by the digital ohmmeter and the
resistance measured by the insulation
tester will be quite different.
To check the accuracy of the
insulation tester, use it to measure the
resistance of the control resistor. (Make
sure to keep your fingers off the leads
because they will corrupt the readings.)
Look up the true resistance of the
resistor on the cross reference table.
The measured resistance of the
control resistor should be about the
same whether the digital ohmmeter is
used or the insulation tester is used.
Water, on the other hand, has a voltage-based, non-linear resistance. That helps
explain why the insulation tester — with its high voltage
battery — detects a definite problem the water on the
insulation, while the digital ohmmeter — with its low
voltage battery — shows uncertain readings.
If you really want to have fun with this experiment, try
shaking a bunch of salt into the puddle of water and
measure the water’s resistance again with both meters.
Try This At Home
Will the insulation tester detect
problems that a regular digital ohmmeter
won’t? Find out! Try this on one type of
insulation problem: water on the wiring.
Spill a little tap water on a clean
counter. The puddle should be about one
or two inches in diameter. Just don’t add
any salt or other contaminants to the
water. Measure the resistance of the
water using a regular digital ohmmeter,
keeping the tips of the probes at least
1/2 inch apart as shown in Figure A.
Write down the resistance indicated on
the ohmmeter. Some digital ohmmeters
will measure infinite resistance, and if that is measured,
that’s what should be written down. As a control, find a
resistor of about the same value and measure its
resistance. Write down that measurement.
Use the insulation tester to measure the same water.
Look up the actual resistance on the cross reference table
and write down that resistance. The resistance measured
■ FIGURE A. Testing the resistance
of a puddle of water. The probes
here are about 1/2 inch apart and
both of them are in the water.
resistance reaches zero ohms (the leads shorted together),
the current through the meter is at maximum and the
needle is all the way to the right. The numbers on the face
of the meter make up a conversion table or cross
reference table. It converts the current flowing through the
device being measured to the resistance of that device.
The numbers on the meter’s face are then multiplied by
the switch setting of RX1, RX10, or RX100 to calculate the
total resistance being measured. A digital ohmmeter does
this cross reference automatically; it converts the test
current to resistance to give a direct reading on the display.
Creating a Cross Reference Table
With the added batteries and limiting resistor, the
numbers on the face of the meter are no longer factory
calibrated. For instance, a resistor under test with my
ohmmeter may show 25 on the ohms scale. Normally, this
September 2010 47