Section III. MEASURING TANK UNIT PROBE INSULATION RESISTANCE

TM 55-4920-325-14&P

S e c t i o n Ill. MEASURING TANK UNIT PROBE

INSULATION RESISTANCE

tive cover from Megohmmeter MIDSCALE ADJ

2-7. Scope.

control (16). Using insulted screwdriver, adjust

This section contains information and in-

Megohmmeter MIDSCALE ADJ control (16) until

structions for measuring tank unit probe in-

MEGOHMMETER indicator (14) pointer is on

sulation resistance on aircraft using liquid fuel

MIDSCALE calibration mark. Do not replace Me-

gaging systems.

gohmmeter MIDSCALE ADJ control (16) cover at

this time.

2-8. Operating Procedure.

WARNING

(5) Repeat steps (1) through (4), above,

as often as necessary until proper indications are

Do not operate ungrounded test set.

obtained.

Proceed as follows to measure tank unit probe in-

( 6 ) Install Megohmmeter ZERO ADJ

sulation resistance.

control (15) cover and Megohmmeter MIDSCALE

ADJ control (16) cover.

a. Check that the grounding lead has been

properly connected (see 2-4 b).

g. To measure leakage resistance between

elements of tank unit main probe, set FUNCTION

b. Check that power cable is connected from

SELECTOR switch (25) to TANK UNIT TEST-

test set to 115 volts, 400 Hz, single phase power

UNSH. To measure leakage resistance between

source (see 2-4c through e).

elements of tank unit compensator, set FUNC-

c. Connect test set to aircraft liquid quantity

TION SELECTOR switch (25) to TANK UNIT

fuel gage system as shown in figure 2-3.

TEST-COMP.

d. Set Power ON/OFF switch (2, figure 2-1)

NOTE

to ON and allow test set to warm up for at least

The three center positions of CAP-RES

five minutes.

CHECK switch (13) have letter designa-

e. Set CAP-RES CHECK switch (13) to EXT

tions A, B, and C which correspond to

RES position.

TANK UNIT connectors (10), (11), and

f. Check that MEGOHMMETER indicator

(12); A designates the coaxial con-

(14) is properly calibrated at ZERO and MID-

ductor, B designates the unshielded

conductor, and C designates the com-

SCALE points of indicator as follows:

pensator conductor.

( 1 ) Set Megohmmeter RANGE SE-

h. Set CAP-RES CHECK switch (13) to posi-

LECTOR switch (17) to ZERO CAL.

tion corresponding to resistance measurement to

(2) Observe that MEGOHMMETER indi-

be made.

cator (14) pointer is deflected to ZERO calibration

i. Set Megohmmeter RANGE SELECTOR

mark on dial. if pointer does not deflect to ZERO

switch (17) to multiplier setting so that ME-

calibration mark, unscrew protective cover from

GOHMMETER indicator (14) displays a reading

Megohmmeter ZERO ADJ control (15). Using in-

closest to midscale. For example, if resistance to

sulated screwdriver, adjust Megohmmeter ZERO

be measured is 50 megohms, the resistance can

ADJ control (15) until MEGOHMMETER indicator

be measured when Megohmmeter RANGE SE-

(14) pointer is on ZERO calibration mark, Do not

LECTOR switch (17) is set to X10, X100, or X1000

replace Megohmmeter ZERO ADJ control (15)

positions. However, an accurate value can be

cover at this time.

measured only at X100 setting. In the example

given, setting Megohmmeter RANGE SELECTOR

( 3 ) Set Megohmmeter RANGE SE-

switch (17) to X100 position causes indicator to

LECTOR switch (17) to MIDSCALE CAL.

read 0.5. Multiply reading displayed on ME-

(4) Observe that MEGOHMMETER indi-

GOHMMETER indicator (14) by setting on Me-

cator (14) pointer is deflected to MIDSCALE cal-

gohmmeter RANGE SELECTOR switch (17) to de-

ibration mark on dial. If pointer does not deflect

termine actual resistance in ohms.

to MIDSCALE calibration mark, unscrew protec-

2-12