(1) The RANGE switch is set to the lowest range giving an on-scale
deflection.
(2) The digital readout dials are set to the value determined in step
(1).
(3) The NULL switch is turned to the lowest position that still
allows excursions to stay on scale.
(4) The voltage deflections are observed on the meter.
Deflections
to the right indicate an increase in amplitude; to the left, a decrease in
amplitude.
b. When recording voltage excursions, it must be remembered that the
leakage resistance, recorder to ground, must be at least 10,000 megohms.
Less resistance will impair the accuracy of the voltmeter. If the recording
instrument has less than 10,000 megohms input resistance, an isolation
amplifier must be used.
The Fluke Model A88 is recommended for this
application as it permits the use of any recorder without regard to input
isolation characteristics.
c. To use the Model 887A with an isolation amplifier and recorder:
(1) The INPUT terminal is shorted to the COMMON terminal and the
voltmeter switches are set to:
RANGE
10
NULL
1
readout dials
1.00000
The meter will indicate full scale deflection (-1.0). This feeds up to a
maximum of 20 mv to the RECORDER output terminals, depending on the
adjustment of the AMP ADJ control.
(2) The recorder and amplifier are energized.
The
isolation
amplifier is connected to the RECORDER OUTPUT terminals.
(3) The AMP ADJ control
is
adjusted
for
the
desired
maximum
deflection of the recorder.
(4) The short from the INPUT and COMMON terminals is removed.
(5) Excessive current leakage is checked as follows:
A voltage is
connected to the input of the Model 887AB/AN and its magnitude is measured
in the most sensitive null range.
The recorder leads are alternately
connected and disconnected to the voltmeter.
A meter deflection of more
than one major scale division (10% of the null range) indicates excessive
leakage.
9.
One of the features of the 887AB/AN Differential Voltmeter is its
ability to be used as a megohmmeter for rapid measurements of high
resistance.
103
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