Operation and use of common test equipment was covered in NEETS Module 16, Introduction to
Test Equipment, NAVEDTRA B72-16-00-95. It is recommended that you review this module before
Most Navy technical manuals provide voltage charts that list correct voltages at all primary test
points in a piece of equipment. Voltage measurements, when compared with these charts, provide a
valuable aid in locating troubles quickly and easily. However, if the sensitivity of the test equipment
differs from that of the test equipment used in preparing the chart, the voltage measurements may not
reflect true circuit conditions. You must keep in mind that a voltmeter with low sensitivity used on a low
range may disturb circuits under test or provide a false indication. Most technical manuals will tell you
what type and model of test equipment was used to prepare the voltage charts. As a rule of thumb, the
input impedance of the voltmeter should exceed the impedance of the circuit by a ratio of at least 10 to 1.
Technicians have spent uncounted hours of wasted time because they have selected improper test
The input impedance of your test equipment should exceed the impedance of the circuit under
test by what ratio?
DC VOLTAGE MEASUREMENTS
Direct current voltage may be steady, pulsating, or have ac superimposed on it. The average value of
a dc waveform depends on the symmetry of the wave and other aspects of the wave shape. It can vary
from 63.6% of peak value for a rectified full sine wave to 50% of peak value for a triangular wave. For a
superimposed sine wave, the average value can be zero. Regardless of whether the dc is steady, pulsating,
or the ac is superimposed on the dc, a rectifier form of measuring device will indicate its average value.
Voltages are usually measured by placing the measuring device in parallel with the component or
circuit (load) to be measured. The measuring device should have an infinite internal resistance (input
impedance) so that it will absorb no energy from the circuit under test and, therefore, measure the true
voltage. The accuracy of the voltage measurement depends on the total resistance of the measuring device
compared to the load being measured. When the input impedance of the measuring device is 10 times
greater than the load being measured, the error usually can be tolerated. If this error cannot be tolerated, a
high input impedance measuring device, such as a vacuum tube voltmeter (vtvm), should be used.
Alternatively, using two voltmeters in series increases the voltage range and, because of the increase in
total voltmeter resistance, provides a more accurate measurement of voltage across the load. If the voltage
to be measured is sufficiently high, more than two similar voltmeters can be connected in series across the
load to provide greater accuracy; the total voltage measurement is the sum of the individual meter
What are the advantages of using two voltmeters in series?
A common piece of test equipment used in the Navy is the Simpson 260 analog multimeter, as
shown in figure 1-2. It is capable of measuring both ac and dc voltages of up to 5,000 volts.