More about the Model 332
Sensor inputs
The Model 332 cryogenic temperature controller features two inputs, with a high-resolution 24-bit analog-to-digital converter and separate current source for each input. Sensors are optically isolated from other instrument functions for quiet and repeatable sensor measurements. The two sensor inputs included in the Model 332 can be configured to measure and control nearly any diode, RTD, and thermocouple temperature sensor.
Sensor inputs for both versions of the Model 332 are factory configured and compatible with either diode/RTDs or thermocouple sensors. The purchaser’s choice of two diode/RTD inputs, one diode/RTD input and one thermocouple input, or two thermocouple inputs must be specified at time of order and cannot be reconfigured in the field. Software selects appropriate excitation current and signal gain levels when sensor type is entered via the instrument front panel.
With NTC RTD sensors at temperatures as low as 500 mK, and with resistance being as high as 75 kΩ, the Model 332 automatically provides an excitation current down to 1 µA. This minimizes sensor self-heating induced errors. At higher temperatures, when resistance is low and concern for sensor self-heating is minimal, the Model 332 provides an excitation current up to 1 mA for a better signal to noise ratio and high measurement resolution. The Model 332 also uses current reversal to eliminate thermal electromotive force (EMF) errors for all resistive sensors.
Standard temperature response curves for silicon diodes, platinum RTDs, and many thermocouples are included. Up to twenty 200-point CalCurves™ for Lake Shore calibrated sensors or user curves can be loaded into non-volatile memory via a computer interface or the instrument front panel. A built-in SoftCal™1 algorithm can also be used to generate curves for silicon diodes and platinum RTDs, for storage as user curves.
Temperature control
For the greatest flexibility in temperature control, the Model 332 has two independent, proportional-integral-derivative (PID) control loops that drive two heater outputs of 50 W and 10 W.
A PID control algorithm calculates control output based on temperature setpoint and feedback from the control sensor. Wide tuning parameters accommodate most cryogenic cooling systems and many small high-temperature ovens. Control output is generated by a high resolution digital-to-analog converter for smooth, continuous control. The user can set the PID values manually or the Autotuning feature of the Model 332 can automate the tuning process.
The Loop 1 heater output is a well-regulated variable DC current source. The output is optically isolated from other circuits to reduce interference and ground loops. The output can provide up to 50 W of continuous power to a resistive heater load, and includes two lower ranges for systems with less cooling power. The second control loop heater output is a single-range, variable DC voltage source that can vary from 0 V to 10 V. The output can source up to 1 A of current providing a maximum of 10 W of heater power.
The setpoint ramp feature allows smooth continuous changes in setpoint and also makes the approach to a setpoint temperature more predictable. The zone feature can automatically change control parameter values for operation over a large temperature range. Values for ten different temperature zones can be loaded into the instrument, which will select the next appropriate zone value on setpoint change.
1The Lake Shore SoftCal™ algorithm for silicon diode and platinum RTD sensors is a good solution for applications that need more accuracy than a standard sensor curve but not traditional calibration. SoftCal™ uses the predictability of a standard curve to improve the accuracy of an individual sensor around known temperature reference points.
Interface
The Model 332 cryogenic temperature controller includes both parallel (IEEE-488) and serial (RS-232C) computer interfaces. In addition to data gathering, nearly every function of the instrument can be controlled via computer interface. Also included is a Model 330 command emulation mode that makes the Model 332 interchangeable with the older Model 330 in software-controlled systems.
Each input has a high and low alarm which offer latching and non-latching operation. The two relays on the Model 332 can be used in conjunction with the alarms to alert the operator of a fault condition or perform simple on-off control. Relays can be assigned independently to any alarm or be operated manually.
When not being used for temperature control, the loop 2 control output can be used as an analog voltage output. It can be configured to send a voltage, proportional to temperature, to a data acquisition system. The user may select the scale and data to be sent to the output, including temperature, sensor units, or linear equation results. Under manual control, the analog voltage output can also serve as a voltage source for other applications.
Configurable display
The Model 332 cryogenic temperature controller includes a bright vacuum fluorescent display that simultaneously displays up to four readings. Frequently used functions can be controlled with one or two keystrokes on the front panel. Display data includes input and source annunciators for each reading. All four display locations can be configured by the user. Data from either input may be assigned to any of the four locations. The user’s choice of temperature, sensor units, maximum, minimum, or linear equation results can be displayed. Heater range and control output as current or power can also be continuously displayed numerically or as a bar graph for immediate feedback on control operation.
Normal (default) display configuration
The display provides four reading locations. Readings from each input and the control setpoint can be expressed in any combination of temperature or sensor units, with heater output expressed as a percent of full scale current or power.
Flexible configuration
Reading locations can be configured by the user to meet application needs. The character preceding the reading indicates input A or B or setpoint S. The character following the reading indicates measurement units or the math function in use.