A Hook gauge evaporimeter is a precision instrument used to measure changes in water levels due to evaporation

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

. The device consists of a sharp hook suspended from a micrometer

Micrometer

A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...

 cylinder, with the body of the device having arms which rest on the rim of a still well. The still well serves to isolate the device from any ripples that might be present in the sample being measured, while allowing the water level to equalize. The measurement is taken by turning the knob to lower the hook through the surface of the water until capillary action

Capillary action

Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

causes a small depression to form around the tip of the hook. The knob is then turned slowly until the depression "pops," with the measurement showing on the micrometer scale. Evaporation rate is determined by a sequence of measurements over a set time interval.