clevelee@cswebmail.com wrote:
>
> I was looking for my carefully misplaced article that was in Sport Aviation
> that
> suggested the 1 meg resister, as part of an overall fuel system discussion.
.
> .and you're right about the shopping time to locate the 1 meg resistor. Not
> very common.
>
> Cleve
I found it on my EAA CD-ROM - April, '87. The author correctly states
that discharge time will extend to several microseconds (1 us per pf
of capacitance), but "probably below ignition temperature of gas
fumes" is unsupported anywhere else I can find. To the contrary,
there's plenty of tech stuff at nasa.gov (as in Space Shuttle), and
they're unhappy with > 10K ohms to control static discharge!
In fact, the many-megohm resistance of the human body, plus 200pf of
capacitance, doesn't suppress the static sparks we experience, where
it does indeed jump the gap before we make contact. It is further the
case that extending the discharge time can have little effect,
provided the voltage is still sufficient to jump a gap and little is
dissipated in the series resistor. What ignites fuel vapor is Joules
- volts-amps-time - however in combination derived and above the
threshhold (.25 milliJoules I think).
I think practical considerations make it all moot. In connecting a
bonding or grounding wire to the airframe, such as the exhaust pipe,
any spark there is harmless. Or even before doing so, touching the
filler cap to open it -- besides the resistor then not in the circuit
-- any spark is also harmless since the cap is unvented.* Since any
charge has then been removed from the A/C, the only remaining hazard
is the flow of fuel, and for that, very low resistance in
bonding/grounding is required, as the literature is consistent in
implying that 1 meg of series resistance there is potentially
hazardous, if the flow rate is high enough.
*And -- on the older top-mount filler, ya didn't put the vents there,
as I did not!
Best,
Fred F.
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