Neptune Type S meters are nutating disc meters, the most common type of positive displacement meter. Also called a “wobble-plate” meter, a nutating disc meter incorporates a measuring element comprised of a flat disc around a central ball.
This element fits inside a measuring chamber with rounded sides that allow it to pivot. As liquid flow through the chamber, the disc nutates, or wobbles, in a regular pattern, causing the shaft attached to the disc to rotate in a conical pattern. The circular motion of the end of this shaft drives the meter’s gear train.