
Lake Shore Model 325 Temperature Controller User’s Manual
8-12
Service
Equipment Required for Calibration (Continued)
Resistor Standards
• Resistor standards with the following nominal values (if standards are not available, 0.25 W 25 ppm/
o
C metal film
resistors can be used) — they should have connectors attached to mate with two dual banana plugs for 4-lead
measurement
• 0
(short), 10
, 100
, 500
1 k
5 k
100 k
Miscellaneous
• Dummy loads for warm up, 1 each for diode/resistor inputs: 6-pin DIN (240°) connectors — plug, with 100 k
resistors configured for 4-lead measurement (calibration cable with 100 k
standard can be used)
• Short length of uninsulated wire, 1 each for thermocouple inputs
8.12.2 Diode/Resistor Sensor Input Calibration
NOTE:
The thermocouple input calibration procedure in provided in Section 8.12.3.
Overview
Each sensor input requires calibration. Sensor inputs contain a current source that can supply 10 µA or 1 mA of current;
only the 10 µA current sources are calibrated. They are calibrated by adjusting pots on the Model 325 main board. The
sensor inputs contain multiple gain stages to accommodate the various sensors the Model 325 supports. The input
circuitry is not adjusted during calibration. Instead, precision voltages and resistors are attached to each input and
mathematical calibration constants are calculated and programmed into the Model 325. Constants are stored to
compensate for both input offset and gain errors.
Calibration Process
8.12.2.1
Sensor Input Calibration Setup and Serial Communication Verification
Allow the Model 325 to warm up for at least 1 hour with 100 k
resistors attached to all inputs configured as
diode/resistor and all thermocouple inputs shorted. Connect the Model 325 to the PC via the serial port. Verify serial
communication by sending the *IDN? command and receiving the proper response from the Model 325. If the input not
being calibrated is diode/resistor, leave a 100 k
resistor attached. If the other input is a thermocouple, leave a short
across the input.
8.12.2.2
10 µA Current Source Calibration and 1 mA Current Source Verification
Purpose
To calibrate the 10 µA current source to be within the specified tolerance and verify operation of the 1 mA current
source.
Process
1. Configure the input for the silicon diode range.
2. Accurately determine the value of the 100 k
resistor using the DMM. Determine the calibration value by
multiplying the actual resistance of the 100 k
resistor by 10 µA.
Example: 100.050 k
× 10 × 10
–6
A = 1.00050 V.
3. Attach the 100 k
resistor to the Model 325 input using proper 4-lead connection techniques, configure the DMM to
read VDC and attach to the resistor.
4. Adjust the current source calibration pot, (R97 for Input A and R103 for Input B), on the Model 325 main board
until the DMM reads exactly the value calculated in Step 2 to ±0.00002 VDC.
5. 1 mA current source verification. Configure the input for the 100
Plat/250 range.
6. Accurately determine the value of the 1 k
resistor using the DMM. Determine the verification value by
multiplying the actual resistance of the
k
7. Attach the 1 k
resistor to the Model 325 input using proper 4-lead connection techniques, configure the DMM to
read VDC and attach to the resistor.
8. Verify the voltage across to resistor to be within ±0.3% of the value calculated in Step 6.
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