Fuel senders..Capacitance, floats or both?

[Mel]

100LL formula does NOT change with the seasons.

[FinnFlyer]

Originally Posted by Mel

100LL formula does NOT change with the seasons.

I didn't think so. So would be interesting if the current reading and discrepancy is due to having other than 100LL in the tank, or if the tank has only seen 100LL.

Finn

[OZCleco]

Originally Posted by FinnFlyer

I didn't think so. So would be interesting if the current reading and discrepancy is due to having other than 100LL in the tank, or if the tank has only seen 100LL.

Finn

No, our particular aircraft only sees 100LL.

I'm the only one that's filled it so far and it's always been from a BP AvGas Bowser or BP AvGas fuel truck. Mogas isn't offered anywhere that would have given it a chance to enter the system, besides delivery via Jerry can, which I haven't done.

I am wondering if the density/SG relative to temperature changes the resistance of the fuel. The calibrated spread sensed by the Dynon system is in the order of only a few millivolts.

[Taltruda]

Originally Posted by FinnFlyer

The cheap RV-8 capacitive sender (an isolated plate close to tank root rib and an isolated plate close to the outboard tank rib) should give you the option of full range readout. The float senders top out and don't show the difference between a full tank and about a 5 gallons from a full tank.

But, as mentioned earlier, you'll have to recalibrate the capacitive senders depending on type of gas used.

However, as I've written many times before, adding a third isolated plate near the inboard bottom of the tank (always covered by fuel), with its own separate output, would solve that issue. The electronics, that read the capacitance and converts it to a voltage for EFIS or other gauges, would have to be improved with a second input to be used as a reference capacitance of whatever fuel is in the tank. (any season of MoGas, AvGas or mixture thereof.)

I wish I'd done that in my RV-4 tanks.

I wonder if there is a market for such an electronic device? If so, I could make and sell them.

Finn

Finn, is this an option with Dynon or Garmin displays? Or is it something that I would have to figure out with an ardwino?

[Taltruda]

Thank you all for your input! I love this forum!

Any other comments about the fuel caps? I heard the upgraded RV14 caps may work if installed while building, but then I read that maybe the curvature of the flange might not match the RV8 airfoil? Andair has a retrofit cap, and Newton may have a nicer quality cap too...

[FinnFlyer]

Tom, I'm not familiar with the EFIS units, but I assume they want something like 0 to 5V.

Using an Arduino Nano or similar would not be a bad idea. It would give you a lot of flexibility in using the measured reference capacitance to correct the measured tank level capacitance (add/subtract/multiply/divide -- whatever you find is needed by experimentation).

But first you'd need two capacitance-to-voltage converters for each tank.
Long time ago I made that with an LM324 for my old RV-3. First a simple oscillator where the tank capacitance controls the frequency. Then a frequency-to-voltage converter using the other half of the LM324. I would have to dig around to find the schematic. However, a brief Google search reveals that there now are chips that will do that for you. CAV444. Actually, you could use the Arduino to measure the frequencies and would just need the capacitance-to-frequency converters. If you look in application notes for LM324, 555 timer IC, or any op-amp, you'll find example of them being used as oscillators.
Whatever chip you decide to use to measure the capacitances, place it real close (few inches) to the capacitive outputs from the tank (one chip/circuit per tank).

I don't have the data, but I suspect the capacitances to be measured are in the 10pF to 1nF range. I certainly could be wrong here. The bigger the plates and the closer together they are, the higher the capacitances.

Finn

[Doggtyred]

No. 100LL is a single specification, year round.

[gereed75]

Any experience with these guys https://ciescorp.net/

Expensive but their sales pitch is persuasive. Big positive article in EAA Sport Aviation and AOPA magazines.

Technology looks good.

My red cube totalizer has been very accurate over the years, but at the end of long cross country legs it would be nice to have a good reading of the real gas left that takes into account leaks. Just had a mechanical fuel pump fail and it leaked at a significant rate

You never know unless you really know

[keitht]

The capacitive fuel gages work on the relative dielectric constant of the medium between the plates. Air has a relative value of 1. Hydrocarbon liquid has a value between 2 and 4 depend on gasoline or kerosene and is different for 100LL and mogas. The big kicker is water which has a relative value of 80 so it doesnt take much water in the fuel to give you a full tank reading. The temperature affects the specific gravity which also affects the relative dielectric constant. The empty tank capacitance of the basic vans plates is relatively low so the range of capacitance from empty to full is also small leading to lack of accuracy and errors due to offsets and processing noise. Transport category aircraft have very sophisticated capacitance fuel gaging system with probes for measuring the dielectric constant of the fuel and shaped multi element concentric capacitance probes that achieve high capacitance values with small volumes. Way out of our price range or needs. The big advantage for us with our simple capacitance systems is reliability. With no moving parts and a multi point look up table calibration we should achieve acceptable accuracy and low failure rates so long as we are careful to ensure absolutely no water in the tank that can reach the probe plates. On balance a float gage system is going to score about even overall so it comes down to personal preference. For more information on the capacitance fuel gaging system go look at the big company players in the game. Symonds, Gull, Smiths Industries and Liebherr Industries . Their web sites all have information on their fuel gage systems.
KT

[Carlos151]

Originally Posted by Taltruda

Thank you all for your input! I love this forum!

Any other comments about the fuel caps? I heard the upgraded RV14 caps may work if installed while building, but then I read that maybe the curvature of the flange might not match the RV8 airfoil? Andair has a retrofit cap, and Newton may have a nicer quality cap too...

I like the Newton caps. Quality item that comes with the option to have them locking or non locking. Down side is that you will have to make the fuel tank opening slightly larger to accommodate these so if you have a quick build wing you would not be able to retrofit these. Lots of room for engraving which I had done later. No curvature or fitting issues whatsoever.