[TG] [TG11] Speed sensor problems at low speed with i70s and DST800

[brownr377]

Hey there, I've got a DST-800 triducer connected to an ITC-5, and an i70s display. At normal cruising speed 5-7 knots everything is fine speed is pretty accurate if speed through water drops to below about 1.5 or 2 knots the speed shown starts fluctuating wildly. Sometimes even tied to the dock the speed shows 100 kts!

I've been looking for some kind of a low speed filtering option or other and can't figure a way to calibrate this out, it's annoying cause it throws the log way off. Anybody got ideas? Is this a sensor issue, software issue, setting issue?

[Tom - Raymarine - Moderator]

Hello brownr377,

This isn't a setup or calibration issue, and there's no setting that will resolve this. I've seen erratic and false high speed readings on both STW and wind-speed in the past be caused by noise signals being picked up on the transducer cabling. In one memorable instance a wind speed issue was being caused by noise from the invertor being used to charge a laptop when racing. The paddlewheel outputs 0..5V square-wave pulses at 5.5Hz per knot - if something else is putting a voltage onto the cable, the ITC5 can end up counting the wrong number of pulses and show the wrong speed.

This isn't a particularly easy one to test. The clearest diagnosis would be for a technical dealer to put an oscilloscope onto the signal wires when the problem is apparent and see what's there (it should be a completely stable 0V or 5V if the boat isn't moving, toggling slowly between the two if there is a little tide and the paddlewheel is spinning slowly). Alternatively, someone could check with a different ITC or transducer., perhaps it's a fault with one of these. The transducer cable run is also well worth checking, in case there's a break in the shielding or something like that.

Regards,
Tom