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?