"Clark" wrote in message
...
"d&tm" wrote in
:


"Clark" wrote in message
...
A minor nitpick on a previous post from Jim:

snip

It was stated that n/V was equal to density. While the stated equality
isn't,
the jist of what was said was ok. The equality would be

(Mass/Molecular Weight)/V = density

Since the molecular weight of air is a constant it can be combined with
the
other constant in the ideal gas law when used for atmospheric calcs.


a minor nitpick on your minor nitpick is that the molecular weight of
air is not constant. It is affected by humidity.
density is better expressed as d = PM/RT
where d will be denity in units of mass/volume
(M is molecular weight), R the gas constant ,P and T press and Temp .
For us SI people R=8.31, P in Pa and T in Kelvin gives density in kg
/m3 ( using M in kg/mole)Terry


I use standard dry lab air in all my calculations. :-)

I vote we call this one a sub minor nit pick on my minor nitpick. Just
what
is the likely range of average molecular weights? Water vapor content
varies from what? pretty darn dry to about 4%? And keep those SI units on
your side of the border, or pond, or whatever socio-economic,
nation-state,
or geographic boundry that thankfully separates us.

4% water could make the density altitude ( the important thing to us pilots)
10 to 20% higher than if the air was dry. Thats not insignificant to me.
But then I know a lot of enjuneers who would be happy with that. ( mostly
the ones who cant spell enjuneer) . and wots wrong with SI units Clark ?
God gave us 10 fingers for a reason.( at least we have 10 fingers on this
side of the pond as you call it, not so sure about you murkins :)
terry