At a recent installation I was involved with, the build included NMEA 183 devices interfaced into NMEA 2000 (N2K). This yacht had multiple fuel tanks, water tanks and a few black water tanks. It had a variety of instrumentation GPS’s, sounders, a weather station, A.I.S, radars, rudder angle indicator, bilge pump monitoring and alarms. When this vessel was being commissioned, a very interesting fault of duelling sensors was debugged.
There were many layers of commissioning from engine data, charts, and the list went on. However, when it came to calibrating one of the last set of tanks, the displayed data for the water tanks were oscillating from 40 to 70% continually. It took a little understanding, even though both tanks were labelled differently, they accidently shared the same instance. The N2K display was actually reading two different tank sensors but had no program error, which allowed this to happen. Some sensors have manual switches to set the instance. Some are set by manufacturer’s software, but regardless, installers watch out with this on N2K installations.
N2K allows using many more devices and the complexity, the system can output data from two devices thinking they are one. The oscillating tanks’ level, check your device instances! The system software was functioning correctly; however, it did not detect two devices sharing the same instance which in this case was set on dip switches on the sender. If you are building your own or an experienced builder, disconnect or isolate common senders and if the oscillation stops, check the instance of this device against the one that was oscillating but now is stable. Change one of the two and now data will get to where it supposes to be displayed, then calibrations can be completed.
Watch for it!