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altimeter calibration (again)

Postby dilbert » Wed Sep 13, 2017 10:17 am

Please pardon my 82 year old foggy brain.
I sat this morning at KNPA (field elevation 15 feet or so) in the PA34.
Had checked the local atmospheric pressure for Pensacola, and it was 29.85 HG
Checked the pressure on the altimeter and it read 29.86 HG, which pretty well agreed with the local forecast.
Yet the altimeter reading was -420 feet. Had to crank the dial to get it to field elevation. When I did so, the pressure then read 30.35 HG, which was clearly incorrect.
Maybe I'm losing it, but still think there's a calibration issue.
It's 55 years since I did any serious (military) flying, but I'd like someone to go through the exact same procedure and explain this.
:? as usual, Best Regards to all.
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Re: altimeter calibration (again)

Postby Thorsten » Wed Sep 13, 2017 11:15 am

Had checked the local atmospheric pressure for Pensacola, and it was 29.85 HG


When you say that, what do you mean? The ATIS said so? Or the actual simulated air pressure at field elevation was like this?
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Re: altimeter calibration (again)

Postby dilbert » Wed Sep 13, 2017 1:00 pm

I checked the local pressure on the internet. I assumed it was accurate because FlightGear downloads METAR and the PA34
altimeter displayed 29.86 HG, which was within 1/100 of an inch HG, and basically in agreement..
That's why I am questioning the fact that with the correct (and it is) atmospheric pressure entered, just as would be if I set it from what ATIS or the tower advised, that the altimeter read 420 feet low-which it shouldn't, if properly calibrated.

A pressure altimeter is just a Bourdan tube with attached needle. To the best of my recollection, it should be physically
calibrated to read 0 feet at 29.92 HG.

P.S.. Just tried it on my old Celeron laptop, and the gauge was correctly calibrated there, so am totally mystified why it's correct on the Celeron and not the I3-both are running the same FG download on WIN10. Will now try uninstalling and reinstalling clean on the I3
to see if the problem goes away.

Thanks for the response. Best Regards
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Re: altimeter calibration (again)

Postby Thorsten » Wed Sep 13, 2017 2:50 pm

I checked the local pressure on the internet. I assumed it was accurate because FlightGear downloads METAR


Unless it for some reason doesn't...

and the PA34
altimeter displayed 29.86 HG, which was within 1/100 of an inch HG, and basically in agreement..


The altimeter shows only the correct pressure if you dial it to the field altitude - it actually doesn't display any pressure value by itself (or rather, it displays whatever pressure value you set).
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Re: altimeter calibration (again)

Postby dilbert » Wed Sep 13, 2017 3:21 pm

Be that as it may in FlightGear, the gauge itself should read 0 feet when the pressure is or is even set at 29.92. When you move the knob, the needle moves in accordance with the pressure.
Regardless of what the atmosphere is, the relationship between pressure and feet of altitude is strictly based on physical calibration of the actual gauge. Therefore, regardless of field elevation, Metar, or whatever, if you turn the knob until 29.91 HG is displayed, the gauge should mechanically read 0 feet. I momentarily thought my Celeron was working correctly, but subsequently found, instead, that the when the gauge came up showing about the right field altitude, the pressure shown next to the knob was excessively high-like 30.46 HG (apparently my Celeron had not downloaded Metar). That's because the relationship between the needle and pressure
on the gauge is a matter of fixed calibration and unvarying. Yes, you can adjust to field altitude with the knob. But in my case, when I did so, the pressure no longer agreed with actual (METAR); and when I adjusted it to METAR, the gauge read -420 feet. Adjust your altimeter to 29.92, and see what your gauge reads. :|
.
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Re: altimeter calibration (again)

Postby legoboyvdlp » Wed Sep 13, 2017 3:27 pm

Are you sure? I thought that it showed field elevation at the pressure setting reported by METAR: altitude above sea level.
This is QNH after all.

QFE is the pressure that gives you zero on the ground: you turn the knob to exactly zero feet.
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Re: altimeter calibration (again)

Postby dilbert » Wed Sep 13, 2017 3:44 pm

Yes, I'm absolutely sure.

Manually adjust the altimeter knob until 29.92 is displayed adjacent to it. In my case the gauge reads -420 feet, not 0, as it should.
This is strictly a matter of gauge calibration. The actual pressure and location have nothing to do with it. The reason for the discrepancy is that the gauge is not calibrated to read 0 feet at 29.92 HG, as it should be. .Best Regards
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Re: altimeter calibration (again)

Postby it0uchpods » Wed Sep 13, 2017 4:15 pm

Are you sure that you have Live Weather on? Environment --> Weather

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Re: altimeter calibration (again)

Postby Thorsten » Wed Sep 13, 2017 4:46 pm

Be that as it may in FlightGear, the gauge itself should read 0 feet when the pressure is or is even set at 29.92.


Let's say I'd be surprised if that were true - because the instrument measures ambient pressure, but what it can't know is field elevation - but in order to consistently display 0 for a certain sea level pressure selection, it would have to have access to that information.

So what you claim is impossible for the instrument to do.

Which may be part of the confusion.
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Re: altimeter calibration (again)

Postby PINTO » Wed Sep 13, 2017 4:46 pm

dilbert wrote in Wed Sep 13, 2017 3:44 pm: Manually adjust the altimeter knob until 29.92 is displayed adjacent to it. In my case the gauge reads -420 feet, not 0, as it should.
This is strictly a matter of gauge calibration. The actual pressure and location have nothing to do with it. The reason for the discrepancy is that the gauge is not calibrated to read 0 feet at 29.92 HG, as it should be. .Best Regards


I don't mean to sound rude, but this is wrong. An altimeter will display 0 feet if set to 29.92 if your aircraft is at 0' MSL and the local pressure outside is also at 29.92. But if your local pressure is, say, 28.90 and you're at an airport that is 1,000' MSL, then setting your altimeter to 29.92 would report somewhere around 800-900'.

If you're at KNPA (field elevation of 28') and you set your altimeter to 29.92, and the altimeter then reports -420, that means the outside pressure would probably be above 30.50. It does not mean the altimeter is wrong. To double check yourself, tune to the KNPA ATIS in-sim frequency and listen to the reported pressure (or read the METAR in the environment->weather dialog). If you tune to that reported pressure, your altimeter should read 28'. Don't rely on outside-of-sim references for barometric pressure in determining if the altimeter is working correctly in-sim.

Setting your altimeter to read exactly 0 while sitting on the tarmac is referred to as QFE, and is entirely separate from what METAR will typically report. It's typically done by simply observing the altimeter, and setting the pressure to where the altimeter reads 0. And the higher your departing airport destination, the lower the pressure required to set your altimeter to exactly 0. Setting it to 29.92 will not have it report as 0, especially at varying altitudes and barometric pressures.
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Re: altimeter calibration (again)

Postby dilbert » Wed Sep 13, 2017 6:55 pm

Your're not at all rude and I appreciate your reply. I may not have explained what I was saying about the instrument very well.
Exactly as you said: If you set the altimeter at 29.92, it will read -420 feet when the atmospheric pressure is 30.5.If I had happened to be at 1000 feet it would have registered +580, by the same token. The only problem
here is that the actual atmospheric pressure was not 30.5, but 29.86 as reported by Metar, so when I set the altimeter to 29.86,
it should have registered the correct elevation, not -420 feet, as it did..

What I was trying to say about the instrument itself was that the relationship between the pressure setting and instrument reading
remains the same, even though readings vary with altitude and pressure. The gauge is a purely mechanical device: when the pressure r ises, the Bourdan tube straightens and the needle moves down. The number to which it points at any given pressure is a matter of calibration. If the pressure it's exposed to is actually 29.92, the needle should point to 0 feet when correctly calibrated.

I suppose one explanation from strictly a code standpoint might be that my programs are stuck on a 30.5 HG pressure and not
reading and employing Metar. In which case the altimeter may actually be getting 30.5 from the program, rather than being incorrectly calibrated..Best Regards
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Re: altimeter calibration (again)  

Postby dilbert » Wed Sep 13, 2017 9:16 pm

Aargh. Checked environment>weather and got "NO SIG" for METAR, which explains the pressure being stuck at 30.5 HG.
Checked the launcher and found "real world weather" unchecked, which explained why I wasn't getting Mettar. :oops: Sorry for the trouble guys, and thanks for your help.
Best Regards, Dilly:
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Re: altimeter calibration (again)

Postby it0uchpods » Thu Sep 14, 2017 12:10 am

No, problem, glad you got it sorted!!!

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