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#1
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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 |
#2
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"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt com" wrote in message news:1105391055.635118@sj-nntpcache-3... .... 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
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"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt
com" wrote in message news:1105391055.635118@sj-nntpcache-3... 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
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"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt com" wrote in message news:1105391055.635118@sj-nntpcache-3... 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
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"Gig Giacona" wrote in message ... "jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt com" wrote in message news:1105391055.635118@sj-nntpcache-3... 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
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"Icebound" wrote in message ... "Gig Giacona" wrote in message ... "jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt com" wrote in message news:1105391055.635118@sj-nntpcache-3... 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
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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
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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:1105397045.6c0b9af7d0985bd99dd3e30aa7ae44ee@t eranews... "Icebound" wrote in message ... "Gig Giacona" wrote in message ... "jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt com" wrote in message news:1105391055.635118@sj-nntpcache-3... 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
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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
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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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