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Altimeters and air pressure variation



 
 
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  #1  
Old January 10th 05, 08:59 PM
jharper aaatttt cisco dddooottt com
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Default Altimeters and air pressure variation

At sea level, the change in atmospheric pressure with altitude is
close to 1"Hg/1000'. Logically, this would mean that the air
pressure would drop to zero somewhere not much above 30000'. It
doesn't, because as the density drops the variation with
altitude also changes.

Which brings to mind the question, how does an altimeter deal
with this? As far as I know, it's just a simple aneroid barometer
with a bunch of linkages and gears to turn its expansion into
pointer movement.

My altimeter is marked "accurate to 20000' ". Is this why? Do
altimeters for higher altitudes have some kind of clever
mechanism to deal with the non-linearity of pressure at higher
altitudes.

I asked my acro instructor (10K+ hrs, airforce instructor pilot,
ex U2 pilot so should know a thing or two about high altitudes).
He explained the non-linearity of pressure to me but was
stumped on how this translates to the altimeter mechanism.

Anyone know?

John

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  #2  
Old January 10th 05, 09:26 PM
Icebound
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Default


"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt
com" wrote in message news:[email protected]
....
Do
altimeters for higher altitudes have some kind of clever
mechanism to deal with the non-linearity of pressure at higher
altitudes.



I am not an expert in the mechanics of barometer/altimeter construction, but
I do not see that it has to be particularly clever.

Typically, the metal "can" (which expands and contracts with air pressure)
is preloaded with a steel-spring against the air pressure. Isn't the force
produced by a steel-spring typically non-linear throughout its expansion...
so could not the spring-steel parameters be chosen such that the
non-linearity matches air pressure non-linearity closely?


  #3  
Old January 10th 05, 09:35 PM
Dean Wilkinson
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Default

"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt
com" wrote in message news:[email protected]
At sea level, the change in atmospheric pressure with altitude is
close to 1"Hg/1000'. Logically, this would mean that the air
pressure would drop to zero somewhere not much above 30000'. It
doesn't, because as the density drops the variation with
altitude also changes.

Which brings to mind the question, how does an altimeter deal
with this? As far as I know, it's just a simple aneroid barometer
with a bunch of linkages and gears to turn its expansion into
pointer movement.

My altimeter is marked "accurate to 20000' ". Is this why? Do
altimeters for higher altitudes have some kind of clever
mechanism to deal with the non-linearity of pressure at higher
altitudes.

I asked my acro instructor (10K+ hrs, airforce instructor pilot,
ex U2 pilot so should know a thing or two about high altitudes).
He explained the non-linearity of pressure to me but was
stumped on how this translates to the altimeter mechanism.

Anyone know?

John


Visit this website and it will answer your questions about the relationship
between pressure, temperature and altitude... altimeters are designed to
take the non-linearity into account...

http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html



  #4  
Old January 10th 05, 09:37 PM
Gig Giacona
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Posts: n/a
Default


"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt
com" wrote in message news:[email protected]
At sea level, the change in atmospheric pressure with altitude is
close to 1"Hg/1000'. Logically, this would mean that the air
pressure would drop to zero somewhere not much above 30000'. It
doesn't, because as the density drops the variation with
altitude also changes.

Which brings to mind the question, how does an altimeter deal
with this? As far as I know, it's just a simple aneroid barometer
with a bunch of linkages and gears to turn its expansion into
pointer movement.

My altimeter is marked "accurate to 20000' ". Is this why? Do
altimeters for higher altitudes have some kind of clever
mechanism to deal with the non-linearity of pressure at higher
altitudes.

I asked my acro instructor (10K+ hrs, airforce instructor pilot,
ex U2 pilot so should know a thing or two about high altitudes).
He explained the non-linearity of pressure to me but was
stumped on how this translates to the altimeter mechanism.

Anyone know?



The following is a WAG but that could be the reason that in the Flight
Levels, above 18k feet everyone sets thier altimeter to 29.90.


  #5  
Old January 10th 05, 09:45 PM
Icebound
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Default


"Gig Giacona" wrote in message
...

"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt
com" wrote in message news:[email protected]


Anyone know?



The following is a WAG but that could be the reason that in the Flight
Levels, above 18k feet everyone sets thier altimeter to 29.90.



Hopefully, 29.92.


  #6  
Old January 10th 05, 10:44 PM
Sriram Narayan
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Default


"Icebound" wrote in message
...

"Gig Giacona" wrote in message
...

"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt
com" wrote in message news:[email protected]


Anyone know?



The following is a WAG but that could be the reason that in the Flight
Levels, above 18k feet everyone sets thier altimeter to 29.90.



Hopefully, 29.92.



That still wouldn't help since the pressure change for a 1000ft change in
altitude at 18k would be smaller than at sea level. It would have to have
some non-linear spring compensation as a function of absolute pressure.


  #7  
Old January 10th 05, 10:45 PM
Jose
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Default

It would have to have
some non-linear spring compensation as a function of absolute pressure.


It could also be a non-linear gear compensation, such as using
non-circular gears. Lotsaways it =could= be done, but I don't know
how it =is= done.

Jose
--
Money: What you need when you run out of brains.
for Email, make the obvious change in the address.
  #8  
Old January 10th 05, 10:57 PM
Rod Madsen
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Default

I couldn't be done easily with a spring. Springs are very linear except in
very special cases. I think it's done with leaver arms that change
effective length with displacement.

Rod
"Sriram Narayan" wrote in message
news:[email protected] eranews...

"Icebound" wrote in message
...

"Gig Giacona" wrote in message
...

"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt
com" wrote in message news:[email protected]


Anyone know?



The following is a WAG but that could be the reason that in the Flight
Levels, above 18k feet everyone sets thier altimeter to 29.90.



Hopefully, 29.92.



That still wouldn't help since the pressure change for a 1000ft change in
altitude at 18k would be smaller than at sea level. It would have to have
some non-linear spring compensation as a function of absolute pressure.




  #9  
Old January 10th 05, 11:50 PM
jharper aaatttt cisco dddooottt com
external usenet poster
 
Posts: n/a
Default



Dean Wilkinson wrote:


Visit this website and it will answer your questions about the relationship
between pressure, temperature and altitude... altimeters are designed to
take the non-linearity into account...

http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html


Nice site, thanks. But presumably there is some standard
atmospheric model that altimeters use? After all nobody actually
cares whether FL300 is really 30000' feet above MSL, as
long as everyone flying there is at the same altitude and,
more importantly, not at somebody else's FL290 or FL310.

Which implies that there must be some standard mechanical
way of making the translation? I'll ask next time I visit my
avionics shop, but considering what each visit costs I
quite hope this won't be for a while.

John


  #10  
Old January 11th 05, 12:05 AM
Peter
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Posts: n/a
Default

jharper aaatttt cisco dddooottt com wrote:

Dean Wilkinson wrote:

Visit this website and it will answer your questions about the
relationship
between pressure, temperature and altitude... altimeters are designed to
take the non-linearity into account...

http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html


Nice site, thanks. But presumably there is some standard
atmospheric model that altimeters use? After all nobody actually
cares whether FL300 is really 30000' feet above MSL, as
long as everyone flying there is at the same altitude and,
more importantly, not at somebody else's FL290 or FL310.


Yes, there is a standard model and if you click on the first
link on the cited page you get to:
http://www.lerc.nasa.gov/WWW/K-12/airplane/atmos.html
which gives the equations describing that standard model.

Which implies that there must be some standard mechanical
way of making the translation?


There's a mathematically defined correspondence
between altitude and pressure under the standard
atmosphere assumption. But I doubt if the specific
mechanical means of achieving that correspondence is
specified anywhere. As long as the manufacturer makes
an instrument that is shown to give the right correspondence
to within a specified accuracy why should it matter exactly
how they do it?

I'll ask next time I visit my
avionics shop, but considering what each visit costs I
quite hope this won't be for a while.


 




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