A magnetic paddlewheel passing by a Hall Effect switching device produces 3 pulses per revolution or 22,500 pulses per nautical mile. Speed range is .5 knots to 50 knots (40 knots on thru-hull models). Linearity is plus/minus 3% over speed range.
The paddlewheel has molded DELRIN bearings and turns on a highly polished stainless steel axle. This combination has an exceptionally long service life. Rarely does the axle or bearings need replacing, but if they do, it is an easy task. Simply remove four screws that hold the paddlewheel to the transducer housing.
A spray shield is an exclusive feature on transom mounted models. This eliminates the water spray that is thrown off by the rapidly spinning paddlewheel. Also, it is effective in preventing debris from fouling the paddlewheel.
Surface water temperature is typically determined by the use of a thermistor embedded in a stainless steel holder using thermal conductive epoxy. The assembly is mounted either in the bottom or side of the transducer. Various thermistor values are available. The most commonly used has a resistance of 10k ohms at 25C. Semiconductor devices are also used for temperature measurement.