There was a good answer on this subject about there not being an explosive atmosphere within a car fuel tank, but I think there may be another safety factor. If I were designing the sensor system, the meter would be configured as an ohmmeter/ammeter, such that there would be a very high series resistance in series with the tank variable element. The meter empty to full range would actually represent a very small current level. I think it could be designed in such a way that there would be little or no possibility of shorting 12 Volts to the sensor element. I don't recall the specifics, but there is a potential below which resultant sparks have insufficient energy to ignite an explosive atmosphere. I used to know it for JP-4, but it will exist for gasoline as well.
on 10/8/02 11:48 AM, Price, Ed at [email protected] wrote: > > 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" > -- Ken Javor EMC Compliance Huntsville, Alabama 256/650-5261 ------------------------------------------- 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"
