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SECTION
I
INTRODUCTION AND SPECIFICATIONS
1-1. INTRODUCTION
a. This instruction manual is for use with the 825A
series Differential
DC
Voltmeters. The 825A voltmeters
are available with either a standard cell or a Zener diode
as the reference element. Models containing the Zener
diode option are denoted by a /AG in the model number.
Both the standard cell option and the Zener diode option
are available in a cabinet model or a model designed to
fit a standard 19 inch rack. Rack models are denoted
by
an
R immediately following the 825A part of the model
number.
b. The high accuracy, portability, and compactness
of the 825A make this instrument ideal for the precise
measurement of almost any DC voltage. Ease of opera-
tion, inherent protection from accidental overload, and
high reliability ccntribute
to
the outstanding performance
that assures suitability for both production line testing
and precision laboratory measurements. This instru-
ment is capable of being used as a vacuum tube volt-
meter, as a precision potentiometer, and as a meg-
ohmmeter for measurement of high resistance.
It
can
also be used to measure the excursions of a voltage
about some nominal value. One feature that should be
emphasized is that no current is drawn from the unknown
source when balance is attained. Thus the determination
of the unknown potential is independent of its source
resistance. The 825A is basically used for measure-
ments of DC voltages from 0 to 500 volts. However,
high accuracy measurements to 30, 000 volts DC are
possible with precision voltage dividers especially de-
signed for use with the F1uke Model 800 Series Differ-
ential Voltmeters. As additional features, this instru-
ment contains a polarity switch for equal convenience
in measuring positive or negative voltages and an ad-
justable recorder output which makes the 825A particu-
larly useful for monitoring the stability of almost any
DC voltage.
Furthermore, the 825A meter has taut-
band suspension which eliminates problems due to meter
stickiness.
c. When used as a differential voltmeter, the 825A
operates on the potentiometric principal. An unknown
voltage is measured by comparing it to a known adjust-
able reference voltage with the aid of a null detector.
An accurate standard for measurements is obtained by
setting the reference supply with a standard cell for the
825A and 825AR and with a Zener diode for the 825A/ AG
and 825AR/ AG. The known adjustable reference voltage
is provided by a 500 volt DC power supply and five
Kelvin-Varley decade resistor strings that are set ac-
curately by five voltage readout dials. In this way, the
500 volts can be precisely divided into increments as
small as 10 microvolts. The unknown voltage is then
simply read from the voltage dials. In the highest null
sensitivity range, potential differences between the un- .
known and the reference voltage of only 0. 001 volts will·
cause full scale meter deflection.
d. The instrument is normally supplied ready for use
on 115 volts. Upon request, instruments are supplied
for 230 volts operation.
If
it becomes desirable
to
con-
vert from one mode of operation to the other, refer to
the instruction decal on the back of cabinet models or
on the cover in rack models.
1-2. DAMAGE IN SHIPMENT
Immediately upon receipt, thoroughly inspect for any
damage that may have occurred in transit.
If
any damage
is noted, follow the instructions outlined on the warranty
page at the back of this manual.
1-3. SPECIF1CATIONS
AS A PRECISION POTENTIOMETER
ACCURACY: ±0. 025% of input voltage from 0. 1 volt
to 500 volts. ±(0. 025% + 25 uv) below 0. 1 volt.
RANGE
Input Resistance
Input
Recommended
per volt of input voltage
Voltage
Null
Range
Range*
At
1% of Full
Null
Scale Off Null
50-500
10-0-10
Infinite
100 Meg
1-0-1
Infinite
1, 000 Meg
5-50
1-0-1
Infinite
1, 000 Meg
0.1-0-0.1
Infinite
10, 000 Meg
P.
5-5
0.1-0-0.1
Infinite
10, 000 Meg
0.01-0-0.01
Infinite
100, 000 Meg
0-0.5
0.01-0-0.01
Infinite
100, 000 Meg
0.001-0-0.001 Infinite
100, 000 Meg
*Any null range may be used with any input vol-
tage range; recommended ranges are those most
useful.
RESOLUTION: Voltage Readout Dial Resolution
Range
Null
Setting
Setting
Resolution
500
any
lOmv
50
any
1 mv
5
any
0.1 mv
0.5
any
0. 01 mv
1-1
_,..:
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