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Capacitive fuel plate issue?

NovaBandit

Well Known Member
My plane hash capacitive plates in the tanks, and a pair of the Princeton Electronics capacitive converters (single point calibration version), wired up to a Garmin G3X touch system.

My plane first flew in 2015, and the fuel level system was rock solid and perfectly accurate throughout the range.

However, recently, we started having some reading issues with one tank (Left Tank). It would read accurately, but after a bit in flight, would slowly show the levels dropping, eventually to near zero. Sometimes, it will pop back to the actual fuel reading, other times not. Most often the correct level will come back after landing.

Then the other tank started behaving strangely, almost in the total opposite way, where it would show full and not come down while fuel was burned out... but only sometimes.

So far, here's what I've tried/checked so far:
  • Re-terminating the BNC plug wired to the Left tank converter
  • Running a dedicated ground to all wing electrical devices (in case a ground loop was causing issues
  • Confirming that the center pin of the tank BNC socket (the pin wired to the plated) was isolated from the fuel tank itself. 0 ohms measured
  • Recalibrated the 0 set point on the converters
  • Recalibrated the G3X Touch in 5 gal increments after the converter calibration
  • Confirmed that the body of the tank BNC socket is grounded to the tank (tested on the ground, obviously)

At this point, I'm thinking I may order a new converter to see if the issue persists.
I may also try to run some sort of ground strap to the threaded part of the BNC plug to ensure that is well grounded.
I'm nervous that there may be some sort of intermittent issue in the BNC socket itself, and I really really don't want to have to do tank surgery!
I also have a hard time something is wrong with the plates themselves. There are no moving parts, and all connectors were totally encapsulated in proseal.

Has anyone heard of any similar issues, or have any ideas on the other things to troubleshoot?
 
One thing you could do is get a multimeter that measures capacitance. Then you could take some direct measurements on the tank BNC and try to isolate the problem between the fuel tank sensor and the converter.

The capacitance is pretty small on the order of 200pF so if you go down this route make sure you get a meter with enough resolution when it is measuring capacitance.
 
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I was able to get a capacitance reading with my DMM, but only on the ground, and the senders seem to be accurate on the ground. I don't recall what the reading was... and I'm not sure the value itself is actually important.


What could cause these to read differently in the air?
 
[*] Confirming that the center pin of the tank BNC socket (the pin wired to the plated) was isolated from the fuel tank itself. 0 ohms measured

You should be measuring infinite, not zero there.
o ohms would mean there's a short from ground to center pin (or plates to ribs).

Finn
 
You should be measuring infinite, not zero there.
o ohms would mean there's a short from ground to center pin (or plates to ribs).

Finn


You're absolutely right. I typed that wrong. It measured Open (OL) when I tested resistance between the BNC socket center pin and the body of the tank!
 
I have the capacative plates in my fuel tanks with Dynon converters (pretty sure they are Dynon). I had fuel leaking tthrough/around the center pin of the BNC connector attached to the fuel tank. This caused all kinds of erratic readings with the converters.
 
I have the capacative plates in my fuel tanks with Dynon converters (pretty sure they are Dynon). I had fuel leaking tthrough/around the center pin of the BNC connector attached to the fuel tank. This caused all kinds of erratic readings with the converters.


I haven't seen any blue around the connector, but that is my worry. I don't think there is any way to correct that without some tank surgery.


Did you replace your BNC connector? If so, what connector did you switch to that's better suited for that environment?
 
I haven't seen any blue around the connector, but that is my worry. I don't think there is any way to correct that without some tank surgery.


Did you replace your BNC connector? If so, what connector did you switch to that's better suited for that environment?

In my case the fuel was leaking around the pin, so in my infinite lazyness and not wanting to remove the tank or repair/replace the bnc connector, I put 2 tiny orings that were a tight fit over the BNC pin. It was very difficult to get the other part of the BNC connector connected, but once connected, no more leak and has been leak free for a couple years now.

Mine was noticably leaking. If you don't see blue stains or can't feel the fuel, then you probably have a different problem.
 
Hi Jeff
I’m just discovered the same leak from the Bnc, I also noted some bue around and checked that leak from inside the connector.
Could you better explain you way to fix it.
Pic?
Kind of o-rings?
Did you ad some sealant or lube like ez-turn?
Thank in advance
 
Similar leak on B&C tank connector 1400hrs ago...
Opened the tank access panel & liberally plastered the connector & about 8" of the wire leading up to it with proseal. Also prosealed the other end of that wire at the crimped loop terminal so no fuel would migrate up inside the wire & insulation. (paranoid, I didn't want to do that repair again)
 
check to make sure your tanks are perfectly clean. anything floating the fuel has the potential to change the capacitance of the plate.

bob burns

RV-4 N82RB
 
Has anyone heard of any similar issues, or have any ideas on the other things to troubleshoot?
Hi Ed -

I have the same sender/avionics setup as you, and unfortunately I'm seeing the same problem with my left tank fuel levels.

Were you able to resolve your problem, and if so, what did you do? Enquiring minds want to know.. :D

Many thanks -

Dave
 
Hi Jeff
I’m just discovered the same leak from the Bnc, I also noted some bue around and checked that leak from inside the connector.
Could you better explain you way to fix it.
Pic?
Kind of o-rings?
Did you ad some sealant or lube like ez-turn?
Thank in advance

sorry, I was out of town for a few weeks and couldn't go look at what I used.

It was a few years ago now, so I don't remember the details. I bought a couple packs of o-ring off of Amazon, 1 set was buna-n, and the other was Viton, slighly different sizes. I found the buna-n pack but not the viton, and since I have all but 1 buna-n in the pack of 12, and it's still not leaking, I'm guessing it was the viton that fit (I used 2 per tank).

viton: .042 ID x .050 OD
buta-n: .053 ID x .064 OD

https://a.co/d/czJVgEZ
https://a.co/d/ckvtjc8

I did not use any other lube or sealant when I added the orings.
I just slipped 2 o-rings around the pin on 1 half of the bnc connector, and with a lot of force got the other half of the connector to connect.
 
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