Capacitive fuel senders, and problems with them

[airguy]

Ok, so I've probably got a great big "Told ya so..." coming for this, and I'll take my lumps as required.

During my build of my fuel tanks, capacitive senders were all the new tech, everyone was talking about them like they were just absolutely gonna be the berries, and I drank the Koolaid. I built my tanks with capacitive senders, and in a wave of unsubstantiated enthusiasm, I did not include the standard float senders as a backup, though they would have happily co-existed in the tank.

The capacitive senders operate by means of an AC voltage applied to a couple of plates in the tank that are insulated from the tank itself, and this forms a capacitor that charges and discharges according to known electrical rules. The fuel is a dielectric between the plates that has a different value from air between the plates, and thus the computer can calculate the level of fuel in the tank from the capacitance characteristics of the plates.

This would probably work fine for most people - but I set my airplane up to run 91E10 autofuel, and it has a different dielectric than 100LL. No problem, I thought - I calibrated them for 100LL, and again for 91E10, and I'll just switch back and forth as needed when I change fuels. Well, as it turns out in practice, I nearly always have some mix of 100LL (due to long trips and fueling enroute) and 91E10 (which I have in the hangar and always top off the tanks with when I shutdown). The result is that I very rarely have a nearly-pure concentration of either 100LL or 91E10 in the tanks, and the capacitive senders are almost always inaccurate to some degree. This has caused me to rely on the fuel flow sender and fuel computer almost entirely, which has fortunately turned out to be incredibly accurate, within 0.5% from my observations over the last 135 hours.

Finally enough was enough - everything else works like it's supposed to on the airplane, and this should too. I ordered replacement access plates, nutplates, and float senders, and then proceeded with surgery this afternoon. I'm going to abandon the capacitive plates in place, no point in trying to remove them, I'm just putting the standard floats in place. They will respond the same no matter what fuel, or mix of fuel, I have in the tanks. Lesson learned and shared for others playing with autofuel.

For the record - it's right at 3 hours to pull a single tank.

[N941WR]

I too installed the capacitance plates in my tanks and use the Dynon converter.

Since I only run 100LL, they are dead accurate. Every time I top off, I'm surprised at how good they are.

One thing to think about, you can plug the capacitance senders into to tanks 1 & 2 and make the floats tanks 3 & 4. Not that you really need to with the floats in place.

[RV7A Flyer]

As I understand it, due to dihedral, won't the floats max out before the tanks are full? Just something to be aware of.

I have the capacative sensors and, while very accurate, my calibration was off just a wee bit...at first, they read about 1 gallon high from the mid-point down to a few gallons. I kept careful logs, plotted the curves, ran the interpolants and adjusted them manually in the Dynon menu. Now, they read about 1 gallon *low* from the mid-point on down. Arrrrghhh...I don't want to recalibrate them, as it is a PITA. So I'll probably just live with it for now, since it's "safe", and the fuel flow/totalizer function is dead nuts on.

[MikeyDale]

Only problem I've got with the float/Dynon is gauges don't bounce! I always knew my Pacer tank was almost empty by the amount of bouncing my fuel gauge needles had to them!.......Actually, the only problem I have with the floats senders on my taildragger is they show about 4 more gallons per tank than they actually have when I'm on the ground with the tail down. But, that just reinforces my resolve to never fully trust a fuel gauge!

[Tankerpilot75]

I think once you've installed the float senders and gone through the calibration process with them you'll figure out they are even less accurate than your capacity sensors. I'm glad your leaving them in place. Also leave the wiring alone and run new wires for your float sensors. That way you can convert back if you want to.

I, like most RV owners, have the float sensor system. It's accurate at empty (I hope) but inaccurate from full to near empty. I use the float sensor readings to estimate my fuel balance in each tank but rely on fuel flow readings (totalizer function) for actual fuel on board. Like the other commentator- it's much more accurate.

Others can comment.

[SledDog]

My wings are not mounted yet and I installed the capacitive senders. Is it a good idea to also install the float senders while it's fairly easy to do? There might be a good reason Vans stopped selling the capacitive sender kit.

[airguy]

Originally Posted by MikeyDale

Only problem I've got with the float/Dynon is gauges don't bounce! I always knew my Pacer tank was almost empty by the amount of bouncing my fuel gauge needles had to them!.......Actually, the only problem I have with the floats senders on my taildragger is they show about 4 more gallons per tank than they actually have when I'm on the ground with the tail down. But, that just reinforces my resolve to never fully trust a fuel gauge!

You partially solve the bounce problem with the Dynon - there is a way to select how much time-averaging is done on the quantity sender between a couple seconds to a couple minutes, the default is a couple minutes to prevent the bounce but you can make it very short.

Originally Posted by Tankerpilot75

I think once you've installed the float senders and gone through the calibration process with them you'll figure out they are even less accurate than your capacity sensors. I'm glad your leaving them in place. Also leave the wiring alone and run new wires for your float sensors. That way you can convert back if you want to.

I, like most RV owners, have the float sensor system. It's accurate at empty (I hope) but inaccurate from full to near empty. I use the float sensor readings to estimate my fuel balance in each tank but rely on fuel flow readings (totalizer function) for actual fuel on board. Like the other commentator- it's much more accurate.

Others can comment.

The capacitive senders are quite accurate when you have the calibration right, and for a single type of fuel. My fueling habits are what was killing the accuracy - and I'm not likely to change those fueling habits - so I went back to the standard floats. I'm using floats now in the outboard auxiliary tanks and I'm quite happy with their response, I just need something in the inboards that responds better to a mixed-fuel condition. The capacitive senders would frequently be off by 3-4 gallons with a mixture of 100LL and 91E10, that was unacceptable. The floats will respond the same to the liquid level regardless of composition.

Originally Posted by SledDog

My wings are not mounted yet and I installed the capacitive senders. Is it a good idea to also install the float senders while it's fairly easy to do? There might be a good reason Vans stopped selling the capacitive sender kit.

I would, yes. They will happily co-exist and might save you grief down the road. To be fair, the capacitive senders were good as long as I kept a single type of fuel in the tank - it's just difficult for me to do that when fueling at home with 91E10 and getting 100LL enroute.

[N941WR]

Originally Posted by SledDog

My wings are not mounted yet and I installed the capacitive senders. Is it a good idea to also install the float senders while it's fairly easy to do? There might be a good reason Vans stopped selling the capacitive sender kit.

IIRC, someone at Van's told me they didn't sell that many.

I love mine and would not want to go backwards in time to floats. The issue the OP had was when switching between fuels.

If unleaded avgas becomes available nation wide, I will recalibrate mine. Doing that is not difficult.

[ppilotmike]

Originally Posted by N941WR

IIRC, someone at Van's told me they didn't sell that many.

I love mine and would not want to go backwards in time to floats. The issue the OP had was when switching between fuels.

If unleaded avgas becomes available nation wide, I will recalibrate mine. Doing that is not difficult.

I'm not flying, but I used capacitors and also installed the floats, as a cheap, lightweight back-up. I expect the cap senders to be very accurate and went to great lengths to make sure they were installed correctly.

[Snowflake]

Knowing how accurate my fuel flow sender is, I wish I had bought two and installed one on each tank. It seems to me that measuring fuel used, and subtracting it from what was there at takeoff, while not *directly* reading the level, is far more accurate than my float-type senders.