Capacitance Fuel System Questions

WSBuilder · #1 12-05-2007, 09:24 AM

I was misbeguiled by Van's. Before I started my -4 fuel tank I questioned a discrepency in the drawings. The wing layout showed the inboard interior tank rib as a flange-in, T-404R while the tank drawing showed it a flange-out, T-404L. Van's said to go by the tank drawing. No big deal, just swap ribs. After drilling, I grew worried when I dry-fit the capacitance plates before sealing the tank (pro-seal virgin and all) because it's "inside" the flanges and rivet shop heads, but I still had the requisite clearance.

Post sealing, I go to install the capacitance plates and the inboard plate interferes on the shop head sealant dollups. I see now that it would have given better clearance to the plate had I installed the rib flange-in .

So questions: can I trim the plate without adversley affecting fuel sensing? Do the plates require equal or identical surface area? (You're supposed to trim the bottom corners for stiffener clearance on the outboard plate so it was going to be different from the inboard, anyway)

Can the plate be mounted on the other side of the rib, in the first fuel bay?

Red Voodoo · #2 12-05-2007, 10:18 AM

As a humble Liaison engineer, this kind of black magic is WAY outside my area of expertise, but here's the way I understand it. The plates in Van's system form a flat plate capacitor. The plates are electrically isolated from the rest of the structure, so that in effect they exist independently in a container of fuel. The system measures capacitance, which is defined by a formula you can find on line if you're interested. What matters is that in normal operations, all the factors are constant, except the area of the plates, which changes as you use fuel and expose more area. This change is what the system allows you to track. Changing the type of fuel changes another factor in the equation, which is why you would need to recalibrate if you went from 100 LL to auto gas.
To answer your questions: as I understand it, the plates need to be identical (or very nearly so) in size. So if you trim one,trim the other to the same configuration.
Not sure about relocating the plate. Another factor in the equation is the distance by which the plates are separated. Changing this spacing would required recalculating the capacitance. Maybe that can be accounted for in the calibration. Someone more knowledgeable than me needs to chime in!

longranger · #3 12-05-2007, 10:56 AM

The plates being the same size helps with the linearity of the output vs. fuel level. Most fuel gauges/EMS systems these days have muliple point calibration capability, so it isn't critical that both plates be exactly the same size. Trimming one for clearance from the structure shouldn't affect the operation in a noticeable way. No need to trim one solely because you had to trim the other. The worst case is that the outboard plate is non-existent, and your gauge doesn't start down until there's ullage in the inboard bay... just like float sensors.

Pirkka · #4 12-05-2007, 02:26 PM

Couple days back I opened by capacitive sender bags and read how they should be installed.

There is one wire going from outer plate to inner plate. From the same electrical point one wire goes to the BNC connector. Is that correct? How does that capacitive sender works then? No ground and plates are connected with single wire which goes to BNC as well. Did I read the plans correctly at all? This doesn't make any sense...

Electrical connection: Outer plate <--> longer wire <--> inner plate <--> short wire <--> BNC

Only thing that comes into my mind how it could work is to fed some pulse into the system and analyze how it reflects back from the system. Sound a bit difficult but only thing that comes to my mind when we have just one wire...

I would appreciate if someone who knows would clear my mind.

Sticky1 · #5 12-05-2007, 02:38 PM

Plates, No plates that is the question........

Whatever you do make sure the fuel sending unit is compatable with the instrument u wish to use to view the level(s).

Brantel · #6 12-05-2007, 02:46 PM

Quote:

Originally Posted by Pirkka

Couple days back I opened by capacitive sender bags and read how they should be installed.

There is one wire going from outer plate to inner plate. From the same electrical point one wire goes to the BNC connector. Is that correct? How does that capacitive sender works then? No ground and plates are connected with single wire which goes to BNC as well. Did I read the plans correctly at all? This doesn't make any sense...

Electrical connection: Outer plate <--> longer wire <--> inner plate <--> short wire <--> BNC

Only thing that comes into my mind how it could work is to fed some pulse into the system and analyze how it reflects back from the system. Sound a bit difficult but only thing that comes to my mind when we have just one wire...

I would appreciate if someone who knows would clear my mind.

The fuel guage or capacitive converter measures the capacitance between the plate and the tank. The fuel or air is the dieletric material. Fuel has a different dieletric constant than air so there is a difference in the capacitance from full to empty. The guage needs to be able to be calibrated at at least two points in order to get this to work. Most modern units have multiple calibration points.

longranger · #7 12-05-2007, 02:49 PM

Each sensor plate forms one plate of a capacitor (electrical charge storage device). Each rib to which the plates are attached (but are electrically isolated from) serve as the other plate. With the two sensor plates being wired together, and with both ribs being grounded by virtue of being part of the structure, the two capacitors formed are in parallel, which means that electrically they act as one larger capacitor.

Capacitance (the quantity of electrical charge that can be held per volt across the plate) is a function of 3 things: the plate area, the distance between the plates, and a property of the material between the plates called "dielectric constant". In a fuel gauge application the only variable is dielectric constant. IIRC, the constant for gasoline is approximately twice that of air. That means that the sensor plates are variable capacitors that depend on the fuel level, and the gauges are capacitance meters calibrated in gallons, liters, etc.

Actually, you do have two wires between the sensor and the gauge. The sensor lates are connected to the center pin on the BNC connector, and the outer shell of the connector is (or should be) grounded to the rib. If you connect your sensors to your gauges with coaxial cable, the ground is carried via the shield part of the cable.
Hope this helps.

Pirkka · #8 12-06-2007, 12:39 AM

Quote:

Originally Posted by longranger

The sensor lates are connected to the center pin on the BNC connector, and the outer shell of the connector is (or should be) grounded to the rib.

Thanks Miles for an excellent answer (it answered exactly and promptly the question I had). I probably would have figured it out if I would figured out that grounding of the BNC. However I might missed that plans say to make sure it's grounded -- the feeling I got from that part of the plans was to seal it well with ProSeal (which might interfere contact between BNC and rib) and it might not have been mentioned that it should be grounded. Therefore I figured that it's not grounded and then my equation became impossible.

bobmarkert · #9 01-23-2008, 10:25 PM

I?ll be ordering my sloooow build wing kit this week. Does anybody have any experience with the Vans fuel capacitance plates and the Dynon 180? I know Dynon now has a device that theoretically lets the capacitive plates work with their product. Thought on using the plates vs. the resistance sender.
Thanks
Bob Markert
N747BM reserved
Littleton, CO

BlackRV7 · #10 01-24-2008, 04:58 AM

Quote:

Originally Posted by bobmarkert

I?ll be ordering my sloooow build wing kit this week. Does anybody have any experience with the Vans fuel capacitance plates and the Dynon 180?
Bob Markert
N747BM reserved
Littleton, CO

Bob, I'll relay my recent experience with my capacitance plates and my Dynon EMS D180. I bought the Dynon converters, there is no what you would call install. The small board has a BNC connector that simply snaps on the BNC at the wing root. Three wires run from the board to your wiring harness to, in your case the 180. Easy, simple install and easy calibration.