements are capsules up to and including the 50 psia range: 100 psia and above use Bourdon tubes. In the former, pressure is applied to the case and is referenced against the evacuated capsule.
In the latter, pressure is applied to a Bourdon tube, which is referenced against an evacuated Bourdon."

 

[btrueblood]

Wups, hydtools beat me to it. Well, part of it - it shows the pressure capsule idea.