CALIBRATING A STANDARD
OHMMETER
(VOM, OR VOLT-OHM-MILLIAMP METER)
Mechanical Adjustment Screw
Zero Ohms Adjustment Knob
Use to adjust the needle to zero ohms.
Selection Switch
Set to the Rx100 Scale.
Ohm
R X 100
+
-
Shorted Together
There are only two things to keep in mind when
calibrating an analog ohmmeter: the two ends of the scale.
The needle points to the left showing infinite ohms (the
meter is at rest) when the probes aren’t touching anything,
or all the way to the right showing zero ohms (maximum
measuring current) when the probes are shorted together.
When the two ends are calibrated, the numbers in
between take care of themselves. Take a look at the figure.
Step one: Place the meter “at rest” by separating the
probes. Turn the mechanical adjustment screw either way
to adjust the needle to show infinite ohms.
Step two: Set the selection switch to the desired
resistance scale: RX1 (resistance times 1), RX10 (times 10),
or RX100 (times 100). In the case of the insulation tester,
always use the RX100 scale. Now, short the test leads
together. Turn the ohms adjustment knob one way or the
other until the needle is exactly over the zero ohms on the
meter faceplate.
The meter is now calibrated. This calibration
adjustment should be done each time the meter is used to
measure resistance, or each time the selection switch is set
to a different RX position.
■ Method for calibrating an analog ohmmeter.
Worst Ground Fault
They tried to use a regular ohmmeter to find the
ground fault. I watched them. Right after sunset, while
there was enough light to see the cityscape and the lit
streetlights, I stood on top of the hill overlooking Duluth,
MN. They were power company linemen and they had just
driven up to a three-phase, 13,800 volt power pole across
the street from a TV station which was off the air because
of a power blackout. The lineman in the cherry picker used
his ohmmeter to check for shorts in the underground line.
A nine-volt battery won’t find much of a problem on a
13,800 volt power line, and that’s what was found — not
much of a problem. Well, they put in a new fuse at the top
of the pole, turned on the switch, and with the sound of a
shotgun and a 15 foot shower of sparks, the fuse blew. All
of the city of Duluth went black. There really was a short. A
proper insulation tester using a 15,000 volt power source
would have detected the problem.
Set meter to the Rx100 Scale.
Set the ohm calibration knob on the
meter to the middle position.
Select the value of the limiting resistor
for full deflection of the meter when
the leads are shorted together.
Ohm
R X 100
9 Volt
-+
9 Volt
-+
9 Volt
-+
9 Volt
-+
+
-
■ FIGURE 8. Electro-mechanical layout of all components
in the insulation tester.
If the meter can’t be zeroed with the ohm adjustment on
the meter, the resistance of the limiting resistor may have
to be adjusted using the experimental method.
Experimental Method
Center the ohms adjustment knob on the meter. This
will allow the knob to be adjusted either way at a later
time. Start out with a 20,000 ohm limiting resistor and
insert the resistor into the circuit (refer again to Figures 7
and 8). The needle probably won’t indicate zero ohms, so
try a resistor whose value is a little different. Remember, a
lower value of resistance will move the needle to the right.
Look at the ohmmeter again. Keep changing the value
of the limiting resistor until the needle is close to zero
ohms. This will become the limiting resistor. The ohms
adjustment on the meter can be used to touch up the
needle to point at zero ohms.
Cutting the Negative Test Lead
Use the black probe wire that comes with the
meter; it already has connectors. Cut it about seven
inches from the end that goes into the meter, and strip
the loose ends (see Figure 5). Seven inches should be
enough to secure the soldered end to the batteries and
still plug into the meter.
Solder this short lead to one end of the limiting
resistor and the other end of the resistor to the positive
battery clip lead. Tape or heat shrink the connections.
Solder the stripped end of the long probe wire to
the negative battery clip lead (see Figure 5). Tape or
heat shrink this connection.
Final Assembly
Tape the limiting resistor and both soldered black
test lead ends to the pigtail to give all solder
connections mechanical strength. This meter is going to
rattle around with the other tools in the tool box, and
September 2010 45
