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 ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
