Best sensors for measuring fuel flow rate in vehicles

The best method for measuring fuel is a capacitive system. This is the same system that is used on aircraft. You need a minimum of two cap sensors, one is in the bottom of the tank and is used for calibration, or weight (you have to know the specific gravity of the fuel). the other sensors are mounted vertically. This sensor is easy to make basically two tubes inside each other and they have holes in them to let fuel flow between and through them. The two tubes are the capacitor, which is measured by a bridge.  here is one blog about a car sensor. The sensor at the bottom of the tank measures fuel temperature, and the dielectric constant, which is performed by a cadensicon.  I found this patent. The rule of thumb is that big planes measure fuel by weight and sport aircraft, and your car measure fuel by gallons. 

The following is from the Federal Register. Caveat: This system has been used on military aircraft for decades, without incident, To be fair the only planes that had incidents were on Boeing 777 and 737. so the moral of the story uses low-voltage and currents in your system.  -CAH

DEPARTMENT OF TRANSPORTATION Federal Aviation Administration 14 CFR Part 39 [Docket No. 2002–NM–119–AD; Amendment 39–13392; AD 2003–25–09] RIN 2120–AA64 Airworthiness Directives; Airbus Model A300 B4–600 Series Airplanes, Model A300 B4–600R Series Airplanes, Model A300 C4–605R Variant F Airplanes, and Model A300 F4–605R Airplanes AGENCY: Federal Aviation Administration, DOT. ACTION: Final rule. SUMMARY: This amendment adopts a new airworthiness directive (AD), applicable to certain Airbus Model A300 B4–600 series airplanes, Model A300 B4–600R series airplanes, Model A300 C4–605R Variant F airplanes, and Model A300 F4–605R airplanes. This AD requires modification of certain components of the 115 Volts Alternating Current (VAC) supply wiring and of the fuel gauging system. This action is necessary to prevent short circuits between 115 VAC wiring and certain fuel system electrical wire runs with subsequent overheating of the cadensicon sensor thermistor or fuel level sensor, which could be great enough to ignite fuel vapors in the fuel tank and cause an explosion. This action is intended to address the identified unsafe condition. DATES: Effective January 27, 2004. The incorporation by reference of certain publications listed in the regulations is approved by the Director of the Federal Register as of January 27, 2004. ADDRESSES: The service information referenced in this AD may be obtained from Airbus Industrie, 1 Rond Point Maurice Bellonte, 31707 Blagnac Cedex, France. This information may be examined at the Federal Aviation Administration (FAA), Transport Airplane Directorate, Rules Docket, 1601 Lind Avenue, SW., Renton, Washington; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC. FOR FURTHER INFORMATION CONTACT: Dan Rodina, Aerospace Engineer, International Branch, ANM–116, FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington 98055–4056; telephone (425) 227–2125; fax (425) 227–1149. SUPPLEMENTARY INFORMATION: A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an airworthiness directive (AD) that is applicable to certain Airbus Model A300 B4–600 series airplanes, Model A300 B4–600R series airplanes, Model A300 C4–605R Variant F airplanes, and Model A300 F4–605R airplanes, was published in the Federal Register on September 8, 2003 (68 FR 52862). That action proposed to require modification of certain components of the 115 Volts Alternating Current (VAC) supply wiring and of the fuel gauging system. Comments Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received. The commenter supports the proposed AD. Conclusion After careful review of the available data, including the comment noted above, the FAA has determined that air safety and the public interest require the adoption of the rule as proposed. Cost Impact The FAA estimates that 70 airplanes of U.S. registry will be affected by this AD, that it will take approximately 29 work hours per airplane to accomplish the proposed actions, and that the average labor rate is $65 per work hour. Required parts will cost approximately $8,938 per airplane. Based on these figures, the cost impact of this AD on U.S. operators is estimated to be $757,610, or $10,823 per airplane. The cost impact figure discussed above is based on assumptions that no operator has yet accomplished any of the requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted. The cost impact figures discussed in AD rulemaking actions represent only the time necessary to perform the specific actions actually required by the AD. These figures typically do not include incidental costs, such as the time required to gain access and close up, planning time, or time necessitated by other administrative actions. Regulatory Impact The regulations adopted herein will not have a substantial direct effect on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government. Therefore, it is determined that this final rule does not have federalism implications under Executive Order 13132. For the reasons discussed above, I certify that this action (1) is not a ‘‘significant regulatory action’’ under Executive Order 12866; (2) is not a ‘‘significant rule’’ under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (3) will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act. A final evaluation has been prepared for this action and it is contained in the Rules Docket. A copy of it may be obtained from the Rules Docket at the location provided under the caption ADDRESSES. List of Subjects in 14 CFR Part 39 ■ 1. The authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. 106(g), 40113, 44701. § 39.13 [Amended] ■ 2. Section 39.13 is amended by adding the following new airworthiness directive: 2003–25–09 Airbus: Amendment 39–13392. Docket 2002–NM–119–AD. Applicability: Model A300 B4–600 series airplanes, Model A300 B4–600R series airplanes, Model A300 C4–605R Variant F airplanes, and Model A300 F4–605R airplanes; as listed in Airbus Service Bulletin A300–28–6066, dated November 8, 2000; or Airbus Service Bulletin A300–28–6070, Revision 01, dated March 22, 2002; certificated in any category. Compliance: Required as indicated, unless accomplished previously. To prevent short circuits between 115 Volts Alternating Current (VAC) wiring and certain fuel system electrical wire runs with subsequent overheating of the cadensicon sensor thermistor or fuel level sensor, which could be great enough to ignite fuel vapors in the fuel tank and cause an explosion, accomplish the following: Modification (a)Within 4,000 flight hours after the effective date of this AD, modify elements of the electrical wiring to separate the cadensicon wiring from the 115 VAC wiring, in accordance with Airbus Service Bulletin A300–28–6066, dated November 8, 2000. (b)Within 4,000 flight hours after the effective date of this AD, modify elements of the electrical wiring to separate the 115 VAC supply wiring of the fuel gauging system, in accordance with Airbus Service Bulletin A300–28–6070, Revision 01, dated March 22, 2002. Alternative Methods of Compliance (c) In accordance with 14 CFR 39.19, the Manager, International Branch, ANM–116, FAA, Transport Airplane Directorate, is authorized to approve alternative methods of compliance for this AD. Incorporation by Reference (d) The actions shall be done in accordance with Airbus Service Bulletin A300–28–6066, dated November 8, 2000; and Airbus Service Bulletin A300–28–6070, Revision 01, dated March 22, 2002. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from Airbus Industrie, 1 Rond Point Maurice Bellonte, 31707 Blagnac Cedex, France. Copies may be inspected at the FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC. Note 1: The subject of this AD is addressed in French airworthiness directives 2002– 172(B) and 2002–171(B), both dated April 3, 2002.

BTW with the above solution you know the volume of your tank at a standard temperature. So the math is not that big of a deal. also, this will give you a fuel rate over time. and in airplanes, you have multiple sensors in a tank as you don't want your meter to change in a climb, or in a bank that is turning, where fluid will move due to both side forces and or centripetal forces, and acceleration. 

BTW with the above solution you know the volume of your tank at a standard temperature. So the math is not that big of a deal. also, this will give you a fuel rate over time. and in airplanes, you have multiple sensors in a tank as you don't want your meter to change in a climb, or in a bank that is turning, where fluid will move due to both side forces and or centripetal forces, and acceleration.