Thermocouple psychrometry

Thermocouple psychrometry is a technique that infers the water potential of the liquid phase of a sample from measurements within the vapor phase that is in equilibrium with the sample. The theoretical relation between water potential of the liquid phase and relative humidity of the vapor phase is given by the Kelvin equation

Ψ = energy/volume = (RT/Vw) ln(p/po) [3.2.3–1]

where ψ is water potential (sum of matric and osmotic potential, MPa), R is the universal gas constant (8.314 × 10^-6 MJ mol^-1 K^-1), T is temperature (K), Vw is molar volume of water (1.8 × 10^-5 m³ mol^-1), and p/p_o is relative humidity expressed as a fraction where p is actual vapor pressure of air in equilibrium with the liquid phase (MPa) and p_o is saturation vapor pressure (MPa) at T.