Capacitive fuel senders

Hi

Does anyone have any idea of the approximate capacitance range of the Vans in tank capacitive fuel senders.

I need it for testing a suspect capacitance to voltage converter.

Thanks

I suspect this will be dependent on the type of fuel you are using as well as the quantity of said fuel. I just installed the capactive senders in my fuel tanks a few weeks ago, and all they are is simple aluminum sheet plates, attached to the ribs (two plates on two ribs), and isolated from grounding on the rib via delrin spacers. There is a wire that attaches to the plates, and runs to the center of the BNC connector on the tank. Your capacitance sender simply reads the capacitance of the fuel using those two metal plates as the anodes, and the tank skin as the cathodes.

I have been flying a while but my fuel level readings on both tanks have become unreliable.

One I traced to a faulty Dynon sender, which I have replaced.

The other now seems to be caused by a full or intermittent disconnection inside the tank. Which just reads zero almost all the time.

To test this I applied a number of different values of capacitor to the bnc connector on the capacitance to voltage converter, I randomly chose the values of capacitor until I found one that caused a reliable and repeatable reading. It seems this has proven that the fault is not from the capacitance to voltage converter to the Dynon, hence I suspect a disconnection inside the tank...deep joy!

I have very little inclination to remove the tank, so will probably rely upon my fuel flow readings on the Dynon Skyview which is very accurate, dipping the tanks and correct fuel planning.

Depending on the details of your construction, the empty value should be somewhere between about 60pf and 150pF. The difference between empty and full (of avgas) should be about 145pF. These numbers can vary quite a bit.

Oops. Finger trouble.

Just expanding a bit, the dielectric constant of air is very close to 1, and the dielectric constant of gasoline is about 2. However, an empty tank will normally have some gasoline vapor in it, which will change the constant slightly. Also, "full" will not replace all the air in the tank with liquid. Together, these suggest that the capacitance ratio of full to empty should normally be a bit less than 2 to 1.

During construction, I measured mine at 180 picofarad while empty. Did not measure while full many months later but the article I was reading indicated a 2x change from air to fuel.

I was building my own freq to volt converter based on Jim Wiers design. The bench model worked but before I built the pc board I found the Dynon converters and bought them.

Thanks for the information, its appreciated and will help me track this down.

Dielectric constant of water

Important to remember that the dielectric constant for water is 80 so it only takes a very small amount of water to cause a large swing in apparent fuel reading. Other contaminants in the fuel can also affect the readings.

KT