Keithley 630 Instruction Manual page 27

CALIBRATION MODEL 630 POTENTIOMETRIC ELECTROMETER
NOTE
The accuracy of the Model 630 calibration will be no greater than the
accuracy of the voltage source used for calibrating. Unless the user is
familiar with techniques for obtaining accuracy greater than 0.002%
(20
PPm) > it is better to return the Model 630 to the factory for range
calibration.
c.
The most critical part in range calibration is establishing a reference source
whose accuracy exceeds 0.002%.. Use the Model SV194B Voltage Calibrator for the 500, 50,
5 and 0.5-volt outputs. Establish the accuracy of these outputs by determining the cor-
rections for the calibrator's CALIBRATION and OUTPUT dials at these outputs. The system's
accuracy can be determined to approximately 10 ppm. Added
to
the accuracy of the standard
cell, total accuracy should be approximately 12 ppm. Note that this depends upon proper-
ly executed procedures.
d.
Procedures to Establish System Accuracy.
1.
Set up the system shown in Figure 10. Use the 9.9-megohm extender for only the
0.5 and 5-volt output.
2.
Establish the corrections for the CALIBRATION dial setting for Model SV194B out-
puts of 500, 50, 5 and 0.5 volts.
Set the Model 241 Voltage Supply to 500 volts.
set
the Model RV722 Divider to a voltage equal to that of the standard cell.
Adjust the
Model SVl94B ratio dial until the null detector indicates a null.
The difference be-
tween the settings of the Model RV722 Divider and the ratio dial is the Model SVl94B
correction factor at 500 volts. With this correction, the 500 volts may be set to
within approximately 12 ppm.
3.
Connect the null detector to the 50, 5 and 0.5-volt taps of the Model SVl94B Cal-
ibrator in that order.
Set the Model RV722 Divider to corresponding voltages.
Use the
9.9-megohm extender for the 5 and 0.5-volt outputs. (See Figure 10). The difference
read on the null detector is the correction factor for each of the three voltages.
These voltages may also be set to within 12 ppm.
4.
Use the four correction factors for calibrating the Model 630 range setting.
f. Procedures for Range Calibration.
1.
Set up the system shown in Figure 11.
The standard cell should be certified to
10 ppm.
Use the dial correction factors found for each output.
If the system accuracy
was properly determined,
the input voltage to the Model 630 should be correct to 20 ppm.
2.
500-volt Range Calibration: Set the Model 630 controls as follows:
DETECTOR INPUT Switch 1010 OHMS
RANGE Switch
500
Reference Voltage Dials 4 9 9.9 9 10
NULL Switch VTVM
POLARITY Switch +
Adjust the Voltage Calibrator to apply 500 volts to the Model 630.
Turn the Model 630
NULL Switch to 10 MV and adjust the 500 V CAL potentiometer, R327 (Figure ZO), for
null on the Potentiometric Electrometer.
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