All currently available atmometers present a moist or partially moist surface to the atmosphere and evaporative loss from these surfaces is measured either as weight change or loss from a calibrated reservoir. The Gas Exchange (GE) atmometer has a dry surface through which evaporation passes in a process where moist air is exchanged for dryer air. Vapour loss is measured in the normal way. Techniques have also been developed for the electronic monitoring of this vapour flow.
Two commonly recurring criticisms of moist-surface atmometers are their over-reaction to wind flow and their variability; the GE atmometer addresses both these problems. Repeated direct comparisons between moist atmometers (eg Livingston, Cantoni) and GE atmometers revealed that evaporative yields per unit surface area were in the ratio of about 4:1 and the design of the latter ensured that its response to short-term wind shifts was appropriately dampened.
Concerning variability, replicates of the GE atmometer were repeatedly compared under a wide range of environmental conditions and produced coefficients of variation arounf 4%. Statistically, this result is very significant.
Comparisons will be presented between evaporation from GE atmometers and a variety of greenhouse plants which are both highly correlated and highly significant. Further comparisons between GE atmometer losses and the normal calculations of ETo (Penman-Monteith, Eddy Correlation) will be presented which are similarly correlated. All available evidence suggests that the GE atmometer reliably reflects an unstressed plant's potential for transpiration.