The CalCOFI tows use a mechanical flowmeter to determine water speed, volume, or distance. The manufacturer is General Oceanics and is model 2030 series. The Flowmeter incorporates a precision molded rotor coupled directly to a six digit counter which registers each revolution of the rotor and displays it in a fashion similar to that of an odometer.
The Flowmeter is usually attached to the front center of the frame containing the net. It helps in detecting the sequential clogging of the net.