the panel. They provide connections to the tank
The Model TF-20 liquid quantity test set
unit probe under test. A pair of binding posts,
(fig. 2-37) contains circuitry that makes the tester
(EXT RES) connect the tester to unknown high
capable of measuring tank unit probe capacitance
external resistances. Use these bonding posts when
and measuring tank unit insulation resistance.
you can't connect the tester to the TANK UNIT
It also simulates total probe capacitance for
receptacles with the accessory cables. Another set
checking aircraft fuel quantity indicators and
of three BNC-type receptacles (COAX, UNSH, and
calibrating the test set capacitance indicator dial
COMP) are located on the bottom right of the panel.
and megohmmeter scales.
These three receptacles let you connect the test
You may test the fuel-gauging system either
set to the aircraft fuel-quantity-gauge indicator.
installed in the aircraft or removed from the air-
All adjustment controls for tester self-
craft. The test set includes a transit case, a shock-
mounted instrument case, and a set of adapter
calibration are accessible on the front panel. The
ZERO ADJ and HIGH ADJ controls provide
cables. The tester has circuitry for measuring
adjustment for zero and the high end of the scale
capacitance and dc resistance. It also simulates
on the CAPACITANCE indicator. The ZERO
the capacitance of compensated and uncompen-
ADJ and MIDSCALE ADJ controls adjust the
sated-type fuel tank probes. The instruments
MEGOHMS indicator so its pointer will position
operate with 115-volt, 400-hertz power.
at zero and at midscale. These four adjustments
All operating controls are on the front panel.
have covers so the calibrating controls will not be
Three BNC-type receptacles (COMP, COAX, and
accidently disturbed when using the test set.
UNSH) are located on the upper middle part of
Figure 2-37.-TF-20 liquid quantity test set.
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