Capacitive fluid level detection is an old and well proven system. I was an avionics maintenance technician for some years and all the aircraft I ever worked on used this technology, to include the B-52 which has been flying since the early 1950s. Aircraft fuel level sensors have the capacitive plates arranged in a coaxial manner. Dan Kinney
> -----Original Message----- > From: George Tang [SMTP:[email protected]] > Sent: Tuesday, October 08, 2002 10:17 PM > To: Ken Javor; Price, Ed; 'EMC-PSTC List' > Subject: RE: Explosive Atmosphere Safety Question > > > 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" ------------------------------------------- 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"
