continually changing the position of the indicating
needle. To do this, a balancing potentiometer
(R128) connects across part of the transformer
secondary. The indicator motor drives this
potentiometer wiper in the direction necessary to
maintain a continuous balance in the bridge.
The circuit shown in figure 6-67 is a self-
balancing bridge circuit. An empty-calibrating
potentiometer and a full-calibrating potentiometer
connect across portions of the transformer
secondary winding. You can adjust these potenti-
ometers so the bridge voltage balances over the
empty-to-full capacitance range of a specific
system.
A test switch (not shown) unbalances the
bridge circuit momentarily when checking the
operation of the system. When the switch
actuates, pin F connects to ground, unbalancing
the circuit. As a result, the indicator drives toward
the empty end of the dial. Opening the switch
should restore the bridge to balance and return
the indicator pointer to its original position. This
proves that the system is operating correctly.
In installations where the indicator shows the
contents of one tank, and the tank is fairly
symmetrical, one probe is sufficient. However,
Figure 6-68.-Fuel quantity tank units: (A) Noncharacterized;
for increased accuracy in peculiarly shaped fuel
(B) Characterized.
tanks, use two or more tank units in parallel. This
configuration minimizes the effects of changes in
aircraft attitude and sloshing of fuel in the tanks.
FUEL CHARACTERISTICS. --The charac-
There are two classes of tank units (fig. 6-68)
teristics of fuel are such that the dielectric constant
used in a typical capacitance fuel quantity
and density deviate because of temperature
measuring system--noncharacterized and charac-
change. Also, the variable factor in the fuel
terized.
composition changes the dielectric constant and
density. The weight by volume of aircraft fuel
Noncharacterized tank units are variable
depends on its density, which, in turn, depends
capacitors, vertically mounted in the fuel cell. As
on its temperature. As the temperature of the
the fuel level changes, the capacitance of the tank
fuel goes down, the density increases. As the
unit changes. This change in capacitance is
temperature of the fuel goes up, the density
uniform the entire length of the tank unit.
decreases. Any change in the dielectric constant
or density of the fuel affects the movement of the
Characterized tank units are similar in con-
indicator pointer. For example, assume that the
struction and mounting to the noncharacterized
indicating system is at balance with the tank unit
tank units. As the fuel level changes, the
immersed to a given depth. The fuel it is immersed
capacitance of the tank unit changes. This change
in has the density and dielectric constant of the
in capacitance is NOT uniform the entire length
fuel for which the system is calibrated. The
of the tank unit.
indicator pointer will then reflect the correct
amount of fuel in terms of pounds. Then, the
Since neither electrode of the tank unit goes
tanks are drained and then refilled to the same
level. This time the fuel has a greater density and
to ground, and one lead to the amplifier is
shielded, capacitance to ground does not enter
higher dielectric constant, which causes the
into the circuit. Therefore, the length of the tank
pointer to show a greater weight. The new reading
unit leads does not affect the accuracy of the
is correct only if the effect of the changes in
system.
density and dielectric constant are proportional.
6-60
