A positive displacement flowmeter is a cyclic mechanism built to pass a fixed volume of fluid through
with every cycle. Every cycle of the meter’s mechanism displaces a precisely defined (“positive”)
quantity of fluid, so that a count of the number of mechanism cycles yields a precise quantity for the
total fluid volume passed through the flowmeter. Many positive displacement flowmeters are rotary
in nature, meaning each shaft revolution represents a certain volume of fluid has passed through the
meter. Some positive displacement flowmeters use pistons, bellows, or expandable bags working on
an alternating fill/dump cycle to measure off fluid quantities.