Capacitive vs. Float style fuel senders

[captain_john]

Well, I suppose it is almost too late for me to make a change in this decision, but I want to be comfortable with my choice.

I ordered capacitive style senders for my tanks. I want to use AVGas and a Lycoming, but there is a chance (this changes routinely) that I may use MOGas and a SOOB. I seem to recall differences in the fuel that affect how each fuel floats the float. They say that calibrating them is different for each fuel. Is that true? What about capacitive?

Seems to me the fuel type won't matter in either instance. I want to use the capacitive style for greater accuracy.



Questions:

1) Are the capacitive senders any more accurate that floats? I assume so.

2) How compatible are the capacitive senders with EI Systems? I haven't yet settled on a brand.

3) Can I swap from one fuel to the next without recalibrating?

4) Last, but certainly not least... What is the operational theory behind the capacitive system and how does it work?

I think I know the basics to #4 but I want to be clear.

Anyone?

CJ

[Snap]

Damn, that is a good question. I would like to know as well. If anyone knows can they tell us or direct us to a site of knowledge

[mustang]

I wouldn't get too technical on this matter. Only the last 15 gallons can be measured by either system. A float system will show the last 15 gallons as accurately as the capacitive system for a lot less money. It won't make any difference as to what kind of fuel you use either. The best fuel gauge is the carefully calibrated dipstick that you will make when first filling your tanks incrementally. You will then measure your fuel with this dipstick for every flight and live a long and happy life.

Even the big iron use dripsticks which are used when the capacitive senders go Tango Uniform. (Which happens when they get wet with water) Some of the new glass EICAS systems will accept either input so you don't need to spend the big bucks on measuring the last 15 gallons in your tank with the capacitive system. You simply input the dipped quantity of each tank into the fuel system of the EICAS and you are good to go.

Cheers, Pete

[Heli-Wrench]

I'm going to try this from memory, so I hope I'm not leading you too far astray.

A capacitive system uses two plates, just like a regular capacitor. The fuel fills the gap between the two plates. Fuel conducts electricity better than air (the empty tank above the fuel). that sets up the di-electric of the capacitor. As the fuel level changes, so does the value of the capacitor and that is reflected on you fuel gauge. (That was about the poorest explination I could come up with. Sorry)

The nice thing is that there is no moving parts, nothing to wear out. One type of aircraft I work on can have either type installed (Resistive, or Capacitive) and by far capacitive causes far less headaces.

[728GD]

Mustang, I could not agree with you more. The fuel gauge on any aircraft is nothing more than a reference tool.

Verify how much fuel is on board, know your fuel burn per hour, and always, always fly by time! If you are going to spend additional money, I suggest a fuel flow monitor. This will save your bacon many more times than a gauge. If you are not paying attention, the monitor will alert you to a pending shortage of fuel giving you time to take action prior to the silence of fuel starvation. The gauge will only confirm your fears after the silence occurs. Best alternative, accurate dip stick and a watch. My two cent's worth.

[darrylDog2000]

Mustang... Without going into a lot of detail as to why, I would challenge your statement that a capacitive sender will only measure the last 15 gallons.

I would like to ask any builders who have completed planes for comfirmation here. Is this true?

[arffguy]

I think it would still be related to the position of the top of the sensor being lower than the top of the outboard part of the fuel tank. Due to wing dihedral. So like the float system some fuel would burn off before the gauge drops. I am using floats simply because of cost. Anybody who has seen the parts know for sure?

But there is this from EI:

http://www.buy-ei.com/Manuals

"Full Fuel Readings - As a tank is filled the fuel sensor may not be able to detect the fuel entering the upper corners of the fuel tank. If this is the case with your sensor, the FL-2 will display lower fuel levels than the actual fuel in the tanks when the tanks are full. When the fuel level drops to a point where the fuel sensors start to detect a change, the displayed fuel level should be accurate."

[mustang]

That is correct. If there were no dihedral then either the float or capacitance system would measure all the fuel from full to empty in our RVs.

Unless you have more than one capacitive sensor installed in each tank and a setup that measures the cumulative capacities of both sensors then the one, single sensor or float will measure only the last 15 gallons of each tank.

On the big iron there may be more than two or three capacitive sensors in each tank which are feeding into a small circuit board with a chip to internally calculate the numerical display in the cockpit. Each tank's capacity will then be added by a totalizer for the total FOB numbers. It only takes a very small amount of water on one of these capacitors to send the whole thing into a tizzy.

For our RVs, a fuel flow sensor and totalizer of fuel burned is a good way to know your exact fuel remaining. You dip the tanks, record the amount. After engine start or "master on",you input the exact amount of dipped fuel into the fuel computer and it starts the fuel burn calculation from that point.

On the big iron, the fuel burn quantity totalizer is continually being compared to the fuel tank quantity indicators to ensure no large anomalies are detected. Even with huge quantities of fuel consumption (300,000 lbs) there might only be a 300 lb discrepancy between the fuel burn totalizer and fuel gauge totalizer at the end of the flight .

Cheers, Pete