Measurement of High Resistance

TM 11-6625 -537-14-1

voltage excursions, carefully complete the

g. Measurement of High Resistance. T h e

following steps:

electronic voltmeter may be used to measure high

(1) Select a recorder with at least 10,000

resistances between 1 and 250,000 megohms. In

m e g o h m s leakage resistance to ground. An

this application, a known voltage (E), indicated

isolation amplifier with at least 10,000 megohms

by the voltage readout dials, is applied to a series

resistance to ground may be used to adapt the

circuit consisting of the unknown resistance (Rx),

electronic voltmeter to recorders with excessive

and the input resistance (10 megohms) of the

leakage resistance.

voltmeter circuit. The voltage (Em) across the 10

mehohm resistance is indicated by the VOLTS

CAUTION

meter. The value of the unknown resistance may

Do not ground the RECORDER OUT-

PUT terminals of the electronic voltmeter

b e exactly d e t e r m i n e d from the following

or the recorder input terminals, Accuracy

equation:

of the electronic voltmeter will be com-

Rx (megohms) = 1 O ( E I }

pletely degraded,

(2) Connect recorder to RECORDER

P r o c e e d with the following steps to rapidly

OUTPUT terminals of electronic voltmeter,

determine the value of an unknown resistance:

initial

operation

(1) Perform

(3) Perform initial operation in accordance

with paragraph 3-5.

accordance with paragraph 3-5.

(2) Set RANGE switch to 500 and NULL

(4) Check for excessive leakage as follows:

switch to 10.

( a ) Connect a dc voltage to input of

(3) Connect unknown resistance to input

electronic voltmeter and differentially measure its

t e r m i n a l s of electronic voltmeter. Use short

potential (para 3-7 b ).

i s o l a t e d leads to prevent measuring leakage

(b) Alternately connect and disconnect

resistance between leads.

recorder leads from RECORDER OUTPUT

(4) Adjust voltage readout dials to obtain

terminals of electronic voltmeter while observing

full-scale deflection of VOLTS meter (meter will

VOLTS meter. More than one small scale division

deflect left). Set NULL switch to 1 or .1 if full

deflection indicates that excessive leakage to

scale deflection cannot be obtained with NULL

ground has been introduced by the recorder and

control set to 10. Determine value of unknown

w i l l degrade the accuracy of the electronic

resistance as shown in table 3-3 when fullscale

voltmeter.

deflection is obtained.

( c ) Disconnect dc voltage from input

(5) If full-scale deflection cannot be ob-

terminals.

t a i n e d , set NULL control to .1 and voltage

(5) Adjust electronic voltmeter controls as

r e a d o u t dials to maximum. Note the VOLTS

follows:

meter indication on lower scale, full scale is 0.1

(a) Set RANGE control to 50.

volt. Determine unknown resistance from the

(b) Set NULL control to 10.

following equation:

Rx (megohms) = 10 E where:

+ (positive).

(d) Set Voltage readout dials to 10,000.

Rx is the unknown resistance

( 6 ) Short electronic voltmeter input ter-

E is the voltage indicated by the voltage readout

minals together. VOLTS meter will deflect full

dials

scale.

(7) Adjust electronic voltmeter GAIN ADJ

Em is VOLTS meter indication.

c o n t r o l (fig. 2-2) to obtain desired recorder

Example: Voltage readout dials A, B, C, D and E

deflection corresponding to fun-scale deflection of

are set to 4, 9, 9, 9, 10, VOLTS meter needle is on

electronic voltmeter

.34. This information could be used to determine

(8) Remove short from electronic voltmeter

that E is 500, Em is 0.034, and that Rx is 143,000

input terminals.

megohms.

(9) The electronic voltmeter and recorder are

(6) Disconnect resistance from input ter-

now ready for combined use. Proceed as in-

minals.

structed in paragraph 3-7 e.

3-9