The head is independent of the volume of liquid in the upper reservoir, depending only on the volume between the timing points. It is therefore much easier to use than the Cannon-Fenske type.
See Goncalves, Kestin and Sengers, Int J Thermophys. 1991, 122, 1013 for a good discussion of the operation of this type of viscometer.
If you can afford it, a commercial Stabinger-type viscometer is a better and more convenient option as it is easy to operate and does not require a thermostat bath.