# Humidity in a closed system

• I
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.

## Answers and Replies

.... 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?

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.

.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? .

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.

G Cooke
... 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?
Not if the volume is allowed to increase. That would be the isobaric heating you mention first. However we can certainly consider the case where the volume is fixed and the pressure increases with temperature if you like.

If so, doesn't that imply that there is no change in temperature that could possibly decrease the humidity of such a system?[\QUOTE]

Well first, just to be sure, I think you are talking about absolute humidity, right? Absolute humidity is the amount of water vapor per unit volume. In your closed system the amount of water is fixed, so the absolute humidity will change if the volume changes or some water condenses out. As I said the volume could change lowering the humidity. An example would be the isobaric heating you already indicated would lower the humidity. So for arguments sake let's say we are talking about a fixed volume, in which case, ok, heating it won't change the humidity. However cooling it certainly can change the humidity. Your premise ("completely filled with humid air") isn't completely clear, but let's take that to mean 100% relative humidity. This would mean that any amount of cooling will condense water and lower the humidity. (Water in droplets on the wall don't count in the definition of humidity). We don't have to assume 100% relative humidity. How much water vapor just changes at what temperature the water starts condensing out.

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.

The pressure can and will decrease if you lower the temperature at constant volume. (Ideal gas law: P V = n R T). Water will condense out if you lower the temperature enough changing the humidity.

G Cooke
Well first, just to be sure, I think you are talking about absolute humidity, right?
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.
Your premise ("completely filled with humid air") isn't completely clear, but let's take that to mean 100% relative humidity.
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.

So perhaps your answer will now change.

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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.

So perhaps your answer will now change.

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).