PSIG Question 1

[dennisb230]

If a spec calls out for a leak test to be done at 5 PSIG,
should I add the 14.7 for atmospheric pressure (Gage reading = 19.7),
or does the PSIG imply that the 14.7 is included when the gage reads O (Gage reading = 5)?

 

[KernOily]

The 5 psig includes atmospheric pressure.

The "g" means gage pressure.

The absolute pressure would be 5 psig + 14.7 (atmosphere, assuming you live near sea level) - 19.7 psia.

Does this answer your question?

 

[dennisb230]

Is the PSIG gage already set for 14.7 at sea level?
What if I were not at sea level?

 

[KernOily]

Yes, most store-bought pressure gauges assumes you are near sea level.

I'm not totally sure how those things are set up in regard to how far off they would be if you are at, say 9500' eelvation. Sure I can caclulate the atmospheric pressure at any elevation, but what I don't know is how the gauge will respond. I suspect if I buy a store-bought gage in Los Angeles and drive up to the summit of Mt Whitney (14,000'+) it won't be accurate, but I don't know by how much because I don't know how the mechanical Bourdon tube mechanism is calibrate dor set up. It might start rotating backwards.

If you need tight accuracy, or if you are at higher elevation, you will have to get a special gauge, one that can be recalibrated for the difference in atm pressure. If you look on the Ashcroft or Wika webpages you will find what you are looking for in the way of special gauges and how they are set up and calibrated. Don't call McJunkin or Kelly Pipe because they won't know.

 

[JLSeagull]

Gauge pressure is at the local elevation. In the case of a bourdon tube pressure gauge, the fluid acts on the inside of the tube and atmosphere acts on the outside.

Consider using lower case letters for the pressure units. Capital letters usually apply to proper names or large SI multipliers.

 

[KernOily]

OK, does that mean that a new Bourdon tube gauge will always read zero when I take it right out of the box, no matter the elevation or atmospheric pressure?

 

[SeanB]

I am pretty sure that a pressure gauge only measure gauge pressure. So if a leak test is to be done at 5 psig - that is the reading on your gauge.

 

[KernOily]

SeanB - I think you might be right. I don't know enough about the workings of the mechanicals inside the gauges to know fer sure how they will be affected BUT my guess is the guts of the gauge only reads the differential between what's inside the Bourdon tube (the process) and the pressure inside the gauge case (the local atmosphere).

If that is so, then 5 psig is measured relative to whatever atmosphere/elevation you are operating in, whether at sea level, 14,000', or in outer space, and NOT relative to 14.696 psia. I still need to look it up to be sure though. Thanks! Pete

 

[JLSeagull]

For a bourdon tube, the linearity is sloppy in the bottom five percent of the range. If the elevation is especially high the zero may require adjustment. My gauges have a zero adjustment on the pointer. However, zero adjustment would not typically be required except for very low pressure applications. I use Dwyer Magnahelic gauges for very low pressures.

Near sea level, the affect of another thousand foot altitude is one-inch Hg or about a half psi. This may not be sufficiently precise for gauges in high mountain locations. BTW, the gauges that I buy have an accuracy near 0.5 percent. If you want something where the precision required is greater than about one psi then don't use a bourdon tube gauge.

 

[JLSeagull]

The generalization for pressure altitude precision for a bourdon gauge applies near sea level. Twenty-thousand foot is "not near sea level".

A private airplane uses an aneroid barometer as the altimeter. Instruments on old private planes are usually called “steam gauges” in contrast to the glass panel or visual display based instruments. Pilot check navigation aids to obtain the current altimeter setting for that area (based upon the barometric pressure). This is done about every 100-mile.

I think that jets use aneroid barometer technology for their basic altimeter. Jets may also have a radar altimeter and a WAAS corrected GPS. The altimeter settings provide consistent adjustment with the steam gauges. Everything changes above 18,000 foot where everyone uses 29.92 as the altimeter setting.

 

[zdas04]

A Bourdon tube gauge should read zero when there is no applied pressure (i.e., it is open to atmosphere). Turning the zero knob (or screw) to make it read exactly zero is perfectly legitimate and is actually required on test gauges. On portable electronic gauges you should hit the "zero" button every time you use it.

Back to the OP's question, if the leak test is at 5 psig, then use a normal gauge and pressurize the system until the gauge reads "5". For some reason that is lost in the mists of time, PSV's, leak tests, and static tests are done at gauge pressure.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[mfitzs]

Could you share the application? If we knew what you were trying to test it could be helpful. I work in a lab and test low pressures to high (50,000) along with some work with vacuum.