Expressions are derived for the fluxes of heat or water vapour to the thermojunction of a Spanner psychrometer from a concentric shell of sample material, or filter paper impregnated with a calibration solution. The sample material is assumed to have a cuticular resistance whereas the filter paper does not. So long as these fluxes are essentially identical in the two cases, the psychrometer retains its calibration sensitivity. The relative difference between the fluxes in the two cases is derived as a dimensionless function which is graphed for a wide range of values of the parameters. The graph allows computation of the cooling period which results in a 1~ difference between the fluxes. The same technique provides a conservative estimate of the maximum cooling period for other sample geometries.
Previous estimates of the equilibration time of the psychrometer chamber with sample material have not accounted for cuticular resistance or adsorption on chamber surfaces. These factors are included in expressions for equilibration times, which are derived for any specified relative or absolute error of water potential. The relevant part of the adsorption isotherm of the chamber should be measureable with the psychrometer, and so the suitability of various chamber materials may be assessed.
For samples of soil or solutions in filter paper, which have no cuticular resistance, the geometry of the apparatus can be arranged to shield the thermojunction from the adsorbing walls of the container. Then, accurate measurements of the water potential of the sample are possible before the chamber has equilibrated. Two shielding geometries are considered in detail: a tube of sample and a pair of parallel discs of sample of equal radii. In each case the thermojunction is located centrally.