You are exactly right. I used to work on automotive fuel electrical systems about 10 years ago. The fuel meter is a current meter, measuring current between 1 to 3 mA (for the one I used). The meter input resistance is greater than 4k ohms. This is the current limit for the fuel sender unit, and the meter is designed to fail in the open circuit mode. The fuel sender does not get enough energy to create a hot enough spark to ignite the gas vapor, since the resistive element is soldered to the sender metal housing, which acts as a heat sink. However, this is not true about the fuel pumps. I have seen the records of several fuel tank explosions due to faulty pumps.
On another note, there is a better design for the fuel sender. One company designed a parallel plate device and submerged it in fuel. The fuel acts as the dielectric between the parallel plates. As the fuel level goes down, the capacitance of the parallel plates changes. The device measures the capacitance to determine fuel level. It was said that the device will out last the car, since it has no moving parts to fail. The current on the parallel plates are about 100uA at 500mV, not enough energy to start a fire. I wonder why this device is not used everywhere. George -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Ken Javor Sent: Tuesday, October 08, 2002 5:39 PM To: Price, Ed; 'EMC-PSTC List' Subject: Re: Explosive Atmosphere Safety Question 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" ------------------------------------------- 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"
