Lake Shore Model 321 Autotuning Temperature Controller User’s Manual
Operation
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3.5.5 Internal Offset Adjustment
When a new or different thermocouple is attached to the controller, you must adjust the offset to
compensate for discrepancies in thermocouple material, leads, and connections. Offset adjustment
trimpots are provided inside the Model 321 to allow offset calibration of the thermocouple.
1. Place the thermocouple in a reference bath of known temperature (liquid nitrogen, ice, etc.). Allow
the system to stabilize to the reference temperature.
2. On the front panel, select the thermocouple input and the desired temperature units.
3. Turn on thermocouple compensation.
4. Remove the controller cover.
5. Adjust the offset adjustment trimpot (R60) so that the displays read the reference temperature.
NOTE:
The offset adjustment compensates for the thermocouple used in the calibration.
If another thermocouple is attached, or the thermocouple has aged, or the
configuration of the system is changed, then the offset adjustment must be repeated.
3.5.6 Curve
Format
The input is hardware limited to reading input between -45 mV and +45mV. All curves should be
limited in temperature so not to exceed these values. If thermocouple compensation is desired, the
thermocouple curve must be normalized to zero in degrees Celsius. Compensation also limits the
practical range of the card by approximately the room temperature voltage of the thermocouple used.
The Model 321 is designed to operate on sensor curve data in the range of 0.00000 to 9.00000 volts
so thermocouple voltage must be converted to this range before it is entered into a curve table. To
obtain the proper table value from a thermocouple voltage, it must be summed with 45 millivolts to
make it positive and multiplied by one hundred to shift the resolution.
A -45.0000 millivolt thermocouple voltage will result in a 0.00000 volt table value and +45.0000
millivolts will result in 9.00000 volts.
