Capacitance fuel gage calibration failure

[bobmarkert]

I am having trouble calibrating the capacitance fuel quantity system in my RV-8. I have the Princeton 2 point capacitance computers and VANs capacitance plates feeding a Gramin G3X. The left tank calibrated perfectly. The right tank failed calibration and does not register the first 8 gallons of fuel. From 8 to 21 gallons, the system can be calibrated. Below is my troubleshooting procedure and thoughts about this problem. Input from those smarter than me appreciated.

When the tank did not register the first 8 gallons (no voltage change) during the first calibration attempt:
1. I recalibrated the Princeton computers. I drained the tank and set the empty point (reading was 0.118 volts), and refilled the tank and set the full point (reading was 4.963 volts). I then drained the tank to start the calibration process.
2. I added fuel two gallons at a time. There was no voltage change for the first 7-8 gallons.

Conclusion:
1. The inboard (lower in the dihedral) capacitance plate is not sensing the fuel.
2. The entire voltage change (0.118 v – 4.963 v) is occurring on the outboard capacitance plate once the fuel reaches it (~8 gallons).

Troubleshooting:
1. I re-drained the tank
2. I used a meter to check the connections. The outer lead goes to ground on the airframe as I believe it should and the inner connection has no continuity…. which I also believe is correct as the fuel bridges the gap between the plates and the fuel tank skin to register the capacitance.

Is it possible that an aluminum oxide has formed on the plate and is acting as an insulator? The tank was built 2 years ago and is just now being put in service. I am considering removing the tank and cutting into it to scuff up the plate in the hope that it will register the fuel…….sigh

Any thoughts or suggestions?

I’ve also sent this to Princeton support and haven’t heard back yet

the first picture below show the failed calibration curve in the right tank. The second picture shows the left tank which calibrated properly


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Thank You
Bob

[wirejock]

Possibly an open connection at the inboard plate?

[9GT]

Contact Steve the G3expert. I had a very similar issue and he walked me through a fix. May not apply in your case but my screen shot was very similar to yours.

[longranger]

Originally Posted by wirejock

Possibly an open connection at the inboard plate?

My first thought as well. You can reach the inboard plate through the cover to check continuity to the external connector:

[DrillBit]

Originally Posted by bobmarkert

Is it possible that an aluminum oxide has formed on the plate and is acting as an insulator? The tank was built 2 years ago and is just now being put in service. I am considering removing the tank and cutting into it to scuff up the plate in the hope that it will register the fuel??.sigh

Oxidation at the joint where the PIDG connector bolts to the plate--yes, problem.

Oxidation of the plate itself--no problem, unless it's turned to Al2O3 completely which seems unlikely.

Scuffing the plate won't make much difference. The thin (about 0.7 nm, IIRC) so-called native oxide on the Alclad surface grows right back. The plate could be anodized (which is simply a thicker oxide layer grown on purpose) and the capacitance change going from air to fuel will still be about the same.

[rzbill]

I measured the capacitance of my tanks when they were empty at 180 picofarad. If yours measure 90 picofarad, then one plate is not in the circuit (for the reasons already posted).

I did not measure them when they were full but I have read that avgas doubles the capacitance (so properly working, a full tank would measure 360 picofarad). If you measure yours full then a capacitance in the 180 ish range would again say one plate is not in the circuit.

[bobmarkert]

Update

I talked to Todd at Princeton and he suggested I swap computers and see what happens. Dooooohhh! That was it, a bad computer; I can now calibrate my right tank. I don't have to bust the tank open. He wants both computers back to asses them and has provided great service. Another lesson re-learned