Page 320 - Electrician - TT (Volume 1)
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ELECTRICIAN - CITS


           Calibration of MI Ammeter and Voltmeter


           Objectives: At the end of this lesson you shall be able to:
           •   define the term ‘calibration’
           •  explain the calibration of  voltmeter and ammeter.



           Calibration
           In  many  industrial  operations,  measurement  instruments  must  be  trusted  to  provide  the  accuracy  stipulated
           by the original design to assure a satisfactory product.  This confidence is provided by a periodic testing and
           adjustment of the instrument to verify the required performance.  This type of maintenance is called calibration.
           Standards
           Before calibration can begin, you must have the accurately known values of the measured quantities against
           which to compare the measurements made by the instrument being calibrated.  Thus, for an instrument that is
           supposed to measure current of 1 milli ampere, you must have, for comparison, a source of current that is known
           to within at least that range or better.  Only then you can say whether the instrument performs satisfactorily.
           A very accurately known quantity used for calibration of instruments is known as a standard.
           Calibration standards
               Quantity          Standard

               Voltage           Standard cell, high precision source

               Current           Voltage standard and standard resistance standard milli volt source, gas filled/
                                 mercury filled thermometers.

           Calibrating DC and AC meters (Ammeter & Voltmeter)
           Both DC and AC meters are calibrated in essentially the same way.  To calibrate a DC meter, a very accurate DC
           current source is connected to the meter.  The output of the current source must be variable, and some means
           must be available to monitor the output current of the source.  Many sources have built-in meter for this purpose.
           The output of the current source is varied in very small steps, and at each step the scale of the meter being
           calibrated is marked to correspond to the reading on the monitoring device.  This procedure is continued until the
           entire scale of the meter is calibrated.
           Same procedure is used to calibrate an AC meter, except that a 50/60 cps sine wave is used mostly.  Also, you
           know that an a-c meter reads the average value of a sine wave, but it is desirable for the meter to indicate rms
           values.  Therefore the rms equivalent are calculated and marked on the scale.
           Thermocouple meters are calibrated on the basis of a sine wave.  But the calibration is made at the frequency
           at which the meter will be used.  At the extremely high frequencies at which it is used, a phenomenon known as
           skin effect occurs.

           At these frequencies, the current in a wire travels at the surface of the wire, the higher the frequency, the closer
           the current moves to the surface of the wire.  This effect increases the resistance of the thermocouple heater wire
           because the diameter of the wire becomes, in effect, smaller.
           Thus the resistance of the heater wire varies with frequency.  Since the resistance of the heater wire varies with
           frequency,  thermocouple meters must be calibrated at specific frequencies.

           Precautions to be observed when using an ammeter in measurement work
           1   Never connect an ammeter across a source of EMF.  Because of its low resistance it would draw damaging
              high currents and damage the delicate movement.  Always connect an ammeter in series with a load capable
              of limiting the current.
           2   Observe the correct polarity.  Reverse polarity causes the meter to deflect against the mechanical stop and
              this may damage the pointer.







                                                           307

 CITS : Power - Electrician & Wireman - Lesson 50-53  CITS : Power - Electrician & Wireman - Lesson 50-53
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