Capacative fuel gauge senders.......... Huh ?

I'm still a builder, and not yet a pilot (saving that for the last year of the build).

Anyway, in finishing up the prep work on the fuel tanks I see this note that says, "If you plan to use the capacative fuel tank senders offered in the accessories catalog, you should do that now".

Problem is, I don't know what that is (I mean, I have an idea, but not if I NEED it or not), and second, I can NOT find this in any keyword on the catalog.

So, DO I need it? And if I do, how to find it on the catalog ?


Appreciate the help

With a capacitive sender, you build a capacitor (think two parallel plates) into the tanks. The tank is one plate, you build the second ‘plate’ into the tank. As the space between them is filled with fuel, the dielectric constant changes, so the capacitance changes. You calibrate the capacitance vs the amount of fuel. Properly done, they are very accurate.
No you don’t need one. The stock float gauges, calibrated thru an EFIS, are quite accurate except they can’t see the top 5 gallons. And, for good engine management, you also really want a fuel flow gauge. They can tell you how much gas is left to very good accuracy.

Edit. Think about speeding up the Private pilot rating. Things change with time, but today, if you approached an insurance company with typical 1 year’s experience for insuring a 10, be prepared for sticker shock.

BobTurner said:

With a capacitive sender, you build a capacitor (think two parallel plates) into the tanks. The tank is one plate, you build the second ?plate? into the tank. As the space between them is filled with fuel, the dielectric constant changes, so the capacitance changes. You calibrate the capacitance vs the amount of fuel. Properly done, they are very accurate.
No you don?t need one. The stock float gauges, calibrated thru an EFIS, are quite accurate except they can?t see the top 5 gallons. And, for good engine management, you also really want a fuel flow gauge. They can tell you how much gas is left to very good accuracy.

Edit. Think about speeding up the Private pilot rating. Things change with time, but today, if you approached an insurance company with typical 1 year?s experience for insuring a 10, be prepared for sticker shock.

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When shopping for insurance on our partnership we were told that most companies want 300 hours total time for insuring a pilot in a 10. One of our pilots didn't have that. Sticker shock was an appropriate term.

I'm not sure VANS is selling the capacative kit anymore (at least I couldn't find it when I was looking a few months ago). Not sure what the story was.

But as Bob pointed out, it's just a different way to measure how much fuel is in your tanks with no moving parts. Downside from what I understand is changes in fuel (100LL to MoGAS or Swift) would require a re-calibration of your fuel gauges due to differences in electrical resistance of the fuels.

Another thing to think about is installing a return fuel line. It's not absolutely needed, but so much easier to install while building the tank than after the tank is finished.

Thank you all for the excellent feedback....

BrianDC, what is the purpose for this return fuel line? I assume it's in the 'intake' fuel line, what is the need for it to not be there?

TXHardWorker said:

Thank you all for the excellent feedback....

BrianDC, what is the purpose for this return fuel line? I assume it's in the 'intake' fuel line, what is the need for it to not be there?

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Depending on your engine and fuel system setup, you may not need a fuel return line.
Some fuel injection systems have the ability to recirculate fuel back to the fuel tank. This has a benefit for hot starts as the fuel lines shouldn't experience vapor lock. Downside is fuel line plumbing gets a little more complicated.

BrianDC said:

Depending on your engine and fuel system setup, you may not need a fuel return line.
Some fuel injection systems have the ability to recirculate fuel back to the fuel tank. This has a benefit for hot starts as the fuel lines shouldn't experience vapor lock. Downside is fuel line plumbing gets a little more complicated.

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You can accomplish this without plumbing the return all the way back to a tank. I have the AFP FI system with purge valve and my return is plumbed back to just below the fuel selector valve.

The simplest and easiest plan is to stay as close to the plans as possible. Most of the mods that you'll learn about are builder preference or "nice to haves" that simply aren't necessary and won't make the plane fly any better.

For clarity, some fuel systems *need* a return line to a (or both) tanks. Most of the automotive style injection systems (SDS, etc) need them. It's optional with 'traditional' dribble injection systems (Bendix, etc) to clear hot fuel from the engine compartment prior to start. If you think you'll ever have an injected engine, planning for a return line now (at minimal pain/expense) will make life a lot easier later.

Capacitive level sensors have the downside that their calibration changes with fuel type. If you never expect to run anything but 100LL, it doesn't matter, but any auto fuel and likely any future no-lead aviation fuel will have a different capacitance, and will therefore affect calibration of the sensors. Switching among or mixing the various fuels (likely to happen frequently in the future, as we transition off 100LL) will cause changes in calibration.

Take a look at the RV14 fuel tanks. Virtually identical to the RV10 and they were designed with a fuel return fitting. Really easy and inexpensive to buy and install the parts when building the tanks. It?s what I did with mine for return lines required by the SDS fuel system design.

Dielectric constant

I didn't install the plates in my RV-4 tanks for the reason Charlie gave (MoGas dielectric constant change with "summer"/"winter" fuel and alcohol content).

Now I regret it. I think that by installing a separate plate near the bottom inboard skin of the tank that's always covered by fuel, one would have a reference capacitance that could be used to correct the output. I realize that there's probably no capacitance-to-voltage-converters (you'll need one if installing the capacitive fuel gauge senders) on the market that have a such a reference input, but I'd have built them myself.

I understand that Van's kit is (was) a plate mounted next to the outboard rib and a plate mounted next to the inboard rib (the two plates wired together), so as to be able to sense from full to empty. In my old RV-3 I made a tube within a tube (with insulating spacers) that ran through holes in the ribs from inboard lowest point to outboard highest point. Outer tube had holes at regular intervals for fuel to enter. That gave a continuous reading from full to empty.

More than you probably ever wanted to know

Finn

This situation sort of addresses a situation I've known would be coming, but not when....

Obviously, these plans are 'stock'. And as an inexperienced aviator and builder, it's difficult to know ahead what options would be best, because I don't even really know what the options are or their pros and cons. I'd like to build as smart as possible, but it's hard to know what you don't know....

TXHardWorker said:

This situation sort of addresses a situation I've known would be coming, but not when....

Obviously, these plans are 'stock'. And as an inexperienced aviator and builder, it's difficult to know ahead what options would be best, because I don't even really know what the options are or their pros and cons. I'd like to build as smart as possible, but it's hard to know what you don't know....

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Here's a thread that will help shed some light on the art of the possible: http://www.vansairforce.com/community/showthread.php?t=149298

You are going to get a lot of opinions here - but don't forget the goal is to build the airplane you want, not someone else's. But like you said, you don't know what you don't know. Getting your ticket will help in that regard. Also it will help to see other's planes, whether local to you or if you can get to OSH or Sun-n-Fun.

Andrew,
I am just finishing up my tanks right now. From what I've read, builders have had problems with the plates. Van's no longer sells their capacitance plate system by many builders here have gone with the Princeton Capacitance system. You can either build your own plates and buy a converter, or simply install a Princeton Capacitance probe in your tank.

Todd at Princeton sent me this email:

"If you order or standard probe at 18? with a 5? bendable section and 1 set-point, it will work for you. Just annotate in the comments if you?d like us to bend it for you."

I ordered a set and am waiting for them to arrive. If you'd like email Todd, here is his email address: [email protected]

Auburntsts said:

Here's a thread that will help shed some light on the art of the possible: http://www.vansairforce.com/community/showthread.php?t=149298

You are going to get a lot of opinions here - but don't forget the goal is to build the airplane you want, not someone else's. But like you said, you don't know what you don't know. Getting your ticket will help in that regard. Also it will help to see other's planes, whether local to you or if you can get to OSH or Sun-n-Fun.

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Yep, nothing that says you can't just follow the plans and keep everything stock. Mods and suggestions here are just that, and as Todd points out everyone will have opinions. Spend time reading threads, and through some of the builder logs that others have posted. Lean from what they have done and make your own decisions.
Will also agree that you should get your license and spend some time flying. You will ABSOLUTELY want that experience to determine what avionics you want to suit how you fly and your mission. Watch "homebuilder Help", "good plane living", "jason ellis", and others on youtube (view their archived videos).
This is why I love this community, lots of knowledge and LOTS to learn!

Jason Ellis is one I watch! But, I keep it in track as to where I am.

Thank you all again.