Help with atmospheric water vapor partial pressure experiment.

Dan Akers · science forum addict Joined: 19 Jul 2005 Posts: 56

In an attempt to "directly" measure (first principles) the partial
pressure of water vapor in air via a non-electronic or psychrometric
method, I have set up an apparatus consisting of a U- tube manometer and
a Salvason tube (I hope that's the correct terminology; it's a
relatively large, glass, cylindrical, test tube shaped device with an
open mouth at one end and a tubing nipple at the other; about 25cm tall,
2.5cm in diameter, with a total volume capacity of 410cc) The tubing
nipple of the Salvason tube is connected to one leg of the water
manometer; the other leg of the manometer is exposed to atmosphere. The
Salvason tube is half filled with pure water and then a piece of filter
paper (of the drip coffee filter variety) is pulled tight over the mouth
of the Salvason tube and secured with elastic bands. The tube is then
inverted (mouth down) and placed on a ring stand such that the now wet
filter paper is exposed to the breeze of a small muffin fan (blowing
upwards) located at the bottom of the ring stand. My idea is that the
steady state differential pressure of the filter paper, as indicated on
the manometer, (once the water column differential of the Salvason tube
is subtracted from the total indicated differential on the manometer)
will be equivalent to the difference between the saturation vapor
pressure at the prevailing dry bulb temperature (the liquid water in the
Salvason tube is at prevailing dry bulb temperature) and the prevailing
partial pressure of water vapor in the air in which the apparatus is
located. So far, my apparatus looks promising. Conditions are: Tair=25C,
RH=52%, Pwater vapor= 16.3cm H20, Psat H2O in Salveson=31.3 cm H20.
Target dP=31.3-16.3=15.0 cm H20. I get to within about 1.5 cm of water
column of my predicted, namely 15.0 cm of differential, before air (tiny
bubbles) starts to penetrate the filter membrane; stopping the partial
vacuum in the top of the Salvason from developing further. But the rate
trend, as I approach my 15.0 cm on the filter membrane seems to indicate
that I am on the right track, ie, the rate of pressure in the Salvason
drops more slowly (it follows that the evaporation rate from the surface
of the filter membrane also slows) as the 15.0 cm target is approached.
Is my thinking correct? If I could stop the air from crossing the filter
membrane, would the dP across the filter paper likely stabilize and be
equal to the difference between the saturation vapor pressure and the
prevailing partial pressure of water vapor in the air? In other words;
would evaporation from the filter paper cease?
Thanks for any constructive advice...
Dan Akers

Uncle Al · science forum Guru Joined: 24 Mar 2005 Posts: 1226

D Akers wrote:

Dan Akers · science forum addict Joined: 19 Jul 2005 Posts: 56

Uncle Al wrote:
"Two mercury barometers and a thermometer. Compare one barometer to the
other. Introduce a known volume of water into one barometer tube so it
rises into Torricelli's vacuum. Read off the vapor pressure at any
ambient temp you choose.
You do know you can look up this stuff, yes? Or calculate it?"
_____________________________________
Re;
Thanks for the reply Al, but that apparatus will not tell me the partial
pressure of water vapor in the air; at least as far as I can ascertain.
The difference between the two barometers will simply be the saturation
vapor pressure of water at the ambient temperature of the barometers.
Look up what? The current partial pressure of water vapor in the air in
my shop? Calculate it from what? That's what I'm trying to determine;
some parameters to work with from an apparatus such that the partial
pressure of water vapor in the air can be determined.
Yes, I know I could measure dew point with a cold plate and look up the
corresponding vapor pressure for that temperature OR use a sling
psychrometer etc. I am looking for a method a bit more "direct" than
that; the vapor pressure actually indicated as a pressure parameter.

Dan Akers

Richard J Kinch · science forum beginner Joined: 26 May 2005 Posts: 36

D Akers writes:

Google