Calibration; Calibration Schedule; Kelvin-Varley Divider Verification - Keithley 630 Instruction Manual

CALIBRATION
MODEL 630 POTENTIOMETRIC ELECTROMETER
Control
Divider Calibrate
Quad Adjustment
Meter Calibrate
DC Balance
Oscillator
Circuit
Fig.
Refer to
Desig.
Ref. Paragraph
R104 15 5-4
R131 16 4-8
R155 16 4-7
R157 16 4-5
R210 18 5-7
500-volt Calibrate R327 20 5-5
50-volt Calibrate R334 20 5-5
5-volt Calibrate R337 20 5-5
0.5-volt Calibrate R339 20 5-5
TABLE 6. Model 630 Internal Controls. The Table lists all internal controls, the fig-
ure picturing the location and the paragraph describing the adjustment.
5-2. CALIBRATION SCHEDULE.
a. Recalibrate the Model 630 yearly. This normally means performing the voltage range
calibration (paragraph 5-5). The other verifications need not be done.
b. Verify the Kelvin-Varley divider (paragraph 5-3) only if trouble is suspected in it,
or if components in the divider have been replaced. Also make the voltage range calibra-
tion after verifying the divider accuracy.
c. Check the reference voltage supply stability (paragraph 5-6) only if trouble is
suspected in the supply or if some of its components have been replaced.
5-3. KELVIN-VARLEY DIVIDER VERIFICATION.
a.
There is no in-field calibration for the Kelvin-Varley divider; its accuracy can
only be verified.
The divider accuracy depends upon matching resistors and switches. At
manufacture, each resistor within the first two Reference Voltage switches, S303 and
S304, is matched to *0.00125%. The resistors in the switches are checked as a set to an
accuracy of better than 0.0025%. Individual resistors cannot be replaced without remat-
ching the string at Keithley Instruments.
b. Kelvin-Varley Divider Accuracy Verification Procedures.
1. Use the Model 662 Differential Voltmeter or equipment with better limit of
error to match the Model 630 under test. Connect the Low end of potentiometer R38L of
the Model 630 to the HI terminal of the Model 662.
Connect both LO terminals.
NOTE
Be careful of high voltages when working within the Model 630. Up to 900 volts
dc is present at various points.
2. Set the dials to random settings on both instruments. Settings should match to
iO.Ol%. This procedure, however, only indicates the Kelvin-Varley divider accuracy.
The errors of the two instruments may be additive, causing a false verification. The
most accurate way is to use standard procedures for checking a Kelvin-Varley divider
or to return the Model 630 to Keithley Instruments, Inc., for checking.
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