Does hump affect average fuel consumption significantly?

Definitively needle is moving according to electromagnetic field affected by current through sender resistor in that diagram, since my gauge is older it is most probable that it is using that kind of circuit, newer ones are, I believe, all digital. By the way, the inductance of coils could produce strange readings during battery voltage decrease which I have encountered, although I would expect that to happen at higher transition speeds, but who knows what noise is in boat conductors at that transition from different connected equipment which could reflect on that, too. Or the circuit inside the gauge has some filter to smooth the needle and prevent it jumping which could also reflect on that. Like I said, without detailed schematic, it is only guessing, the circuit that you gave is just a principal diagram.

Because the gauge is showing very roughly amount of fuel in tank only graphically as shown in attached link:

VDO Fuel Tank Gauge 12 Volt http://www.boatpartsandspares.co.uk/vdo-fuel-tank-gauge-12-volt-3075-p.asp

so you can maybe evaluate that there is 1/8 of fuel left in tank or something similar, but you cannot read like 6.8 liters consumed during ten minutes ride or one hour ride, so I measured the values of the sender to acquire the table with more precise values according to the voltage on sender. The table that I made is in the attachment, so I can check very accurate amount of fuel in tank, well, accurate enough actually for my purpose. Values for 20, 40, 60, 80,... liters were written as the fuel was poured in tank and the values in between are calculated using piece wise linear approximation, easy task in Matlab. Amounts of fuel in the table are not the whole values at measured points due to rounding of the sender/battery voltage ratio to three decimals. So I can always know amount of fuel in tank in liters rather than only in percents and fractions of full tank, which helps me plan the voyage more precisely and how much fuel do I have. Of course, reserve is still reserve and not to be used until the tank is empty. This also enabled me to measure/evaluate the fuel consumption in short test period at given RPM, i.e. 10 minutes at planing speed as explained earlier in the thread.

About the increasing/decreasing resistance value of sender with amount of fuel, yes, one must be careful when buying to use appropriate sender for the chosen gauge or vice versa, and for the resistance range, not all have the same resistance either.

 

In order to take out the battery voltage and any transient effects, perhaps it would be better to disconnect the sender from the gauge and just measure the resistance through the sender
After all the sender is a variable resistor, not requiring any input. Just use the ohm setting on your multimeter

 

That is the truth, I could measure resistance in a couple of specific points known from calibrated voltage (to avoid repeating of measuring while pouring 20 by 20 liters in tank) and test it, but the multimeter has the worst precision at lowest ohm scale I think (possible error of a couple ohms), but it could give good result, since resistance is mostly above 20 ohms in used range.

 

You confuse precision with accuracy. Ie if you are out a couple of ohms from a true value of ohms, it does not really matter so long as you are calibrated correctly and the ohmmeter produces the same precision in presenting the same number every time.;
You said that the multimeter has the worst precision, but you mean accuracy. Accuracy is the reference to the actual true value of an ohm.

So you do the calibration, ie by pouring a fixed amount of fuel into the tank, and read the resistance, it does not really matter what it says anywhere along the filling of the tank., Only that it will repeat itself every time.

You could in essence, put a piece of tape over the scale, so long as you can see the needle, and put a line on the tape at each 20 litres or gallons of fuel that you put in to develop the scale

or get a 270 degree sweep to have each change in the sender give you a more visible representation of the amount of fuel used. Of course you would have to calibrate it so each division on the gauge would represent some actual amount fuel in the tank. Note that this is fuel psi but gauges are available in 270 degree sweeps for levels

 

Well, good point, I did mix accuracy and precision according to definition and yes I meant accuracy.

Yes, I agree, accuracy of ohmmeter will not be of importance in this case, but at least 0.5 ohm measurement resolution needs to be used so that fuel amount steps are small enough because sender resistance range is, if memory serves me well, 10 through 90 ohms (well, this resolution is important only for consumption measurement, not for fuel tank level in general though).

I did mark a couple of specific spots on the gauge so I can know some specific values, but all the fuss about accuracy was to measure fuel consumption once, that is not something I intend to do every time I ride the boat, so investing time and money for gauge with wider scale is out of question, good idea though, I have not seen one, although I was only watching vehicles market available in my country, there are others, of course.