Pressure Gauges
Pressure Gauges
(OP)
Hi,
Would anyone know a manufacturer of pressure gauges in psia (absolute)?
I know of Winter's. Anyone else?
Thanks
Gabriel
Would anyone know a manufacturer of pressure gauges in psia (absolute)?
I know of Winter's. Anyone else?
Thanks
Gabriel





RE: Pressure Gauges
and look under "Gauges, Pressure and Flow, Hydraulic"
Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING
RE: Pressure Gauges
RE: Pressure Gauges
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
RE: Pressure Gauges
http:
Ted
RE: Pressure Gauges
http://www.heise.com/products.cfm?doc_id=31
RE: Pressure Gauges
David
RE: Pressure Gauges
"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.
RE: Pressure Gauges
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
RE: Pressure Gauges
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.
RE: Pressure Gauges
In Denver, CO, I had to remember to correct for 12.2 atmospheric pressure to get absolute pressure.
Ted
RE: Pressure Gauges
RE: Pressure Gauges
David
RE: Pressure Gauges
RE: Pressure Gauges
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
RE: Pressure Gauges
RE: Pressure Gauges
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
RE: Pressure Gauges
http://www.wika.com/WIKAWeb/Product/pdf/532.5X.pdf
Ted
RE: Pressure Gauges
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."
RE: Pressure Gauges