Measurements made with water vapor diffusion porometers, constructed of plexiglas and equipped with lithium chloride sensors, can be influenced by the mode of storage, the dehydration procedures adopted prior to each measurement, and the temperature at which the measurements are made. There also appears to be a general ageing effect on the calibration relationships.
It is recommended that standard storage procedures be adopted, which avoid exposure to room air. For the instruments tested the simplest and most convenient procedure is one in which the instrument is stored with the porometer cup dosed to outside air but connected to the silica gel dehydrator. In use it is recommended that instrument drift, which is frequently observed, should be reduced to a low and relatively steady value before measurements commence, periodically checked during measurements, and allowed for in calculations of leaf resistance.
The temperature dependence of the calibration curve did not agree with a theoretical expression developed elsewhere. It is recommended that the temperature dependence be determined by direct calibration and the resultant curve used to calculate resistances from measurements made at different temperatures.