The Hook gauge evaporimeter measures the rate of evaporation by the change in level from a free water surface in a pan or tank. It is used by meteorologists and water engineers throughout the world, especially in hot, arid countries where water conservation is of vital importance.
The water level in a 'Class A' pan is measured, usually every 24 hours, by adjusting the height of the hook until its point just breaks the surface. The measurement is carried out inside a 'still well', which provides a small area of water surface free from ripples. The still well, made of brass, stands on three levelling screws and has an opening in the base to equalise water levels inside and out. The hook gauge, also brass, rests on top of the well supported by three horizontal arms and has a micrometer head to permit very fine adjustment.
In some applications, the evaporation measurement is correlated with the surface temperature of the water and the wind-run past the tank. When this is required, the floating Maximum/Minimum Thermometer and Cup Counter Anemometer are also used.
