TM 9-4935-282-34
803D
2-9. MEASUREMENT OF HIGH RESISTANCES
deflection.
(5) Multiply the amount set on the voltage readout
One of the important features of the 803D voltmeter is its ability
dials by 100 to find the resistance of the unknown in megohms.
to be used as a megohmmeter for rapidly measuring high
resistances from 1 megohm to 250,000 megohms. The
d. FROM 50,000 MEGOHMS TO 250,000 MEGOHMS.
following equation may be used to compute the resistance in
To determine the value of an unknown resistance between
megohms of an unknown connected to the input binding posts:
50,000 megohms and 250,000 megohms, proceed as follows:
(1) Perform preliminary procedure as stated in
Rx
=
10( E - 1) megohms,
(2) Set RANGE switch to 500 and NULL switch to
Em
0.1.
(3) Connect unknown resistance between input
where:
terminals.
Rx
=
unknown resistance in megohms
(4) Adjust voltage readout dials for a convenient
E
=
voltage indicated by voltage readout dials
meter deflection.
Em
=
voltage indicated on meter
(5) The resistance in megohms may be calculated
by substituting the meter reading in volts (Em , 0 to 0.1 volt on
10
=
megohms of input resistance of VTVM
circuit on 10, 1, and 0.1 null range.
bottom scale) and this voltage readout dial setting (E) into the
following equation:
When connecting the unknown resistance to the input
terminals, use short isolated leads to prevent measuring
Rx =
10 E Megohms
leakage resistance between the leads.
Em
a. FROM 1 MEGOHM TO 500 M GOHMS. For rapid
E
measurement of resistances from 1 megohm to 500 megohms,
2-10. NOTES ON MEASURING AC OR DC VOLTAGES
proceed as follows:
a. ADJUSTMENT OF 500 VOLT REFERENCE
(1) Perform preliminary procedure as stated in
SUPPLY. The 500 volt DC reference supply may be adjusted
(2) Set RANGE switch to 500 and NULL switch to
heed to the position of the switches and without removing any
10.
input or output connections. However, until the instrument has
(3) Connect unknown resistance between input
warmed up to an equilibrium temperature (about 1/2 hour), it
terminals.
should be adjusted prior to each specific measurement for best
(4) Adjust voltage readout dials for full scale meter
accuracy. When making prolonged measurements, allow one
deflection.
hour warmup time to insure that 500 volt reference supply does
(5) Subtract 10.00 from the amount set on the
not shift during the final warmup phase.
voltage readout dials to find the resistance of the unknown in
b. GROUND LOOP PRECAUTIONS.
Ground loop
megohms.
currents should be avoided to assure accuracy when making
b. FROM 500 MEGOHMS TO 5,000 MEGOHMS. For
measurements. Potential differences are often found at
rapid measurement of resistances from 500 to 5,000
different points on power system grounds. When this is the
megohms, proceed as follows:
case, current may flow from the power system ground through
(1) Perform preliminary procedure as stated in
the 803D and the equipment under measurement and back to
the power system ground. To avoid this when system being
(2) Set RANGE switch to 500 and NULL switch to
measured is grounded, do not connect shorting link from lower
1.
input binding post to chassis ground post.
(3) Connect unknown resistance between input
c. USE OF SHORTING LINK. A 0.47 uf capacitor (C1)
terminals.
is connected from the lower input binding post to the chassis
(4) Adjust voltage readout dials for full scale meter
ground post to reduce the effect of circulating AC currents from
deflection.
the transformer. In some cases, it is possible for C1 to acquire
(5) Subtract 1.00 from the amount set on the
a charge. For example, C1 will become charged through
voltage readout dials and multiply the result by 10 to find the
leakage resistance over a period of time if there is no external
resistance of the unknown in megohms.
connection to the input terminals and the controls are set as
c. FROM 5,000 MEGOHMS TO 50,000 MEGOHMS.
follows: range to 500, null to any null range, polarity to +, and
For rapid measurement of resistances from 5,000 to 50,000
voltage readout dials to several hundred volts. This condition
megohms, proceed as follows:
may cause an error on low level measurements (under 5 volts)
(1) Perform preliminary procedure as stated in
due to C1 discharging through the Kelvin-Varley divider.
Connecting the shorting link from the lower input post to the
(2) Set RANGE switch to 500 and NULL switch to
ground post will discharge C1 and thus prevent an inaccurate
0.1.
indication.
(3) Connect unknown resistance between input
terminals.
(4) Adjust voltage readout dials for full scale meter
2-6
