The basic principle of the Swoffer fiber-optic sensor is simple and unique: Multiple bundles of fiber-optics, assembled into a propeller driven rotor, gate a beam of infrared light from a photo diode to a photo-sensitive transistor producing electronic pulses. These pulses, generated by the propeller rotor over a precisely measured amount of time, are directly proportional to water velocity.

The electronic half of the sensor is the latest opto-electronics and is epoxy encapsulated in a ½" (12.7mm) diameter acetal resin housing for protection from chemicals and the elements. It can be purchased separately and attached to and used with many in-water devices in addition to those offered by Swoffer. The sensor uses a two-wire signal system requiring only 3 volts for operation and can generate a square wave output signal through over 3000 feet of cable.

The rotating portion of the sensor consists of a very low friction fiber-optics rotor rotating around a polished & hardened stainless steel shaft, all driven by a glass-reinforced nylon propeller. All rotor parts are easily replaceable in the field and spares are provided with every new instrument.