# I Humidity in a closed system

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1. Dec 25, 2016

### G Cooke

We know that for a closed system, isobaric heating decreases the humidity and isothermal compression increases the humidity. But assuming that we start with the volume completely filled with humid air, is it not true that any increase in temperature must also increase the pressure? If so, doesn't that imply that there is no change in temperature that could possibly decrease the humidity of such a system? Doesn't that then imply that there is no way to decrease the humidity of such a system? Of course, no molecules are entering or leaving, and the pressure cannot decrease since it starts with the volume completely filled, right? So I think that exhausts everything.

2. Dec 25, 2016

### oz93666

Certainly ... humid or not , any temp increase will increase the pressure .... if no liquid water in the system the pressure increase must be less, but still increases.

Depends how you measure humidity ....relative humidity , is a % and compares to the total saturation possible at a particular temp. ... so any temp increase will decrease relative humidity .

Absolute humidity is a measure of mass of water per cubic meter of air , this will never change unless the total volume changes.

3. Dec 25, 2016

### Cutter Ketch

4. Dec 25, 2016

### G Cooke

Well, I want to refer to humidity as it contributes to the conductivity of the air...which I think would be the absolute humidity since that measures it in g/m^3 rather than a percentage which doesn't necessarily mean anything electrically. That's just my logic though.
Well no, what I meant by that was that the entire volume of the closed system is filled with the air, which happens to be humid (enough to conduct electricity given some electric field), not that the volume of the air is completely filled with water. So the volume cannot increase then, since it starts already at its maximum.

Last edited: Dec 25, 2016
5. Dec 26, 2016

### Cutter Ketch

Air being a gas always fills the available volume. However gasses are compressible, so you can always stuff in more air (well, at least until you reach the vapor pressure of Oxygen or the vessel bursts).