Always 100% relative humidity over water? Supersaturation?

[Roger44]

Hello

Is there any way that RH over water in a closed container could not be 100%? Assuming that the walls of the container are at the same temperature as the contents, that sufficient time has gone by for uniform diffusion of water vapour, that we are at usual room temperatures and atmospheric pressures, and RH errors of the order of 1 or 2%.

I'm getting values over 100%, can there be the notion of supersatured air in this case?

 

[256bits]

What does 100% relative humidity mean to you?

If it is a closed container, why would usual atmospheric pressure apply?

 

[Roger44]

usual atmospheric pressures INSIDE the container, in order to keep it simple. At extremely high or low pressures other phenomena may become important, I don't wish to diverge onto these domains.

 

[gmax137]

usual atmospheric pressures INSIDE the container, in order to keep it simple...

The pressure inside the container is the sum of the partial pressures of the air and the water vapor. Do you know how to figure out the partial pressures?

 

[russ_watters]

I'm getting values over 100%..

How?

... can there be the notion of supersatured air in this case?

No.

 

[Roger44]

So can we say that if RH >100% in this case, there can be absolutely no other conclusion than errors in the measuring device?

 

[russ_watters]

So can we say that if RH >100% in this case, there can be absolutely no other conclusion than errors in the measuring device?

Or in the setup of the experiment, yes.

 

[Roger44]

Good comment Mentor. Oh too true! Set up errors like drops of water around the ouside of the bowl containing a saturated NaCl solution, so the measured RH was obviously much too high.

But I now know that there is no theoretical reason not to use "over water" as a 100% RH calibration reference.

Thanks everybody

 

[Roger44]

In fact there is a very good reason why you cannnot use "100% RH over water" to calibrate polymer capacitive RH sensors, It's simply because they are not spec'd for use at 100% RH.
The Honeywell sensors I'm using are barely sensitive to temperature in the normal operating range, whereas at 100%RH the output voltage rises linearly with falling temperature, leading to aberrations like 112% RH at 12°C.