Second try with reduced image:
There are other possibilities-
AVgas is nonconductive as far I could tell by testing with a megohmeter. I
have seen electrical fuel senders with coils and wipers that are directly
exposed to fuel.
Sloshing around in a fiberglass
tank, I believe the fuel could build a static charge on the surface. Fuel
plumbing would normally be submerged so wouldn’t help by wicking off any
buildup. An isolated metal object such as the filler neck could collect the
charge.
Normally this wouldn’t be a problem
as the fuel tank is filled with such a high concentration of fuel vapor that
it’s too rich for combustion. I used to be a firefighter, eventually fire
chief and this is what we were taught.
During the filling process
concentrated fumes are displaced by fuel- the danger zone is the interface
between the concentrated fumes and the outside air where the mixture becomes
combustible. When the nozzle first touches the filler neck it would most
likely be inside the noncombustible zone and would remain that way until
fueling was finished. Cars aren’t grounded and yet they usually don’t catch
fire when being fueled. Even so, touching the cap with the ground wire before
opening wouldn’t hurt. Fuel nozzles are grounded so once a connection was made
static should no longer be a problem.
OTOH, I figure better safe than
sorry. I thought about this quite a bit while building the Velocity and the
picture shows my solution for the IV-P. Rather than try to ground the entire
tank I made a cylinder of expanded copper mesh, sewed with copper wire and
soldered. The lead formed is attached to the filler neck and also to the cap,
using a braided ground strap to the adjusting nut. That connection isn’t
perfect but it also serves to keep the cap from falling off the wing. The mesh
not only collects any charge but prevents trash from going into the tank- I’ve
heard of people sticking wrappers etc. into fuel tanks during airshows. The
mesh was attached to the tank with Jeffco and slightly compressed when the
tank was closed up.
In use, the procedure would be to
touch the ground wire to the cap before opening, and I’ll placard the filler.
A ground wire could also be connected to the cap lead once it was open. In
addition, I think Pete Cavitt’s method to remove surface static makes a lot of
sense. -Bill Wade