Units for passed parameters

[CR2O7]

Hi Timo,

I've got a couple of question regarding units:

1. As far as I understand the NMEA2000 are using SI-units in the network, but in your code I see liters uses for volume.
   It that an exception?

2. I did a Google search on the parameters of the PGN127497 message and came up with the following:

   Engine Instance: Discrete Identifier (typically 0-3 for up to 4 engines).
   Trip Fuel Used: Liters ((L)). In the underlying binary message, this is often represented as a volume with a resolution of 1 liter.
   Fuel Rate Average: Liters per Hour ((L/h)). This represents the average consumption rate for the trip.
   Fuel Rate Economy: Liters per Nautical Mile ((L/nm)) or Nautical Miles per Liter ((nm/L)),
   depending on the display configuration; the raw data typically uses volume per distance.
   Instantaneous Fuel Economy: Liters per Nautical Mile ((L/nm)) or Nautical Miles per Liter ((nm/L)). 

For the last two the unit is said to be: L/nm. However, to strictly follow the SI-standard it would be: m3/m or perhaps l/m.

I your code you mention litres/hour for these.

I would very much appreciate if you can bring some clarity into this matter.

Best regards,
Göran

[ttlappalainen]

That is not true. NMEA2000 PGNs has units like RPM, m3/h, minutes, degrees, days, etc.

My goal was to use SI in all parameters, but did stupid mistake with l and l/h. So just live with it.

[CR2O7]

Thanks for the reply!
I still don't understand how the parameters Fuel Rate Economy and Instantaneous Fuel Economy in PGN127497, as per your code, is given the unit liters/h.

In the attached photo of my chart plotter you see the heading "Bränsleek. (L/nm) (eng. Fuel Economy), in the right-hand corner,

Unless you do some magics behind the scene, I can't see how l/h can be converted into l/nm.

Best regards,
Göran

20250617_105052.zip

[ttlappalainen]

PGN127497 has all values m3/h with 10-4 accuracy. That translates to l/h with 0.1 accuracy.

Since MFD handles several PGNs, it can calculate l/nm by using your speed. l/nm makes more sense finding best speed. Fuel sensors normally measures just simple flow and so does not calculate flow/distance. On the other hand there is no PGN to carry flow/distance so MFDs does the calculation.

[CR2O7]

Thanks, I now see the light!