not heat the gauge blade sufficiently to bend it, and the
resistance in the sending unit. When the float reaches
blade holds the pointer at the empty reading.
the bottom, the sending unit presents a near zero
resistance. This provides a maximum current flow
When the tank is full, the float is at the top and the
through the limiting coil and little through the operating
cam takes the position shown in view B of figure 6-76.
coil. The limiting coil draws the pointer to the totally
In this position, the cam pushes the grounded contact
against the insulated bimetal contact, bending the
bimetal blade in the sending unit. Since the bimetal is
This type of gauge consumes very little current,
then under a strain, the current must flow longer to bend
about one-eighth of an ampere. Since the operation of
it sufficiently to open the contacts. The longer flow of
this gauge depends on the difference in the magnetic
current causes a bending of the bimetal blade in the
effect between two coils, variations in the battery
gauge unit, pushing the pointer over to the full position.
voltage will not cause an error in the gauge reading.
The contacts open and close fast enough to give a
THERMOSTATIC FUEL GAUGE.--The ther-
steady reading by the pointer. The maximum current
mostatic electrical fuel gauge has bimetal blades in the
requirement for a full reading is less than one-fourth
gauge and the sending unit. This type of gauge may also
ampere. This type of gauge is not affected by variations
be used as a temperature or oil pressure gauge by using
of battery voltage, and is compensated for outside air
changes in temperature or pressure to operate the
movable grounded contact in the sending unit.
When the tank is empty and the float is down, as
Oil Pressure Gauge
shown in view A of figure 6-76, the two contacts in the
sending unit are just touching. Current flows through
The oil pressure gauge indicates the oil pressure in
the resistance heater wires of both gauge and sending
the system. Usually, such gauges are mounted on the
units, causing the bimetal blades to bend. Bending of
instrument panel and are calibrated to read oil pressure
the bimetal blade in the sending unit separates the
in pounds per square inch.
contacts to break the circuit. The heater wire cools
BOURDON TUBE OIL PRESSURE
when the current stops flowing, and the bimetal blades
GAUGE.-- Bourdon tube oil pressure gauges are
return to their original position. Contact is again made,
actuated by the pressure of air trapped above the oil in a
and the cycle of operation is repeated approximately
very small copper tube connected from the gauge to the
every second. Opening and closing of the contacts
lubricating system. Air pressure in the connecting tube
produces an intermittent flow of current, which does
O P E R AT I O N W I T H TA N K E M P T Y
O P E R AT I O N W I T H TA N K F U L L
Figure 6-76.--Thermostatic fuel gauge circuit.