transistorized amplifier, bridge circuit, and

adjustment potentiometers. As the quantity of

fuel in the tank changes, the capacitance value of

the tank probe changes proportionately. The tank

probe is one arm of a capacitance bridge

circuit. The change of capacitance of the probe

unbalances the bridge circuit of the amplifier

power unit. The unbalance in the circuit causes

an error voltage. The amplified error voltage goes

to the motor. The motor drives the pointer

mechanism and the rebalancing potentiometers to

restore the bridge to a balanced condition. The

direction of change in the capacitance of the probe

unit determines the phase of the error voltage. The

phase determines the direction of motor rotation

and, therefore, the direction of pointer movement.

A tank probe and a simplified version of a

tank circuit are shown in figures 6-64 and 6-65.

The capacitance of a capacitor depends upon three

factors--the area of the plates (A), the distance

between the plates (d), and the dielectric constant

(K) of the material between the plates, or

where A = the area of the plates,

the voltage between the center tap and point P

d = the distance between the plates, and

is zero. As the fuel quantity increases, the

K = the dielectric constant of the materials

capacitance of the tank unit increases. This

between the plates.

increase causes more current to flow in the tank

unit leg of the bridge circuit. A voltage, which

The only variable factor in the tank probe is

is in phase with the voltage applied to the

the dielectric of the material between the plates.

transformer, now exists. As the quantity of fuel

When the tank is full, the dielectric material is

in the tank decreases, there is less current in the

all fuel. Its dielectric constant is about 2.07 at 0°C,

bridge tank unit leg. The voltage across the

compared to a dielectric constant of 1 for air.

When the tank is empty, there is only air between

the plates, and capacitance is less. Any change in

fuel quantity between full and empty produces a

corresponding change in capacitance.

Look at figure 6-66, which shows a simplified

capacitance bridge circuit. The fuel tank capacitor

and a fixed reference capacitor connect in series

across a transformer secondary winding. A

voltmeter connects from the exact center of the

transformer winding to a point between the two

capacitors. If the two capacitance are equal, the

voltage drops across them are equal. Therefore,

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