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Re: atmos to elevation conversion


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Posted By<" ">Allen Forsman on August 29, 2002 at 08:19:52:

In Reply to: atmos to elevation conversion posted byMark Cralle on August 28, 2002 at 16:49:44:

: My roommate were talking about at what elevation
: you would have to be at for water to boil at room temp. (75 deg f for us)
: I figured out that P1/P2=T1/T2 and came up with 0.796 atmos.
: But I dont know how to get that to an elevation above sea level.
: Any one no the answer and can you explan a little.

: P.S. Keep it fairly simple im just a draftsman!

: Thanks,

: Mark W. Cralle


Actually, per published steam tables, the boiling pressure of water at room temperature is about .43 psi, which is approx 0.029 atm. The P1/P2=T1/T2 relationship is only valid for an "ideal gas", which doesn't really exist, but works quite well for most calculations involving P1/P2 or T1/T2 ratios which aren't too large. This equation is only valid for H20 when it's in a superheated vapor or "steam" condition. Now, back to the original problem - the conversion of atmospheric pressure to altitude is not as straight forward as calculating the pressure gradient of a liquid, such as water. This is because, as you go higher in elevation, the air becomes less dense, which means that the change in pressure for each foot change in altitude gets smaller and smaller. Below 10,000 feet or so, a decent approximation is 1 psi for every 2,000 ft. in elevation change. I plugged some altitudes into a simple software calculator and came up with around 75,000 ft. to get the .43 psi necessary to boil water at room temp. Just an aside while I'm rambling - the air temp. at 75,000 ft. is around -70 degrees Farenheit, which means you'd probably freeze any water before you boiled it if you were to take it that high :-)


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