The Simpson 260 multimeter (fig. 3-35) is a
much higher potential than that supplied by the
typical vom used in the Navy today. It has a meter
battery of an ohmmeter. This potential is placed
movement for you to use to take the readings.
between the conductor and the outside of the
You must look at the switches to determine which
insulation. A megohmmeter (Megger) is used for
scale to use on the meter. A disadvantage of this
these tests. Figure 3-37 shows a portable megohm-
type of vom is it tends to "load" the circuit under
meter with two main elements: (1) a hand-driven
test.
dc generator, which supplies the necessary
voltage for making the measurement, and (2) the
ELECTRONIC DIGITAL
instrument portion, which indicates the value of
MULTIMETERS
the resistance you are measuring.
The electronic digital multimeter can do the
same job as the multimeter. The main difference
between the two types of multimeters is the
electronic digital multimeter display is digital and
Many different types of oscilloscopes are
it has a higher input impedance. Because of the
available. They vary from relatively simple test
higher input impedance of the electronic digital
instruments to highly accurate laboratory models.
multimeter, the loading effect is less and the
measurements taken are more accurate.
One example of a typical electronic digital
multimeter in use within the Navy is the Model
8000A electronic digital multimeter. Most
electronic digital multimeters overcome the
disadvantage of requiring a continuous external
power source by combining an external ac source
with an internal rechargeable battery. Figure 3-36
shows the Model 8000A electronic digital
multimeter.
MEGOHMMETER
You cannot use an ordinary ohmmeter for
measuring multimillion ohm values of resistances,
such as those in conductor insulation. To test for
such insulation breakdown, you need to use a
Figure 3-37.--Megohmmeter.
Figure 3-36.--Digital voltmeter operating features.
3-27
