Lake Shore 321-01 User Manual
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Lake Shore Model 321 Autotuning Temperature Controller User’s ManualApplication Notes D-1APPENDIX DAPPLICATION NOTESD1.0 GENERALThis appendix includes these Lake Shore Applications Notes:1. Fundamentals For Usage Of Cryogenic Temperature Controllers – Application Note ................ Page D-12. Standard Curve 10 – Technical Data ........................................................................................... Page D-83. DT-470 Series Temperature Sensors Installation and Operation – Application Note................ Page D-104. Measurement System Induced Errors In Diode Thermometry – Article Reprint ........................ Page D-14FUNDAMENTALS FOR USAGE OF CRYOGENIC TEMPERATURE CONTROLLERSbyDr. John M. Swartz • Lake Shore Cryotronics Lawrence G. Rubin • MIT National Magnet Laboratory575 McCorkle Blvd. • Westerville, OH 43082 170 Albany St. • Cambridge, MA 02139I INTRODUCTIONCryogenic temperature controllers have been available for years, but users often have an incomplete understanding oftheir operating principles and of the closed-loop interactions between the controller and the controlled low temperatureenvironment. The object of this primer is to address this problem by presenting some fundamental and practical conceptsof control at low temperatures. The so-called "three-mode" or "PID" controller, utilizing Proportional (gain), Integral (reset),and Derivative (rate) functions, will be discussed and examples given of its operation and adjustment. While the emphasiswill be placed on analog control systems, the advantages and disadvantages of digital versus analog control will also bepresented.II CHARACTERISTICS OF CRYOGENIC TEMPERATURE CONTROL SYSTEMSThe adjective "cryogenic" as applied to temperature control systems defines a set of conditions that distinguishes suchsystems from those for which the great majority of applications exist, i.e., industrial processes in which temperatures areabove—and often well above—room temperature. There are at least five factors which crucially affect temperature controlperformance when one compares a cryogenic system with that existing inside a furnace, for example:1. The values of heat capacity (lower, Cp , and thermal conductivity (often higher, κ, are such that much shorter thermaltime constants (τ α Cp/κ) are the rule at low temperatures.2. The temperature sensor used in a furnace is almost always one of a variety of thermocouples with sensitivities in the10-100uV/°C range. In the cryogenic regime, resistance thermometers (both metallic and semi-conductive), diode,and capacitance thermometers provide from one to three order-of-magnitude higher sensitivity.3. The heat input for furnaces is almost always derived from a line frequency source, and is controlled by relays,variable transformers, saturable reactors, or SCRs. Experiments performed in a cryostat usually involve low levelsignals, and hence require a low noise background. For that reason, ripple-free direct current, usually controlled by aseries transistor bank, should be used to power the heater.4. As one traverses the cryogenic regime from the liquid helium range up towards room temperature, there can be quitelarge variations in both the thermal time constants and thermometer sensitivities.5. In the case of the furnace in which the load does not experience large endo- or exothermic reactions, the heat inputrequired to maintain a set point temperature is approximately constant. This is because the heat loss through a fixedthermal conductance to the room temperature environment outside the furnace is also constant. However, there arecryogenic systems where the low temperature environment provided by, e.g., a surrounding cryogen such as a liquidhelium or liquid nitrogen bath, may vary drastically as the level of the cryogen changes. In addition, the thermalconductance to the outside world is highly dependent on the gas pressure (vacuum) maintained in the cryostat. Theresulting variations in "cooling power" will cause the heat input requirements to be anything but constant. A fewcryogenic systems employ a controller cooling loop, but this type of system will not be discussed.Most of the difficulties in cryogenic control applications are associated with factors 4 and 5, where changes in parametersare involved. |
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