Altimeter Setting Question
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- mckenziepiping
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Altimeter Setting Question
Does anyone know what physically happens inside an altimeter when adjusting the altimeter setting? Is more pressure applied to the aneroid capsules? Is there a moving mechanical arm that actions something? When you turn the knob, are you pumping some air into the altimeter to bring the local atmospheric pressure back to standard pressure?
This would be useful in answering those questions about whether you will clear a mountain top when flying from point A to point B with an incorrect altimeter setting....
This would be useful in answering those questions about whether you will clear a mountain top when flying from point A to point B with an incorrect altimeter setting....
Re: Altimeter Setting Question
I apologize for begging the question, but this got my attention
I can claim, with absolute certainty that you do not need to know the answer to your question to determine whether you will clear a mountaintop. And, BTW, you might have actually put in the correct setting at the departure airport...the error can be caused by non standard pressures and temperature.
The important thing here is to recognize when you are flying into an area of lower pressure or the OAT is far below the standard.
Now I will leave it to others to answer your original question...maybe even explain how it works with EFIS.
How do you know if it would be useful without knowing how it works?This would be useful in answering those questions about whether you will clear a mountain top when flying from point A to point B with an incorrect altimeter setting.
I can claim, with absolute certainty that you do not need to know the answer to your question to determine whether you will clear a mountaintop. And, BTW, you might have actually put in the correct setting at the departure airport...the error can be caused by non standard pressures and temperature.
The important thing here is to recognize when you are flying into an area of lower pressure or the OAT is far below the standard.
Now I will leave it to others to answer your original question...maybe even explain how it works with EFIS.
Accident speculation:
Those that post don’t know. Those that know don’t post
Those that post don’t know. Those that know don’t post
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Re: Altimeter Setting Question
As far as I understand it, turning the knob only turns the needle(s) in relation to the rest of the linkage. It doesn't change the pressure in the unit or put any additional pressure on the capsules - they react to absolute pressure and that is converted mechanically into the dial indication. On an electronic system the static pressure sensor will be an accurate absolute pressure sensor, and the conversion to altitude is done electronically taking into account the specified altimeter setting.
"People who say it cannot be done should not interrupt those who are doing it." -George Bernard Shaw
Re: Altimeter Setting Question
In very plain and easy terms:
1) The Altimeter senses the ambient pressure.
2) The Altimeter setting is for YOU to tell the instrument what altitude that pressure equates to.
3) The Altimeter assumes standard atmospheric conditions (ie that the temperature will drop 2deg for every 1000')
So no there is no air pumping or pressure on the capsule when you set the altimeter setting. You only move the needles in order to set a base reference for the instrument to provide you the correct altitude information.
JP
PS.: Like Posthumane said
1) The Altimeter senses the ambient pressure.
2) The Altimeter setting is for YOU to tell the instrument what altitude that pressure equates to.
3) The Altimeter assumes standard atmospheric conditions (ie that the temperature will drop 2deg for every 1000')
So no there is no air pumping or pressure on the capsule when you set the altimeter setting. You only move the needles in order to set a base reference for the instrument to provide you the correct altitude information.
JP
PS.: Like Posthumane said
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Re: Altimeter Setting Question
All it does is move the needles and the altimeter setting in the window. The bellows stays the same.
It's a lot like a watch. A watch only knows there is x number of pulses per second from a quartz crystal or mechanical movement and a rate to advance the hands...... but it has no idea what time it is until you set it.
It's a lot like a watch. A watch only knows there is x number of pulses per second from a quartz crystal or mechanical movement and a rate to advance the hands...... but it has no idea what time it is until you set it.
Geez did I say that....? Or just think it....?
Re: Altimeter Setting Question
When you turn the setting knob, the entire innards of the instrument are turned around inside the case, other than the front frame, with the pointers, face and pointer gearbox. And, the altimeter scale is also turned separately, though with the same gears. If the knob is moved differently during set up by the shop, the altimeter setting in the instrument is adjusted.
This is why very old altimeters, long from overhaul or function check, can seem stiff, there's quite a bit of surface contact, and potential for friction in there. A small spring wire keeps the rotating part of the chassis snug against the opposing side of the case, if that wire is missing, broken or too soft, it will not hold effectively, and turning the knob will cause the entire innards to jump around a bit.
This is why very old altimeters, long from overhaul or function check, can seem stiff, there's quite a bit of surface contact, and potential for friction in there. A small spring wire keeps the rotating part of the chassis snug against the opposing side of the case, if that wire is missing, broken or too soft, it will not hold effectively, and turning the knob will cause the entire innards to jump around a bit.
Re: Altimeter Setting Question
I think you should explain your thought process here.mckenziepiping wrote:This would be useful in answering those questions about whether you will clear a mountain top when flying from point A to point B with an incorrect altimeter setting....
The fact that you think your initial question is of importance to know if you'd clear a mountain or not is a sign that there is one or some concepts you do not understand correctly.
Olivier
Think ahead or fall behind!
Re: Altimeter Setting Question
A screw can be taken out of the front of the altimeter, after the screw is removed you can take the adjustment knob and pull it back so when you rotate it it moves the scale and not the needles. This is how they are calibrated... by certified engineers with calibrated references and up to date technical documentation of course
Re: Altimeter Setting Question
Yes, of course. I chose to not mention that, as too many "other people" have tried it. Letting alone upsetting the calibration of the altimeter, it is easy to have the screw come right out, and you'll have real difficulty getting it back in, so the instrument will have to be removed, and go to the instrument shop anyway. Best not fiddled with.....certified engineers with calibrated references and up to date technical documentation of course
Re: Altimeter Setting Question
it is easy to have the screw come right out, and you'll have real difficulty getting it back in, so the instrument will have to be removed, and go to the instrument shop anyway. Best not fiddled with
Now how would you know that?
Accident speculation:
Those that post don’t know. Those that know don’t post
Those that post don’t know. Those that know don’t post
Re: Altimeter Setting Question
I used to apprentice with Vic leBlanc, the instrument guy at YYZ.Now how would you know that?
- mckenziepiping
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Re: Altimeter Setting Question
Sorry for the delay is replying, for some reason I'm not getting any e-mail notifications for thread activity. The reason I say it would be useful in answering those questions is because sometimes, when figuring out how to correct altitude, it's easier to simply understand how an altimeter is affected by pressure changes rather than memorizing phrases like "from high to low, look out below". Because if you understand what's happening physically inside, then you can just work out what the correction needs to be based on what's happening inside the altimeter that might "trick" it. So yes, I know I can solve these kinds of questions without having an answer to my question, but I'd rather be able to work it out from first principles rather than simply memorizing "steps" in solving the problem.trey kule wrote:I apologize for begging the question, but this got my attention
How do you know if it would be useful without knowing how it works?This would be useful in answering those questions about whether you will clear a mountain top when flying from point A to point B with an incorrect altimeter setting.
I can claim, with absolute certainty that you do not need to know the answer to your question to determine whether you will clear a mountaintop. And, BTW, you might have actually put in the correct setting at the departure airport...the error can be caused by non standard pressures and temperature.
The important thing here is to recognize when you are flying into an area of lower pressure or the OAT is far below the standard.
Now I will leave it to others to answer your original question...maybe even explain how it works with EFIS.
- mckenziepiping
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Re: Altimeter Setting Question
OMG wow I never ever thought of this as being a possibility.... big fail on my part for not applying Occam's Razor.Posthumane wrote:As far as I understand it, turning the knob only turns the needle(s) in relation to the rest of the linkage. It doesn't change the pressure in the unit or put any additional pressure on the capsules - they react to absolute pressure and that is converted mechanically into the dial indication. On an electronic system the static pressure sensor will be an accurate absolute pressure sensor, and the conversion to altitude is done electronically taking into account the specified altimeter setting.
Re: Altimeter Setting Question
Quick question. What are the bellows filled with? Is it dry air? Nitrogen?
- mckenziepiping
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Re: Altimeter Setting Question
Not sure if this is a legitimate question or if you're attempting to make fun of my original question.... at any rate, I don't think the "filling" affects its operation since all gases respond to pressure changes the same way (since the difference in molecular size between different types of gases is negligible compared to the size of the space between molecules). That's the foundation for the ideal gas law and the concept of "partial pressures" exerted by each gas in a mixture, which you can read about here: https://en.wikipedia.org/wiki/Partial_pressurecgzro wrote:Quick question. What are the bellows filled with? Is it dry air? Nitrogen?
I'm honestly not sure what's in the bellow (I think you mean aneroid capsule). All I know is that it's a partial vacuum. That said, for the reasons stated above, knowing the contents of the bellow probably wouldn't help your understanding of how the instrument works - a valid question, nonetheless.
Re: Altimeter Setting Question
Lol.mckenziepiping wrote:I'm honestly not sure what's in the bellow (I think you mean aneroid capsule). All I know is that it's a partial vacuum.
Peter knows perfectly well what's in the aneroid capsule. As should every pilot from the aspiring PPL upwards. A bit of research for you to do ...
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
- mckenziepiping
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Re: Altimeter Setting Question
Well FTGU states "these capsules are sealed and are 99% evacuated of air"... so I'd have to assume that the bellow has dry air inside of it, no nitrogen... but like I said, I don't see what difference it makes.photofly wrote:Lol.mckenziepiping wrote:I'm honestly not sure what's in the bellow (I think you mean aneroid capsule). All I know is that it's a partial vacuum.
Peter knows perfectly well what's in the aneroid capsule. As should every pilot from the aspiring PPL upwards. A bit of research for you to do ...
But of course, if you still see a need for more research, let me know.
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Re: Altimeter Setting Question
I think the following is somewhat related to this thread, so I won't start another:
I seldom fly into transponder airspace, but I've always been curious (and confused) about the relationship between my transponder's pressure sensor and the altimeter on the panel. Let's say ATC wants me at 3,000. I set my altimeter to the airport setting and fly accordingly with reference to the altimeter. But what pressure altitude does the transponder see and display on the scope in the tower? I know my transponder has a "QNH" setting, so do I have to set the QNH in the transponder to the airport altimeter setting as well? If not, wouldn't I be flying at 3,000 but the tower see a different altitude for me on their radar?
Yeah .... I probably should know this, but I normally fly in the boonies where I don't have to worry about such things. Up here, as long as 5T sees me on their TCAS I'm happy!
I seldom fly into transponder airspace, but I've always been curious (and confused) about the relationship between my transponder's pressure sensor and the altimeter on the panel. Let's say ATC wants me at 3,000. I set my altimeter to the airport setting and fly accordingly with reference to the altimeter. But what pressure altitude does the transponder see and display on the scope in the tower? I know my transponder has a "QNH" setting, so do I have to set the QNH in the transponder to the airport altimeter setting as well? If not, wouldn't I be flying at 3,000 but the tower see a different altitude for me on their radar?
Yeah .... I probably should know this, but I normally fly in the boonies where I don't have to worry about such things. Up here, as long as 5T sees me on their TCAS I'm happy!
Re: Altimeter Setting Question
Altitudes displayed to ATC are adjusted for altimeter setting; they should see what your altimeter shows if it's set correctly and your pressure sensing encoder and transponder are working ok.
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
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