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Can you explain the different types of fuel quantity sensors used in most recent aircraft?

Are ultrasonic tank sensors used these days? How does an ultrasonic sensor work?

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From this PPRune forum thread, the Boeing 777 does use ultrasonic sensors (some 20 per tank)

This airplane has some 20 sensors per tank. Each of these is effectively a "radio doppler" and works on the same principle. The sensor calculates the height of fluid by measuring the difference in the speed of the sonic signal sent out thru one sensor to another,as against the ACTUAL local speed of sound (which is a preset value) thru standard temp air.This difference is then converted very accurately into height in the tank in mm.Which translates into quantity of fuel AT THAT sensor given the actual specific gravity of fuel at that point. - King on a Wing

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In small aircraft, fuel is measured simply with a floating fuel gauge. In large commercial aircraft, this is accomplished by measuring the capacitance of the fuel.

Ultrasonic fuel sensors seem to be prevalent in stationery ground fuel tanks, not in aircraft.

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    $\begingroup$ This is interesting information, but doesn't really address the question of how they work. $\endgroup$ – Dan Hulme Nov 13 '17 at 14:13
  • $\begingroup$ It's a partial answer @DanHulme, the question asks about the different types of quantity sensors used. $\endgroup$ – GdD Nov 13 '17 at 14:15
  • $\begingroup$ Basically this type of sensor is not used in aircraft. I hope this clarifies things. Hard to explain, if they are not in use. It like asking how control surface work on cars. $\endgroup$ – mike Nov 13 '17 at 14:47
  • $\begingroup$ @mike From what i know B777 uses ultrasonic fuel sensors. Capacitance types arent used anymore $\endgroup$ – Pritam Nov 13 '17 at 15:20
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Ultrasonic fuel sensors use a transmitter/receiver to transmit a sonic pulse inside a pipe filled with fuel. The pulse is reflected back to the transmitter/receiver by the fuel surface and the time taken by the pulse is the measurement of fuel height which is then converted to fuel quantity using the fuel density and specific gravity.

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