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Vacuum chamber demo

by windshear
Tags: chamber, demo, vacuum
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Oct2-13, 07:42 PM
P: 1
I did a demo for my students, using a simple vacuum pump and a vacuum chamber. I put a burning candle in the vacuum chamber. This generates numerous CCNs (cloud condensation nuclei) (I believe) and perhaps some larger-than-normal CCNs. A cloud forms in the vacuum chamber for a few seconds, then disappears, as the air gets sucked out. I don't believe the heat output of the candle significantly affects the situation.

Why did the cloud form if the temperature did not change much? The relative humidity did not shoot up to 100%, according to the pocket sized meteorological instrument Kestrel 4000, it went down! Clouds form when relative humidity goes up to near 100%, right?? Was there some not-so-simple thermodynamics going on here??

In the chamber, the pressure decreases quickly and I believe it drops over 50mb in the first few seconds. I got the pressure to change from 850 to 180mb in 30 seconds to a minute. (note: 850mb is normal for my altitude.) The temperature only decreased a couple of degrees F, according to the Kestrel 4000. I had a temp. of about 70 and a dew point of about 50. The temp went down to 68, and the dew point decreased. The RH decreased, because dew point started going down rapidly and is in the 20's (F) after 15-30 seconds. The Kestrel can measure a several degree change in a couple of seconds, and the same is true for dew point.
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Oct2-13, 10:36 PM
P: 12
CCI... Very high tech.

When I was in school we had 2 liter soda bottles, add a match, water cover, shake, then squeeze the bottle and see a cloud formation :)

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