How to calibrate STW from transducer?

[LifePart2]

I have an airmar STW transducer that goes through a Raymarine ITC5 converter to turn it into NMEA2000 data that can then display on OpenCPN.

But, there is no way to calibrate the data, so my STW shows in OpenCPN as 2.1kt when it should show 4kt.

Is there some way to apply a calibration factor to the data so that it displays the 4kt? I use this only for checking on how much current we have, so even just applying a factor within the dashboard display would do the job.

The same applies to the depth finder where I just want to subract 1m from the data to bring it down to below the keel instead of below the transducer.

I am on a RPI5 with macArthur HAT and openPlotter4

Any suggestions welcome!
Thanks

[Hakan]

Noel..
The spec for PGN128259 says "WaterReferenced: Speed over water in m/s" so in Dashboard we convert received value from m/s to knots (* 1.943844) when parsed to the instruments.
I'm sorry there is no GUI-way to change this in O.
Could it be that your source is accidentally using knots?

[Hakan]

Originally Posted by LifePart2

The same applies to the depth finder where I just want to subract 1m from the data to bring it down to below the keel instead of below the transducer.

That's possible to adjust in Dashboard->Preferences->Advanced->Depth offset...
Håkan

[wholybee]

Calibration of the ITC-5 is done on a Raymarine i70.

[LifePart2]

Originally Posted by Hakan

That's possible to adjust in Dashboard->Preferences->Advanced->Depth offset...
Håkan

Thanks. Perfect. Nothing like that for the STW though?

[LifePart2]

Originally Posted by wholybee

Calibration of the ITC-5 is done on a Raymarine i70.

Yes, but that would mean buying a new display instrument which I don't need just to do the adjustment. I figured there must be a way in either SK or O to tweak the numbers.

I could do it with the nmea converter plugin, but then the dashboard alternates between the raw and tweaked values, and I don't know how to filter out the raw value so it just shows the derived value $IIVHW

Noel

[Hakan]

Noel..
Hmm. Do you mean you are receiving both N2k PGN128259 and N0183 VHW?
(VHW uses knots so no need to convert)
Anyhow, Dashboard prioritizes N2k over N0183, for the same value, and we also presume it's following the NMEA standard using SI-units, i.e m/s in this case.
So, whoever uses knots in PGN128259 is doing it incorrectly.

[stevead]

As Warren & Hakan have respectively stated, calibration of the ITC-5 is performed from a Raymarine display and all NMEA 2000 data is in SI units.

Suggest you log/verify the NMEA 2000 data received by your MacArthur HAT using the candump utility. IIRC the CAN interface supports socketCAN and default is can0.

Or verify in the SignalK Data Browser.

Extremely unlikely that the ITC-5 doesn't transmit PGN 128259 using SI units so perhaps the previous owner misconfigured the ITC-5. Also, although extremely unlikely that the SignalK to NMEA 0183 plugIn is at fault.

Also double check the Speed Units for the Dasboard Preferences.

[barcoMeCasa]

signalk has an app to do just that, I use it to calibrate for nmea compass deviation on a steel boat.
https://github.com/SignalK/calibration

Easiest way to get the sigK data to Opencpn could be send it out as nmea0183 from the sigk to nmea app.

[Corsair63]

STW is really difficult to say what is really the correct reading, it depends of several factors, what is read from transducer should be taken as correct unless you find that there are errors.

what is stated in the second message of this thread is that the device is giving in m/s and O is displaying in knots, with plugin nmeaconverter_pi it's really easy to do the conversion and if you know the error you could do some trick with that plugin and got new nmea sentence with the "correct" reading.

I do recommend only to do the conversion from m/s to knots.

while SOG is not depending of another factors just from GNSS positions STW is the reading from a paddle or ultrasonic transducer that reads too much environmental speeds and that's the value displayed plus in the paddle ones that could be something in the mechanism that could lead into non trust-able readings.

sometimes you could notice it while sailing at constant speed that SOG is more or less stable but STW is showing little jumps in the value.

[Hakan]

- We must assume every manufacturer does follow the NMEA standards. What else could we do?
- Plugin nmeaconverter can't handle N2k, what I know.


And don't confuse this discussion with desired unit to display in individual instruments. That's managed in Dashboard's preferences.

[wholybee]

Originally Posted by LifePart2

Yes, but that would mean buying a new display instrument which I don't need just to do the adjustment. I figured there must be a way in either SK or O to tweak the numbers.

I could do it with the nmea converter plugin, but then the dashboard alternates between the raw and tweaked values, and I don't know how to filter out the raw value so it just shows the derived value $IIVHW

Noel

Before doing anything in SK or anywhere to tweak the numbers, how about asking another boater to borrow a Raymarine i70? Connect it to your ITC-5, and see
1) if it is reading the correct speed, and
2) calibrate it so that it does.

If you don't do those things, you are asking for trouble down the road. For example, the STW from the ITC-5 is also used to calculate TWS and TWA. So if you tweak STW in SK or O, then you will be left with inexplicitly wrong TWS and TWA values.

In theory someone could have written some PC software that would correctly calibrate the ITC-5, and that would also work. But that would require a pretty good understanding of Raymarine's proprietary PGNs, and I don't think anyone has decoded that and written the SW.

[LifePart2]

Originally Posted by wholybee

Before doing anything in SK or anywhere to tweak the numbers, how about asking another boater to borrow a Raymarine i70? Connect it to your ITC-5, and see
1) if it is reading the correct speed, and
2) calibrate it so that it does.
.

Is the callibration done by the i70 persistent, ie it changes something in the transducer itself? Or just transient while receiving data which it modifies on the fly?

The transducer was previously calibrated with an ST60, but that went when I switched over to NMEA2000. So that calibration was obviously transient. I presumed that the i70 would be the same, thus necessitating having it permanently connected.

[LifePart2]

Originally Posted by Corsair63

STW is really difficult to say what is really the correct reading, it depends of several factors, what is read from transducer should be taken as correct unless you find that there are errors.

what is stated in the second message of this thread is that the device is giving in m/s and O is displaying in knots, with plugin nmeaconverter_pi it's really easy to do the conversion and if you know the error you could do some trick with that plugin and got new nmea sentence with the "correct" reading.

I do recommend only to do the conversion from m/s to knots.

while SOG is not depending of another factors just from GNSS positions STW is the reading from a paddle or ultrasonic transducer that reads too much environmental speeds and that's the value displayed plus in the paddle ones that could be something in the mechanism that could lead into non trust-able readings.

sometimes you could notice it while sailing at constant speed that SOG is more or less stable but STW is showing little jumps in the value.

The dashboard is displaying in knots. I don't know what units the airmar is sending it as. But presumably the ITC-5 indicates which units are used when it converts that to NMEA2000 (actually seatalkNG). And presumable dashboard can read that unit indicator and make the conversions necessary to display the desired units.

I am aware of the second to second variations of STW and how it compares to SOG.

Hmm. Do you mean you are receiving both N2k PGN128259 and N0183 VHW?

signalk has an app to do just that, I use it to calibrate for nmea compass deviation on a steel boat.
https://github.com/SignalK/calibration

The ITC-5 converts to N2K. I then used the SK Calibration Plugin to try to tweak that. The problem with that plugin is it seems to use a lookup table to apply conversions, so I would have to enter a line for every single speed, I could not find anywhere in it to just put in a single multiplication factor. Maybe I missed something there, as that would seem to be the right solution.

Failing that I then used the SK to NMEA0183 plugin to convert that to nmea so that I can try using the O plugin to convert it. But that didn't work, presumably because dashboard prioritises SK data.

BTW, thank you everyone for chiming in here. Your thoughts are definitely welcome.

Noel

[stevead]

Don't confuse Seatalk and ST60 Displays with NMEA2000 (Seatalkng), ITC-5 Converters and i70 displays.

When you were using ST60 Displays, each analog transducer was connected to a display and the calibration was stored in the ST60 displays.

Now that you have migrated to NMEA2000 the analog transducers are connected to the ITC-5 and all calibration settings are stored in the ITC-5.

You need to buy/borrow a Raymarine display to calibrate the ITC-5 for all of the sensors connected to it (Depth Offset, Speed through Water, etc.)

Who knows what values the ITC-5 is generating for PGN 128259 for the analog pulses received from the speed transducer when it has not been correctly calibrated. As we have stated umpteen times, all NMEA 2000 Data is in SI units. The ITC-5 supports the NMEA 2000 standard and the units are not selectable.

As the old saying goes, "Garbage in, Garbage out"

It is the displays themselves be it OpenCPN, or an I70 where you choose to display knots, kilometres per hour or m/s.

And if you did perform the calibration it would be great if you could capture the NMEA 2000 Data so that we can reverse engineer the proprietary Raymarine PGN's.