Capacitive Fuel Level Bulletproof 1000 hrs - Until Now

[Noah]

My RV-7A has just shy of 1000 hours and the capacitive fuel level gauges (Vans plates in fuel tanks) have always been spot-on, until a couple of months ago, when I noticed they are BOTH reading HIGHER than the levels in the tanks by a few gallons. Dangerous! I have reverted to old-school timing of each tank and bookeeping. My Princeton converters have 5 setpoints feeding AFS 4500 displays. Always dead nuts. Thankfully the red cube fuel flow totalizer has always been spot-on so that's some insurance. I ran a tank dry with the gauge showing 1.5 gal remaining.

Has anybody else had a problem like this? I verified my calibration setpoints haven't changed since Phase I. What kind of failure mode should I be considering? I only use 100LL, so this isn't a mogas fuel mixing issue / capacititve differences with different types of fuel. Should I be looking at the converters? The plates in the tank? Plots of fuel level over time in flight are very linear & consistent, no jumpiness or noisy signals.

[Aluminum]

Originally Posted by Noah

they are BOTH reading HIGHER than the levels in the tanks by a few gallons.

Was there a software update coinciding with the onset of symptoms?

Check the reference voltage to your frequency-to-voltage converter units, and the signal ground on your EMS module (i.e. the analog-to-digital converter).

I can also imagine a one-time contamination event that left a high-dielectric film on the insides of your tanks, but that would likely have caused headlines and lawsuits if it came from the 100LL supply.

[Noah]

Good question, but no, no software updates. Good idea to check the reference voltage to the converters and ground connections. I was wondering about surface corrosion of the plates.

[Captain_John]

Noah... so weird...

What are you going to do?

Assuming corrosion on the plates (or a poor connection to the plates???) re-calibrating and monitoring would probably be a good ongoing procedure.

My converters have been solid for 1,000 hours. Can't see a solid state component failing like that.

Let us know if you find a smoking gun.

CJ

[Mark Albery]

Assuming the geometry of the capacitance plate hasn't changed, then the capacitance being measured will be a function of fuel level and the fuel's dielectric property.

Have you put motor fuel in? that could upset it considerably if there's even a small amount of ethanol mixed in the fuel (ethanol has a constant 12 times higher than gasoline).

Do you know the actual capacitance values measured when you did the calibration? If so you should be able to cross check with a capacitance meter.

[N941WR]

Odd that they would both go out at the same time. Are you sure its not a setting on your engine monitor that was reset somehow?

[Noah]

Originally Posted by Captain_John

Re-calibrating and monitoring would probably be a good ongoing procedure. CJ

Good idea CJ, if I don't find anything wrong, I will do this

Originally Posted by Mark Albery

Have you put motor fuel in? Do you know the actual capacitance values?

No, never. And unfortunately, I do not know the initial raw capacitance values. Would have been a good idea to record those.

Originally Posted by N941WR

Odd that they would both go out at the same time. Are you sure its not a setting on your engine monitor that was reset somehow?

That's what I thought too, but no, I have ruled that out. Same calibration values since initial calibration.

But your point that this happened to both tanks makes me think that concentrating on the individual converters or the tank plates is probably the wrong place to focus. The engine monitor is the only thing common to both. So I think the first thing to check is the reference voltage supplied from the engine monitor to the converters (& ground) as suggested by Dan.

Thanks for the input.

[bkervaski]

Any electrical system upgrades since calibration? i.e., EarthX battery, etc.

[airguy]

Originally Posted by bkervaski

Any electrical system upgrades since calibration? i.e., EarthX battery, etc.

The Dynon EMS outputs a regulated 5V source for sensors that should be completely independent of bus voltage, within reason. Does the AFS EFIS do the same? I agree that I would find this to be a likely suspect.

[Noah]

Originally Posted by bkervaski

Any electrical system upgrades since calibration? i.e., EarthX battery, etc.

Yes, interestingly. Earth X Battery. Can't imagine how this would affect things though, as nominal bus voltages haven't changed.

Originally Posted by airguy

The Dynon EMS outputs a regulated 5V source for sensors that should be completely independent of bus voltage, within reason. Does the AFS EFIS do the same?

Yes, AFAIK.