Hook Gauge Micrometer: used to obtain exact depth of pan water to nearest .001" (0.0254 mm). Observed as follows: full inches and tenths of inches read from staff; hundredths and thousands of inch read from circular wheel; knob is twirled until points of hook forms small depression (capillary) of the water surface -- knob is slowly turned until capillary depression "pops" (disappears); located on north side of pan.
Used to indicate changes in the level of water in the evaporation pan. The gage consists of a hook in the end of a stem that is graduated to tenths of inches over a range of 4 inches (10.16 cm). The stem is constructed with double threads throughout its range of adjustment. The threads have a pitch of one-tenth of an inch (2.54 mm). A three-legged spider and adjusting-nut assembly supports the hook and provides for adjustment of the height of the hook when the gage is installed on the stilling well. Accurate to within one-hundredth of an inch.

The hook gauge micrometer is placed on top of the still well when measurements are taken as shown in the photo above. The best method of measurement is to keep the pan filled at the same level each day, recording the water lost to evaporation each time since keeping the pan filled to the same level makes for a better control of water temperature.