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.
Tank Probe
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
Figure 6-64.-Fuel quantity transmitter.
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 0C,
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.
System Operation
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
Figure 6-65.-Simplified tank circuit.
voltage drops across them are equal. Therefore,
