Measurement Of Voltage Excursions About A Nominal Value - Fluke 825A Instruction Manual

FULL-SCALE RANGE NULL VOLTAGE METER
DEFLECTION SWITCH SWITCH DIALS SCALE
..
DC
500-0-500 500 VTVM No effect Upper
50-0-50 50 VTVM No effect Upper
5-0-5
5 VTVM
No effect Upper
0.5-0-0.5 0.5 VTVM No effect Upper
10-0-10 No effect 10 All zero Lower
1-0-1 No effect 1 All zero Lower
0. 1-0-0.l No effect 0.1 All zero Lower
0.01-0-0.01
No effect 0.01 All zero
Lower
0. 001-0-0. 001 No effect 0.001 All zero Lower
Figure 2-3. VTVM RANGES
2-6. MEASUREMENT OF VOLTAGE EXCURSIONS
ABOUT A NOMINAL VALUE
a. Perform preliminary operation as stated in para-
graph 2-3.
b. Connect voltage to be observed to input binding
posts. If meter reads to the left, the voltage being
measured is negative; set polarity switch to the negative
position in this case. This will cause meter pointer to
deflect to the right.
c. Set RANGE switch to lowest range which will give an
on-scale meter indication and note nominal value of vol-
tage indicateq.
d. Set the five voltage readout dials to nominal voltage.
e. Turn NULL switch to lowest position that will allow
voltage excursions to remain on scale.
f. Read voltage excursions from meter. Note that full
scale right and left meter deflections are equal to the
NULL voltage setting. Meter deflection to the right in-
dicates that the voltage under observation has increased
above the nominal value while deflection to the left in-
dicates it has decreased.
2- 7. USE OF 825A WITH A RECORDER
Recorder output binding posts and an output level control
are provided on the 825A for monitoring the excursions of
an unknown voltage from the voltage indicated by the vol-
tage dial settings. If the leakage resistance between the
recorder and ground is less than 10, 000 megohms, the
accuracy of the 825A will be impaired. Also, both input
posts of the recorder should
be
able to stand 500 volts
DC
to ground. Therefore, the John Fluke Model A-70 Po-
tentiometric Recorder (manufactured by the Texas Instru-
ment Co. ) is recommended for this application. Set up
the recorder as follows:
a. Co!Ulect A- 70 recorder to 825A voltmeter with teflon
leads as shown in figure 2-4.
2-4
NOTE
Do
not ground either of the 825A output ter-
minals or either of the A-70 input terminals.
If any of th.ese terminals are grounded, cur-
rent will be drawn from the 825A Kelvin-
Varley divider and the accuracy of the 825A
will be completely destroyed.
b. After connecting the recorder, perform preliminary
operation as stated in paragraph 2-3.
c. Check for excessive leakage as follows:
(1) Connect a voltage to the input of the 825A and
differentially measure its potential.
(2) Alternately connect and disconnect the recorder
leads from the output terminals of the 825A while noting
the meter needle deflection. More than one-quarter of a
small scale division deflection indicates that excessive
leakage has been introduced by the recorder. This will
impair the accuracy of the 825A voltmeter.
(3) Disconnect the voltage.
d. After the leakage test has been successfully com-
pleted, short the input terminals of the 825A.
e. Set switches on the 825A voltmeter as follows:
RANGE
NULL
voltage read-
out dials
50
10
10.000
The 825A meter will indicate full scale deflection. This
provides
up
to a maximum of approximately 20 millivolts·
at the output terminals depending on the setting of the
output level control.