Equipment for measuring water use of a greenhouse crop of up to ± 1 kg h-1 over 30 m2 is described. It is based on growing a crop in nutrient film, with a nutrient tank replenished from a water tank, and controlled by accurate level sensors. The water tank is suspended from a load cell interrogated at frequent intervals by a data logging computer. Examples of data collected are given. Peak daytime transpiration rates varied from 50 mg s-1 m-2 to 150 mg m2 s-1 with a maximum error of 5%. With low transpiration rates, the errors were increased, but accuracy could be improved by calculating the rates over a prolonged time interval.