facts about capcitance fuel level system

[bird]

Hi, Im about to start my rv8 wings, I have ordered the capacitance type senders. I am interested to hear from others about the accuracy and compatability of vans senders. I will probably have a mgl or similar glass. I have read some previous threads about it so no need to repeat. I guess my most conern is about the variations in fuel. Does it really make a big difference in fuel types. Keep in mind that I will most likely be using the same fuel such as 100 ll avgas for lycoming. Is this difference in name brand fuel? Or is it like if I went to auto fuel? thanks

bird

[longranger]

The output of a capacitance fuel quantity measuring device depends on a property of the fuel called dielectric constant, which is a measure of how much electrical charge the fuel can hold per volt of an applied voltage, compared to air. The dielectric constant is dependent on the chemistry of the fuel. Once you calibrate for 100LL, you it's not likely you'll see much difference between different name brands of 100LL, as all brands must meet a spec which doesn't allow much variation in the chemistry.

I'm not sure how the dielectric constant of non-ethanol mogas compares with 100LL, but I think both are in the vicinity of 2 (that is, fuel holds about twice charge per volt than does air). The dielectric constant of ethanol is 24.3 (per wikipedia) so it greatly affects the dielectric constant of the fuel. The effect of ethanol-laced fuel on a sender calibrated for 100LL would be to show a significantly higher fuel level than is actually present.

I've toyed with the idea of adding a separate small plate to the bottom inboard bay of the fuel tank so that it is (almost) always immersed in fuel. It's function would essentially be to measure the dielectric constant of the fuel and enable compensating the output the measuring plates. The trick would be to come up with the compensation circuitry. My guess is that someone has already done this, but I haven't been able to find much info on it. I've taken a couple of stabs at designing one, but haven't yet come up with anything practical.

[Geico266]

Do yourself a favor and install the float type. As we phase out 100ll you will be glad you did. Any mixture of mogas (unleaded fuel) and 100LL will result in VERY in accurate readings. Up to 7 gallons per tank. Unacceptable in any plane I have owned / flown with capacitive fuel gages.

[hendrik]

Originally Posted by longranger

I've toyed with the idea of adding a separate small plate to the bottom inboard bay of the fuel tank so that it is (almost) always immersed in fuel. It's function would essentially be to measure the dielectric constant of the fuel and enable compensating the output the measuring plates. The trick would be to come up with the compensation circuitry.

I'm considering this, too, and since I have the wing crates sitting in the garage I need to come to a conclusion in the not-so-distant future. The easiest way of doing the compensation would be to combine the information of the two sensors in software, I think. I started a thread about this question recently, but haven't received any helpful replies yet: http://www.vansairforce.com/communit...ad.php?t=68259

If you come up with something, please let us know!

[billgill]

fuel readings with Vans capacitance set-up with the Advanced AF-2500 engine monitor and the Princeton converters. The gauge readings often mirror the fuel computer reading. If 100LL goes away, one could simply recalibrate the tanks with the new fuel.

Bill
RV-7 N151WP
Lees Summit, MO

[longranger]

Originally Posted by hendrik

I'm considering this, too, and since I have the wing crates sitting in the garage I need to come to a conclusion in the not-so-distant future...

You can add the compensation probes now, in anticipation of a solution for their use being found.

Originally Posted by hendrik

... The easiest way of doing the compensation would be to combine the information of the two sensors in software, I think. ...

I've also pretty much come to the conclusion that the software solution would be the most straightforward, especially if one of the EIS boxes would provide a couple of extra inputs and a way to implement some custom software.... But in the meantime a hardware solution is still ricocheting around in my head...

Originally Posted by hendrik

If you come up with something, please let us know!

Will do!

[rocketbob]

Originally Posted by longranger

I've toyed with the idea of adding a separate small plate to the bottom inboard bay of the fuel tank so that it is (almost) always immersed in fuel. It's function would essentially be to measure the dielectric constant of the fuel and enable compensating the output the measuring plates. The trick would be to come up with the compensation circuitry. My guess is that someone has already done this, but I haven't been able to find much info on it. I've taken a couple of stabs at designing one, but haven't yet come up with anything practical.

That's a fantastic idea. Since the distance between the plates is known it would be straightforward to solve for the dielectric constant. Assuming that one is developing their own microcontroller circuit for fuel quantity readings. I'd do this but my tanks are closed up and done.

[longranger]

Originally Posted by rocketbob

That's a fantastic idea. Since the distance between the plates is known it would be straightforward to solve for the dielectric constant. Assuming that one is developing their own microcontroller circuit for fuel quantity readings. I'd do this but my tanks are closed up and done.

You wouldn't even have to find the dielectric constant numerically. If I've done the math right (NOT a slam dunk), the ratio of the outputs of the measuring probe and the compensation probe would be constant for a given fuel level regardless of the dielectric constant. EDIT: This is only true when the tank is full. I need to give this some more thought...

[rocketbob]

Originally Posted by longranger

You wouldn't even have to find the dielectric constant numerically. If I've done the math right (NOT a slam dunk), the ratio of the outputs of the measuring probe and the compensation probe would be constant for a given fuel level regardless of the dielectric constant.

Correct. You can call it what whatever, ratio, dielectric constant, etc. Its just a number to correct the fuel level reading. So when you calibrate the fuel readings for the first time, the baseline value would be established based on the capacitance value. Then during the normal course of operation when that number is different compared to the calibration number, is when you apply the correction based on the difference.

[Janekom]

I urgently need the setup/calibration procedure for a Princeton capacitance probe. It is the one with three LEDs and small push switch on the pug.
I will appreciate a document or a text reply.
Thanks.