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CALIBRATION
MODEL 630 PO'TENTIOMETRLC ELECTROMETER
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I
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Oil
Bath;
I
I
H
Model
150B
t
Null
Recorder
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FIGURE 12.
Circuit
Diagram
for Model
630 Reference
Voltage
Supply
Stability
Test.
The
voltage
across
the 530-ohm resistor
is slightly
higher
than
the standard
cell.
Use the
lo-kilohm
potentiometer
to shunt
the divider
voltage
down.
All
resistors
are wirewound.
See Table
5 for
recommended
equipment.
Adjust
the Voltage
Calibrator
to apply
0.5 volt
dc to the Model 630.
Adjust
the 0.5 v
CAL potentiometer,
R339 (Figure
ZO), for
an off-null
reading
on the Model
630 equal
to
the correction
factor
at 0.5 volt.
5-6.
REFERENCE VOLTAGE SUPPLY STABILITY
TEST.
a.
The reference
voltage
supply,
consisting
of the power transformer
and the main sup-
ply
(printed
circuit
PC92),
is
factory
calibrated
for
an output
of 500 volts
dc greater
than 0.002%.
The 500-volt
output
is adjustable
to meet specifications.
If
the stability
of the supply
is not within
specifications,
then
troubleshoot
for
a faulty
component.
b.
Routine
calibration
of the Model
630 does not require
a stability
test
of the ref-
erence
voltage
supply.
However,
a stability
test
is recommended if
one of the components
in the supply
is replaced.
c.
For the 24-hour
test,
the 500-volt
output
of the reference
voltage
supply
is divided
and compared
to a 1.02-volt
saturated
standard
cell
using
a sensitive
null
detector.
Var-
iations
between
the reference
voltage
supply
and the standard
cell
are detected
by the
Model
150A and are recorded
on a recorder.
Refer
to Figure
12 for
the block
diagram
of
the test
circuit.
d.
In using
the test
circuit,
the following
points
are important.
1.
Saturated
standard
cells,
though
extremely
stable
with
time,
have a high
tempera-
ture
coefficient
and require
a controlled
environment
during
use.
Therefore,
the Wes-
ton Oil
Bath,
which
is maintained
at ~350 M.OloC,
is used for
the test.
Unsaturated
standard
cells
have a lower
temperature
coefficient,
but
they
do not have the long term
stability
required
for
this
test.
2.
The resistor
divider
network
is
constructed
from wire
of the same spool
for
an
extremely
close
temperature
coefficient
match
(4 ppm, typically).
Additional
stability
26
1067R
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