Keithley 630 Instruction Manual page 15

OPERATION MODEL 630 POTENTIOMETRIC ELECTROMETER
of the shunt resistor depends upon the
current to be measured and the setting of
the Model 630 (Refer to Table 2).
b.
Current measuring procedure:
Shunt the Model 630 input with a
seikcted resistor R. (See Table 2 for
resistor values).
2.
Set the Reference Voltage Dial Set-
ting to zero and the DETECTOR INPUT
Switch to OPEN.
3. Switch the Model 630 sensitivity
to the lowest.setting to obtain an on-
scale reading.
Current Range Shunt Resistor NULL Setting
in amperes in ohms
in millivolts
10-13
10 10 1
10-13 1011
10
10-12
109 1
10-12 1010
10
10-12 1011
100
10-11
108
1
I
10-11 10-11 10-11 109 1010 1011
10
100
1000
I
TABLE 2. Recommended Shunt Resistors for
Use in Current Measurements.
4. The Meter reading is the voltage drop V across the shunt resistor R.
5.
Use the Model 630 as a potentiometer and bring the meter to a null with the Re-
ference Voltage Dials. Record the Reference Voltage Dial reading Ed.
6.
Calculate the current I, using the equation,
1
I, =-
Ed v
R (- Ed - V
where I, is the current to be measured in amperes;
R is the shunt resistance in ohms;
V is the meter reading (from step 4 above) in volts:
Ed is the setting of the Reference Voltage Dials in volts.
NOTE
It is recommended that shunt resistors no larger than 1011 ohms be used
a larger resistor the response is extremely slow.
An offset which 1s largFth
.
enough
to
affect accuracy may not show up immediately due to the slow response.
c.
The accuracy of the measurement is i3% plus the accuracy of the shunt resistor, if
the above procedure is followed. However, when using a 1011 shunt resistor the accuracy
of measurement may be *4% plus the accuracy of the shunt resistor.
It is necessary to
calculate the current to compensate for the loading effect of the shunt resistor.
12
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