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[TG11] Speed sensor problems at low speed with i70s and DST800
05-28-20, 10:55 AM (This post was last modified: 06-02-20 02:08 AM by Tom - Raymarine - Moderator.)
Post: #1
[TG11] Speed sensor problems at low speed with i70s and DST800
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?
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06-02-20, 02:17 AM
Post: #2
RE: [TG11] Speed sensor problems at low speed with i70s and DST800
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

Raymarine since 1999.
Interests: Diagnosis of problems in sonar/fishfinders, NMEA2000, ethernet comms, autopilots, thermal cameras
Location: Sydney, Australia.

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