[TG] ST70 and DST800 speed calibration issue

[Azura]

My boat is equipped with full Raymarine instruments including ST70 displays and DST800 transducer. Recently the boatspeed reading has been indicating faster than actual speed by over 1 knot at full speed, although it seems very close to gps speed at lower speeds. I have gone through the calibration process multiple times, and the more data points I add, the less accurate it becomes. I even tried a factory reset and went through the whole commissioning process from scratch.

My most recent attempt at calibration attempt resulted in a boatspeed reading far below SOG, even though 1 of the calibration points I used was exactly the speed I was going.

All of my calibration was done in minimal current conditions. The impeller is clean and spins freely.

I also noted that the water temperature displays much lower than actual temp, and outside the offset range.

I have read about testing the transducer with a multimeter, but my DST800 connects directly to the STng bus, with no pod or ITC-5 module.

My questions:

Is there a way to test my transducer?
Could the problem be caused by a problem on the SeatalkNG bus, and if so how can I troubleshoot that?
Could the problem be related to the ST70 displays?

I welcome any suggestions you might have!

[Tom - Raymarine - Moderator]

Hello Azura,

I'm sorry to say that there's no way to test an NMEA2000 transducer: they're sealed and potted and no diagnostics on the analogue signals can be done. I can rule out some things though: it's definitely not either the network or the display. NMEA2000/SeatalkNG, in common with all modern networks, has extensive and effective error-checking that means that corrupted data will automatically be rejected at the hardware level by the CAN transceiver chip. If you have a network problem the result you will get will be missing or intermittent data rather than wrong data. Similarly, the display shows only what it receives over NMEA2000, it does no modifications or corrections to that speed data. It does calculate True wind from Apparent+STW, or True heading from magnetic+variation, but can't read (for example) 7 knots STW and display 5. The only way to do that would be a re-write of the software.

If there's a fault - and I have to say that your large temperature error makes it sound like there could well be - then it's inside the transducer, you can be sure of that. I'd guess that it's got water inside, I'm sorry to say.

Regards,
Tom

[Azura]

Thanks for the quick response. I suspected the transducer may be failing, but want to make sure before I order a new one.

So am I to understand that the calibration points that are entered via the ST70 are actually stored in the transducer and then applied to the signal before it is sent out over the Seatalk network?

When I order a new DST 800, would I be better off buying an ITC-5 module and a standard transducer, or should I stick with the SeatalkNG version?

[Tom - Raymarine - Moderator]

Quote:the calibration points that are entered via the ST70 are actually stored in the transducer and then applied to the signal before it is sent out over the Seatalk network?

Yes, that's it. The transducer just outputs a calibrated speed value. If it were otherwise and the displaying device applied the calibration then different products in the system could end up displaying different values depending on what local calibration they applied.

Quote:would I be better off buying an ITC-5 module and a standard transducer, or should I stick with the SeatalkNG version?

Personally I prefer the analogue versions rather than NMEA2000/STNG. The 'Smart' versions are harder to diagnose if there's a problem and more expensive to replace if there is (all of your eggs in one basket) compared to analogue.

Regards,
Tom