Kahuna

Kahuna

Moderatoring
When you build the capacitive sending units into the tanks and close em up. Thats it. Game over. There is no repairing them without complete removal of the rear baffle. ARGH!

Im curious as to the life of the wire and plastic stand offs. I have no idea how long this stuff will last in fuel and fumes.

Are there other non-rv planes in the field that have used this material in the field where we might know the life span? RV'ers are not a good source for data as most are young relatively speaking.

Please no debate here on cap vs resistive units. Im only interested in field data on this material used in fuel tanks, aviation or otherwise.

Thanks
 
Mike,
I had the same concerns. That's why I substituted a quality [made in USA] Amphenol brand electrical bulkhead connector for the import unit supplied by Vans. I also substituted one piece spacers made from Delrin for the 3 piece nylon washers Vans supplies.
Regarding difficulty of repairing those units, you don't have to completely remove the rear baffle. You will have to cut two access holes [in the 2nd and 5th bays] to facilitate repairs. That will require fabrication of doubler rings and access covers, though.
Make sure you apply ProSeal to the wire which is wound around the vent tube. This will keep it secured to the tube. Having the wire loose in the tank is sure to cause problems.
Charlie Kuss
 
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Charlie,
I had one of those Vans supplied bnc-ish connectors leak years ago in my RV-6. THe plastic insulator was dried and cracked. Lesson learned. Encapsulate the connector and wire to prevent fuel getting to it.

Building 4 tanks right now for a replacement set of wings for my 8 for greater range. ARGH!
 
Cool. I'm currently converting the outboard leading edge into an extra set of fuel tanks. After the airplane is flying, I'll build a set of tip tanks. IIRC, you currently had 60 gallons so your range will be a bit more than double.

I think someone has made the outboard tank removable so you could, in theory, fix potential problems. I'm riveting mine on and put a float in. I'm not even sure you're required to have a sender in there. I believe the Cessna 340 doesn't in the nacelle tanks.

Going back to your original question, when I researched the subject, it wasn't as long as a float but long enough that it's not something to worry about. BMW decided to use capacitive senders in their vehicles which failed for other reasons. You may be able to find out how long it took for components to fail on their forums.
 
chaskuss said:
Regarding difficulty of repairing those units, you don't have to completely remove the rear baffle. You will have to cut two access holes [in the 2nd and 5th bays] to facilitate repairs. That will require fabrication of doubler rings and access covers, though.
Make sure you apply ProSeal to the wire which is wound around the vent tube. This will keep it secured to the tube. Having the wire loose in the tank is sure to cause problems.
Charlie Kuss
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I dont think you could replace the wire between bays 2&5 with only access to 2&5. With the wire prosealed in the snap rings to prevent movement and chaffing, the entire rear baffle, or access panels for bays 2-5 would be in order. No way to get the wire through the other center bays. Not fun.
 
You could always use a small fuel line as a conduit. You could then just use an access panel on each side; you wouldn't even have to remove the tank.
 
Kahuna said:
I don't think you could replace the wire between bays 2&5 with only access to 2&5. With the wire Prosealed in the snap rings to prevent movement and chaffing, the entire rear baffle, or access panels for bays 2-5 would be in order. No way to get the wire through the other center bays. Not fun.
Click to expand...

Mike,
If the failure is in the wire between the sender plates, you are correct. For the wire to fail, the builder would have to fail to adequately secure the wire to the vent tube. I wrapped my wire around the tube and then ProSealed it in place. I did this to ensure that sloshing fuel would not dislodge or damage the wiring. Note that I also added a support bracket on the roof of the inboard bay to support the wire where it comes off the vent tube.

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I feel that the most likely source of failure is in the spacers, which insulate the sender plates from the tank. Vans use of "off the shelf" nylon washers is less than ideal. These washers must be stacked 3 deep to obtain the needed 3/8" separation of the plates from the ribs. Another issue with these washers is that their internal diameter is 1/4", not .166" [clearance hole for the #8 mounting screws]. Vans uses .040" thick, 1/4" outside diameter, vinyl tubing to take up this space. The vinyl tubing is placed inside the 3 stacked washers. This vinyl tubing reduces the ID of the spacer assembly to a reasonable level. It also stabilizes the 5 piece [4 washers and the tubing] spacer assembly. Remember that there is a 4th nylon washer that goes between the head of the mounting screw and the tank rib.
My fear is that this vinyl tubing is not close to being fuel proof. What will happen to this tubing over the years, while it is submerged in either 100 LL or auto fuel? I fear that it will degrade to the point where the edge of the .025" thick tank rib could cut through it. When any of the 6 mounting screws [ 3 on each plate] contacts the mounting screw, the sending unit will ground out. The fuel gauge will than fail to function until the spacers are replaced. That of course would be a fair amount of work.
I created spacers made of a single piece of Delrin, as my research indicates that it is a better material, compared to nylon for immersion in gasoline. It also eliminates the need for the use of vinyl tubing. My spacer design would require the plate to cut through 7/16" of Delrin to ground out the plate.
Charlie Kuss
PS If you plan on using auto fuel with ethanol, only certain variants of Delrin should be used.
 
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An outstanding post Charlie and thanks for the thoughtful detail and information.
Highly useful.
Guess I was not the only one worried about it.
Course now that my tanks are closed up, I get to worry MORE!:confused:
 
Where you planning on going???

Kahuna said:
An outstanding post Charlie and thanks for the thoughtful detail and information.
Highly useful.
Guess I was not the only one worried about it.
Course now that my tanks are closed up, I get to worry MORE!:confused:
Click to expand...

Mike,
I originally had no plans to use an engine monitor. Thus, I felt the need for accurate fuel gauges. I later fell into a good deal on a used Rocky Mountain MicroMonitor. I purchased a fuel flow transducer for it. This provides me with very accurate fuel flow measurements, as do all of the available engine monitors.
If I had known I would end up with this capability, I would have forgone the capacitance fuel gauges. With your plan for enormous fuel capacity, I suspect that you "may" be planning to fly over a large body of water! In that case, I'd want ALL methods of checking fuel levels to be as accurate as possible.
I can provide CAD drawings for my spacer design, if desired. Just contact me via email. Since grounding of the sender plates will not damage the other parts of the gauge system, I would not worry to much about your spacers. You could always create the two access points I mentioned earlier, to update the spacers at whatever time [if any] that problems developed. I made my spacers with a shoulder. This shoulder passes through a 7/16" hole in the tank rib. This means that there is almost no chance of the edge of the sender plate wearing through the rib.

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The pieces without the shoulder go between the tank rib and the mounting screw. This spacer fits over the shoulder of the main spacer. The thin piece at the bottom replaces the Viton O-ring on the weighted end of my flop tube.

Charlie
 
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