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At atmospheric pressure and between temperatures of $\sim 0$ to $100^\circ C$, well known relationships between established humidity and given saturated salt solution are available. I.e. ASTM E104 describes this.

I am looking at doing some experiments in high pressure, and potentially high temperature conditions (measuring the sorptivity of porous samples). Up to ~5MPa say. Is there any chemical or physical reason why such saturated salt solutions would not work? Are there specific relationships which I could use to calculate or estimate the relative humidity inside such a chamber?

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@BrandonEnright: While it's on topic there as well, I can't migrate it unless it's off topic her (which AFAICT it isn't). I'll wait for others to chime in first.. –  ManishEarth May 15 '13 at 7:53
    
This might be one of those questions that could be asked on both sites, slightly differently. Perhaps this one could be edited to ask if there is some thermodynamical equation that could be used to extrapolate the behavior of a solution up to high temperatures and pressures, and on Chemistry you could ask if there are specific properties of a saturated salt solution that would affect the trend, or if there is any data about such solutions in high-T,P conditions. As written I think it is barely (and only partially) on topic here. –  David Z May 15 '13 at 19:45
    
In practice physicits nowadays are inapt to such simple questions :=) Those salt solutions mentioned maintain a certain partial pressure of water vapour. This partial pressure is independent from total pressure at first approximation. You have to calculate the rel humidity from the partial pressure. –  Georg May 21 '13 at 19:01
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migrated from physics.stackexchange.com May 17 '13 at 15:18

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