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TM 9-254
8-1.
Types of Meters - Continued
c.
Digital Multimeter. The digital multimeter performs the same basic functions as the multimeter and the
TRVM. The major difference is that the digital multimeter uses all electronic components and the measured quantity is
displayed as individual numbers or digits. This accounts for a faster operation than that of the multimeter and TRVM and a
more accurate indication since the quantity is displayed in numerical form.
(1)
Circuit loading effects. Like the TRVM, the digital multimeter contains input amplifier circuits which gives
the meter a high input impedance. The high input impedance draws less test circuit current which
reduces the effect of circuit loading. Most digital multimeters are also protected against high input
voltages which might be accidentally applied to the input terminals during normal use.
(2)
Accuracy. Greater measurement accuracy is obtained when using a digital display which eliminates the
need to interpret a meter indication. The measured quantity is expressed in direct numerical form.
8-2.
Meter Usage.
a.
General. Meters are designed for a variety of uses such as the wattmeter for measuring power or the field
strength meter used to measure the strength of transmitted radio signals. However, this section will deal only with the
three most commonly used meters, the voltage meter, the current meter, and the ohmmeter.
(1)
Voltage measurements (fig. 8-1). When attempting to measure a voltage, the operator should always
remember that a voltmeter is connected in parallel to the voltage source being measured. This is
necessary because the voltmeter has a very high internal resistance. If the voltmeter were connected in
series, the circuit would become inoperative. Prior to connecting a voltmeter, the operator should
determine a few basic facts about the circuit.
(a)
If the voltage to be measured is DC voltage, the polarity must be determined before the connection
is made. This is necessary since most meter movements operate on a direct current basis and
applying a reverse polarity voltage to the meter could damage the meter movement.
(b)
If quantity of the voltage to be measured is unknown, always select the highest voltage range
available on the meter. If the selected range does not give an adequate indication, reduce the
range setting one range at a time until an on scale indication is obtained.
(c)
When making an AC voltage measurement, disregard the polarity since this type of voltage has no
definite polarity. However, most voltage meters measure AC voltage in terms of the root-mean-
square (RMS) value. Thus all AC voltage measurements will be the RMS value of the sine wave.
8-2
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