What are fuel measuring systems, types, functionality and limitations?

022 Instrumentation theory.
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CaptainMFT
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What are fuel measuring systems, types, functionality and limitations?

Postby CaptainMFT » 17 Jun 2016 01:53

Resistive fuel measuring system and its' limitations:-
In this system the mechanical linkage is replaced with an electrical resistance and a gauge is operated by DC. Any amendment within the fuel level causes the float arm to move a wiper across the resistor. This leads to a varying signal being sent to the gauge. The system solely offers indication of volume. Indications will fluctuate when the airplane is manoeuvring. 

A major disadvantage for volumetric indication is that the fuel contracts when it's cold and expands once it's hot. So a variation in temperature alters the contents reading. Fuel weight are often obtained from fuel quantity if the specific gravity is known. This can be calculated, or there could also be an adjustable specific gravity compensator within the gauging system. 


Capacitance fuel measuring system & its' limitations:-
The capacitance system weigh the fuel on board and displays the quantity of fuel as a mass or weight. A capacitor is basically two (2) plates holding completely different electrical charges. The plates are separated by an insolent known as a dielectric. The charge that can be held on the plates is named the capacitance. It is measured in farads. 
It depends on the:
1) area of the plates. 
2) Distance between the plates. 
3) Insulating qualities of the dielectric (dielectric permittivity). 

As a formula: 
Capacitance = E x A/D 
where
A = the area of the plates 
D = the space between the plates 
E = the dielectric permittivity 

Capacitance systems need AC electrics. In this case the capacitance unit or capacitor consists of three (3) concentric tubes one inside the other. The outer tube is an earth screen, the inner tube at low potential and therefore the innermost at high potential. The dielectrics here are fuel and air. Fuel has roughly double the capacitance of air. The amount of capacitive reactance (opposition to AC because of capacitance) indicates how much of the tube is covered by fuel. This is a volume measurement. Fuel has completely different capacitance for different densities. The capacitance will increase as density will increase. The reference unit measures the actual capacitance for the fuel within the tank. It is in the unusable fuel and always remains submerged. This information permits the gauge to convert the output from volume to mass of fuel. Several tube units are placed in each tank wired in parallel for redundancy. Any modification in density caused by temperature changes can still have an effect on the volume within the tank. However the mass or weight displayed are going to be the same. Fuel gauges solely indicate the usable fuel and electrically operated gauges can always move to zero once the power is removed or if the system fails. 


Venturi fuel flow indicators & its' limitations:-
The fuel feed is narrowed at a degree. Velocity increases and pressure decreases. The instrument uses a pressure capsule fed from the venturi. This is within a container that is controlled with a feed from the normal dimension fuel lines. Reduced pressure within the venturi compared to the normal pressure within the pipe is a sign of flow rate. 
The indication on the instrument is typically litres an hour. It may be calibrated in kg/hour (in this case it'll be solely correct for one fuel density). 


Variable orifice flow indicators & its' limitations:-
A hinged measuring vane is placed inside a shaped chamber. It is slightly offset from the centre. The fuel flow exerts a pressure on the vane. Pressure is counteracted by a spring force. Under high fuel flow rates the vane is deflected more. The orifice that enables the fuel to slip past the vane becomes larger. Vane displacement is converted into electrical signals. Signals are fed to the instrument to indicate fuel flow. Variable orifice flow indicator truly shows volume flow. 
However an indication of mass flow will be obtained by using temperature sensitive resistors to catch up on changes in fuel density. 


Turbine volume flow indicators & its' indications:-
A turbine is inserted within the fuel lines. Blades of the turbine are designed with magnetic inserts. As the turbine rotates the blades pass an induction coil within the casing. This produces AC pulses (like phonic wheel of induction tachometers). These pulses will either be squared off to transmit digital data or averaged to form a current flow proportional to the speed of turbine rotation. These don't cope well with the large rate and temperature changes. 


Mass flow indicators & its' limitations:-
True mass flow indicators rely on deflecting the fuel. Due to deflection the fuel swirls within the pipes. Angular momentum is detected. As momentum could be a product of rotational speed and mass the true mass flow will be indicated.

1) Stator torque principle: 
Impeller rotates at a constant speed. Puts a swirl in to the fuel. A turbine anchored to a spring (in other words a stator) is found downstream of the impeller. It takes the swirl out of the fuel. The greater the momentum the more the twisting force on the stator as it does this. 

2) Rotor torque principle: 
The blade (the rotor) is turned by a motor at a fixed revolutions per minute. Rotor is connected to the motor by a twisting spring. Greater the resistance the more the rotor lags behind the motor. That means, more the spring is twisted. Both the rotor and therefore the motor have magnetic inserts that produce pulses in inductive pick-ups. The interval between the passage of the motor and rotor pickups indicates the amount of twist on the spring. Hence the mass flow. Fuel flow is integrated to point out fuel used over a period of time. This is combined with the fuel flow indication the system and is termed an integrated flowmeter system.
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Good luck,
CaptainMFT.

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