I saw someone put X litres into a CJ4 and the FMS showed the total weight of the fuel within 10kg of his calculation, which is pretty good.
Fairly obviously they must have fuel totalisers (a turbine in the fuel flow) on the way to the engines but I am pretty certain they don't have one on the pipe from the central refuelling point.
So it must work by measuring the fuel level in the tanks, or perhaps the pressure at the bottom of the fuel tank(s).
With a clever multiple level sensor arrangement one could get the fuel level accurate but it will be very sensitive to aircraft movement. Whereas pressure can be measured very accurately, and it would indicate weight directly so no need to compensate for the fuel temperature which you would need to do if converting a fuel level to weight.
I wonder how they do it...
The Meridian uses capacitative sensors in the tanks. They are very accurate.
And it uses flow transducers not impellers/turbines in the fuel flow.
The Meridian uses capacitative sensors in the tanks. They are very accurate.
Most jets do it the same way. There are several sensors in each tank whose individual measurements are averaged so that the result is almost unaffected by aircraft attitude and sloshing. The inaccuracy lies more in the very variable density of jet fuel than in the measured fuel level, especially when the contents is displayed as mass (e.g. Pounds). The rather coarse tape indicators of our Citations allow reading the fuel quantity within +/- 50lb (of a total of around 5000lb) which is an accuracy of one percent. Good enough!
As well as cost, I assume the reason that these sensors are not used so much in GA is that turbine engines (at least mine) are far more critical wrt to fuelling vs range. ie for a given fuel load there is very little you can do to affect range other than fly at the optimum altitude. You can slow down - that increases time aloft but does very little to improve your range. As a result you have to pay very careful attention to fuel - more so than in my piston time.
PS - and what next mine gives it to me in 10lb increments on an 1150lb load. So pretty similar levels of accuracy.
The rather coarse tape indicators of our Citations allow reading the fuel quantity within +/- 50lb (of a total of around 5000lb) which is an accuracy of one percent. Good enough!
The resolution is not a problem to achieve. Resolution is cheap 
It is the absolute accuracy which is impressive.
I have "expensive" capacitive fuel gauges in the TB20GT and the indicators are accurate to about the thickness of the needle i.e. a few % of full scale deflection.
But the CJ4 accuracy was of the order of 0.5%.
What sensors do they use?
More sensors and averaging?
In the Meridian there are three tanks on each side - a main (outboard tank) holding 64 USG, an inner tank holding 12 USG and a header tank holding 8.8 USG. There are three sensors in the outboard and one each in the two inboard tanks. So 5 per side. They are wired in parallel so the total capacitance is summed and used to compute the fuel load by the DAU.
I have "expensive" capacitive fuel gauges in the TB20GT
Peter, are they standard or aftermarket? Float gauges in my tanks are totally useless, and I am thinking of replacing them with something better - any recommendations?
There is an interesting STC fuel sender being developed by www dot ciescorp dot com. They are currently available for the Cirrus. The Mirage people are quite excited as the senders in the PA-46 (ex-Meridian) are notoriously bad.
