Vacuum Chamber Demo: Investigating Temperature & Humidity Changes

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Discussion Overview

The discussion centers around a demonstration involving a vacuum chamber and a burning candle, exploring the formation of clouds within the chamber, the behavior of temperature and humidity, and the underlying thermodynamic principles. Participants examine the relationship between pressure, temperature, relative humidity, and cloud formation in this experimental context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant describes a vacuum chamber demo where a burning candle generates cloud condensation nuclei (CCNs), leading to temporary cloud formation despite a decrease in relative humidity.
  • The participant notes that the pressure in the chamber drops significantly while the temperature only decreases slightly, raising questions about the thermodynamics involved.
  • Another participant recalls a simpler cloud formation experiment using a soda bottle, suggesting a common interest in demonstrating similar principles.
  • A later reply questions whether the observed cloud is indeed due to condensation or if it could be smoke from the candle, indicating uncertainty about the nature of the cloud formation.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms behind cloud formation in the vacuum chamber, with some questioning the role of humidity and others focusing on the thermodynamic aspects. The discussion remains unresolved regarding the exact cause of the cloud formation.

Contextual Notes

Participants mention specific measurements from a Kestrel 4000 meteorological instrument, including pressure, temperature, and dew point, which may influence interpretations of the results. There is also a lack of consensus on whether the cloud is a result of condensation or smoke.

windshear
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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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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 :)
 
2-gallon-vacuum-chamber-w-mastercool-6cfm-11.gif

this is simple vaccum chamber demo.
 
windshear said:
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.

So, do you know for sure that it is cloud condensation and not just smoke from the candle?

Zz.
 

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