On Dec 14, 5:52 pm, "Bill Habr" wrote:
"I R A Darth Aggie" wrote in . ..

On Fri, 14 Dec 2007 14:03:29 -0600,
Bill Habr , in
t wrote:

+ The reason the relative humidity in
+ the atmosphere doesn't get much above 100%

You need to qualify that statment.

There are conditions where supersaturation occurs, and the values are
significantly larger than 100%.

"Supersaturation" is used to mean relative humidity above 100%. Relative humidity, with
respect to water, in the atmosphere rarely gets above 103 % in clouds, the reason is that
droplets have something to form on. If you take a container with smooth enough sides you
could create 300% to 400% relative humidity before you would SEE the condensation.

This has been very instructive,
but also confusing!

As I understood it, relative humidity was the ratio of how much water
was in the air to how much water the air COULD hold at the current
temperature. This is easy to understand as the ratio of water vapor
pressure to "saturation" pressure. I'm also concerned with ABSOLUTE
humidity since that effects the density of the air in a particular
chamber and is important in calculating air flow from differential
pressure in a venturi or a nozzle.

Since relative humidity is the ratio of current water pressure to
"saturation" pressure, absolute current vapor pressure will be (RH/
100)*Psat, where Psat is a known for a particular temperature.

I intend to measure in a duct with smooth sides, so I suppose 300% to
400% relative humidity is theoretically possible, but not very likely
since I will be operating from 0 to 100 C with a closed system vented
to room atmosphere to prevent pressure buildup when heated (or vacuum
created when cooling).

I have already developed an inexpensive method of reading a T-type
thermocouple with a constant reference temperature which should give
me resolution to .01 C. My plan was to take a peltier effect cooler
and two gold-plated, copper bars spaced .01" apart with the air
flowing around and between the bars. Attached to each bar will be a
thermocouple. I will cool the bars with the pelter effect cooler
until there is conduction between the bars and measure the
temperature. I will then reverse the current through the peltier
effect device to heat the bars at a MUCH slower rate and record the
temperature when conduction stops. I will then remove current from
the cooler until bar temperature matches abient and stabilizes, and
start again. The dew point will be the mean of the four recorded
temperatures.

From dewpoint, I can find relative humidity. From relative humidity,
absolute humidity. From absolute humidity I can find pressure due to
water vapor, and from this and temperature, I should be able to find
the density of the air in the duct. With density, static pressure and
differential pressure I should be able to calculate the volume (mass)
of air passing through the venturi or nozzle with high accuracy.

Sound like a waste of time?