Originally Posted by HighFive
Does BMW not know how to measure volume progressively in the tanks they build? Seriously....not being sarcastic. Seems they would use the same mold to make the tanks the same way, and then be able to measure volume (by sensor height incrementally, very accurately. But what do I know?
As Joel explained, the tank is not a perfectly regular shape. If it was, the sensor could span the entire vertical extent of the tank and be very accurate. However, it's full of bumps and protrusions and curves, and the floor is not level. So while the tank volume can be calculated quite precisely, the quantity of fuel within it cannot.
The fuel level sensor only occupies the lower portion of the tank. For simplicity, let's say it can measure the bottom half of the tank. It will thus tell the bike the tank is "full" if you have half a tank, a full tank, 5/8ths of a tank, 9/10ths of a tank, etc. Because it has no idea how much fuel is above its highest reading, it (politely) declines to guess, and thus you'll only see the fuel level begin to dip when you've used that first half of the tank. This is the same on my Wee.
It would, hypothetically, be possible to calculate instant fuel consumption (the OBC already does this), subtract that from the full tank volume, and display a full-tank level gauge. However, to be accurate this would rely on the rider filling the tank to the same level every time and then resetting the fuel gauge manually - and can you imagine the whining if BMW made that
I use a hybrid OBC/brain-o-matic distance-to-empty method - assume the tank is 15 litres (giving me a 1-point-something-litre margin for error), and use the OBC's calculated average fuel consumption (which I reset each tankful). If it says, for example, 5.0L/100km, then my range is 300 and I'll start thinking about fuel at 250.
This is less easy for those of you saddled with miles, gallons and cubits, but still do-able