Sensor inputs

The Model 325 cryogenic temperature controller features two inputs with a high resolution 24-bit analog-to-digital converter and separate current sources for each input. Constant current excitation allows temperature to be measured and controlled down to 2.0 K using appropriate Cernox® RTDs or down to 1.4 K using silicon diodes. Thermocouples allow for temperature measurement and control above 1,500 K. Sensors are optically isolated from other instrument functions for quiet and repeatable sensor measurements. The Model 325 also uses current reversal to eliminate thermal EMF errors in resistance sensors. Sensor data from each input is updated up to ten times per second, with display outputs twice each second.

Standard temperature response curves for silicon diodes, platinum RTDs, ruthenium oxide RTDs, and many thermocouples are included. Up to fifteen 200-point CalCurves® (for Lake Shore calibrated sensors) or user curves can be stored into non-volatile memory. A built-in SoftCal®1 algorithm can also be used to generate curves for silicon diodes and platinum RTDs for storage as user curves. The Lake Shore curve handler software program allows sensor curves to be easily loaded and manipulated.

Sensor inputs for Model 325 are factory configured and compatible with either diodes/RTDs or thermocouple sensors. Your 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.

1 The Lake Shore SoftCal™ algorithm for silicon diode and platinum RTD sensors is a good solution for applications requiring more accuracy than a standard sensor curve but not in need of traditional calibration. SoftCal uses the predictability of a standard curve to improve the accuracy of an individual sensor around a few known temperature reference points.

Temperature control

The Model 325 cryogenic temperature controller offers two independent proportional-integral-derivative (PID) control loops. A PID 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. A high resolution digital-to-analog converter generates a smooth control output. The user can set the PID values or the Autotuning feature of the Model 325 can automate the tuning process.

Control loop 1 heater output for the Model 325 is a well-regulated variable DC current source. The output can provide up to 25 W of continuous power to a 50 Ω or 25 Ω heater load, and includes a lower range for systems with less cooling power. Control loop 2 heater output is a single-range, variable DC voltage source. The output can source up to 0.2 A, providing 2 W of heater power at the 50 Ω setting or 1 W at the 25 Ω setting. When not being used for temperature control, the loop 2 heater output can be used as a manually controlled voltage source. The output voltage can vary from 0 to 10 V on the 50 Ω setting, or 0 to 5 V on the 25 Ω setting. Both heater outputs are referenced to chassis ground.

The setpoint ramp feature allows smooth continuous setpoint changes and can also make the approach to setpoint more predictable. The zone feature can automatically change control parameter values for operation over a large temperature range. Ten different temperature zones can be loaded into the instrument, which will select the next appropriate value on setpoint change.

Temperature limit settings for inputs are provided as a safeguard against system damage*. Each input is assigned a temperature limit, and if any input exceeds that limit, all control channels are automatically disabled.

*Firmware version 1.5 and later


The Model 325 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. Sensor curves can also be entered and manipulated through either interface using the Lake Shore curve handler software program.

Configurable display

The Model 325 offers a bright, easy to read LCD display that simultaneously displays up to four readings. Display data includes input and source annunciators for each reading. All four display locations can be configured by the user. Data from either input can be assigned to any of the four locations, and the user’s choice of temperature or sensor units can be displayed. Heater range and control output as current or power can be continuously displayed for immediate feedback on control operation. The channel A or B indicator is underlined to indicate which channel is being controlled by the displayed control loop.

normal display configuration

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

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.

curve entry

Curve entry

The Model 325 display offers the flexibility to support curve, SoftCal™, and zone entry. Curve entry may be performed accurately and to full resolution via the display and keypad as well as computer interface.

325 rear panel