The answer is that despite what it seems, a gasoline tank is not an explosive
atmosphere. The vapor pressure of gasoline is high enough so that the atmosphere
in the tank has too much gasoline vapor to burn. This takes care of possible
arcing during normal operation of the fuel level sender and possible arcing of
the electric fuel pump that is now commonly found inside the gas tank. There may
still be concern about the sender during a fault condition, but the concern is
not about igniting an explosive atmosphere inside the tank.
There IS ongoing concern about fuel level senders in the tanks of jet aircraft.
At sufficiently high temperatures (hot summer days or tropical areas), an
explosive atmosphere can form inside tanks. Several explosions have occurred,
despite the energy limited design of the fuel level sensors.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
"Price, Ed" <[email protected]> on 10/08/2002 08:48:44 AM
Please respond to "Price, Ed" <[email protected]>
To: "'EMC-PSTC List'" <[email protected]>
cc: (bcc: Don Borowski/SEL)
Subject: Explosive Atmosphere Safety Question
I have recently been pondering the safety design of the typical automobile
gasoline tank fuel quantity sensor (or "sender") assembly. The several
examples that I have seen consist of a float attached to a mechanical pivot
arm. As the fuel level varies, the arm moves, changing the resistance of the
sensing element.
My first question is how is the mechanical sliding resistive contact
isolated from the explosive fuel/air mixture? Certainly, when the tank is
nearly full, the entire sensor element is submerged in the fuel. But what
happens when the tank is nearly empty, and external air replaces the fuel.
The sensor is then hanging in the fuel/air mixture. In short, how are sparks
avoided at the mechanical sliding sensor contact?
Secondly, how is the problem of sensor self-heating (during a single-fault
condition) avoided? I can imagine a scenario where the hot vehicle bus is
faulted to the sensor lead. Since a typical sensor element varies between
about 100 Ohms to just a few Ohms, the sensor element could dissipate 25
Watts or more. This would cause rapid heating of the sensor element,
possibly ending in the burn-out of the resistive element (inside the fuel
tank). Again, how is this ignition scenario prevented?
I am hoping to get a few informed comments before I go out and buy one for a
sacrificial dissection. (I think I became an engineer because I never could
get those frogs to work again after re-assembly.)
Regards,
Ed
Ed Price
[email protected]
Electromagnetic Compatibility Lab
Cubic Defense Systems
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Is Our Specialty
Shake-Bake-Shock - Metrology - Reliability Analysis
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