Weekend Update 3/8/20

[Iain]

Okay, we pulled out the sheets, and both gauges are documented by the calibrators as reading 1 psi high.  Considering the age of the gauge, that's not too bad, and also the two compund gauges we have don't need to be highly precise, anyway.

Interesting.  I didn't realize there were that many types of air pressure gauges, or that they used different units for pressure and vacuum.

This gauge is for steam.  You get a vacuum in the condensor, and compound gauges are used for things that could go negative (for example, the feed heater)

[nkalanaga]

OK, a steam gauge.  I can understand the difference between a compound gauge and a simple one, if the pressure can go negative.  But what is the difference between a steam and air gauge, except that the steam gauge has to be heat and water resistant?  Or is that the only difference?

[aikorob]

we use oil filled gauges for steam and hydraulics; along with a snubber---helps dampen any pressure spikes in the system

[nkalanaga]

OK, still learning new things!  Thank you.

[Spades]

But in this case it is a combination gauge, indicating both vacuum and pressure. In this case ambient air being present at the gauge's inlet should have it show zero, since neither vacuum or pressure is present at the inlet. Inlet and ambient pressures are equal.  At least that is how I understand things, and that is how a similar gauge I have works.

I bet when the engine is running the needle moves in an anti clockwise direction indicating vacuum units.  The stem on the bottom of the instrument is open to the atmosphere and that is why it's not zero.

From time to time I still fly a piston twin from a sea level airport. The manifold pressure gauges (MP) are marked up to 30"  With a local altimeter setting o' 29"Hg, engines off the MP gauges will indicate 29".  Once you start engine the maximum MP will be is about 25" and the engines will be down to about 13" during various fate of flight.  The engines are normally aspirated so lose manifold pressure as gain altitude.

I know ablatisque testiculis flama medicus aka Dr. V could explain this a lot better than I can.  I failed latin. Duh

[narrowminded]

... I failed latin. Duh

Latin failed me.

[peteski]

I bet when the engine is running the needle moves in an anti clockwise direction indicating vacuum units.  The stem on the bottom of the instrument is open to the atmosphere and that is why it's not zero.

No need to bet - that is exactly how that gauge works.  The pointer should be at zero when the gauge's inlet and ambient pressure are equal (no differential).  If there is pressure applied to the inlet, the pointer will move clockwise, and if there is vacuum, it will move counterclockwise.

[Spades]

No need to bet - that is exactly how that gauge works.  The pointer should be at zero when the gauge's inlet and ambient pressure are equal (no differential).  If there is pressure applied to the inlet, the pointer will move clockwise, and if there is vacuum, it will move counterclockwise.

It won't read zero, because the gauge stem is open to the atmosphere right now. Ok, maybe a diaphragm or fitting is there but that gauge is open to the atmosphere, and that is what is reading.  You could evacuate the instrument and cap it and could get it read zero.  I would guess there are Aneroid barometers to measure ambient barometric pressure attached to gearing so the needle moves in a precise calibrated manner.  The instrument just a pressure-sensing of air mass.  It doesn't distinguish between vacuum or compressed air.

Also the tool just came out of calibration, you can see the sticker.  That sticker means it is a precision instruments with in tolerance.

[peteski]

It won't read zero, because the gauge stem is open to the atmosphere right now. Ok, maybe a diaphragm or fitting is there but that gauge is open to the atmosphere, and that is what is reading.  You could evacuate the instrument and cap it and could get it read zero.  I would guess there are Aneroid barometers to measure ambient barometric pressure attached to gearing so the needle moves in a precise calibrated manner.  The instrument just a pressure-sensing of air mass.  It doesn't distinguish between vacuum or compressed air.

Also the tool just came out of calibration, you can see the sticker.  That sticker means it is a precision instruments with in tolerance.

I beg to differ. I doubt there is vacuum inside the gauge's case.  If it was adjusted properly (the ambient pressure and the pressure inside the Bourdon tube are equal, it should read zero).

This looks to be to be a gauge based on the Bourdon tube. If you were to evacuate air and seal the gauge's case, it would read 14.7 psi (as that is roughly atmospheric pressure at the sea level), or maybe bit less if it is above sea level.




See https://en.wikipedia.org/wiki/Pressure_measurement

This brings up another point - to me when an instrument is calibrated it means that it is adjusted to work properly and its readings are verified to be within tolerances.  If the person that did the calibration noted that the pointer is on 1psi, it means that they didn't actually perform full calibration.

[Steveruger45]

That’s pretty much what I was going to say Pete.

[tehachapifan]

I seem to recall there were PSI gauges and PSIG gauges...at least with water gauges. One took into acct. atmospheric pressure and one did not. Been a while though.

[Steveruger45]

I seem to recall there were PSI gauges and PSIG gauges...at least with water gauges. One took into acct. atmospheric pressure and one did not. Been a while though.

Yep.  A psi gauge is measuring actual pressure whereas a psig gauge is measuring pressure relative to atmospheric pressure at sea level.
So a psi gauge open to atmosphere would measure 14.7psi whereas a psig gauge would read zero.

[up1950s]

Maybe it shows this when it is cold , but it is perfect when at normal working temperature ?

[peteski]

I'm starting to regret that I started this OT pressure gauge discussion by asking why the pointer was not sitting at zero. Sorry. 
That's what I get for being too anal. 

[Roger Holmes]

To, co dostaję za to, że zbyt anal.  Google translate doesn't have Latin so I thought this might entertain our friend, Peteski.