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CHAPTER 8 ELECTRONIC TEST EQUIPMENT
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TM-9-254 Gearcase Transfer M548 M548A1 (
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Figure 8-1.  Proper Voltmeter Connection
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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