Abstract: The impeller and drum of a motorless in-line mass flowmeter are constructed with small optical targets having high reflectivity. Light is pulsed toward the path of rotation of the targets through optical fibers. The pulse width is small in comparison to the delay between pulses. Reflections of the pulses from the optical targets are conveyed by a single output fiber to a remote location where the optical pulses are converted to electrical pulses. The electrical pulses can be analyzed to determine the speed of the impeller and the deflection angle of the impeller relative to the drum, for accurate determination of the mass flow rate of liquid passing through the flowmeter.