Lake Shore Model 321 Autotuning Temperature Controller User’s Manual
Application Notes
D-1
APPENDIX D
APPLICATION NOTES
D1.0 GENERAL
This appendix includes these Lake Shore Applications Notes:
1. Fundamentals For Usage Of Cryogenic Temperature Controllers – Application Note ................ Page D-1
2. Standard Curve 10 – Technical Data ........................................................................................... Page D-8
3. DT-470 Series Temperature Sensors Installation and Operation – Application Note................ Page D-10
4. Measurement System Induced Errors In Diode Thermometry – Article Reprint ........................ Page D-14
FUNDAMENTALS FOR USAGE OF CRYOGENIC TEMPERATURE CONTROLLERS
by
Dr. John M. Swartz
• Lake Shore Cryotronics
Lawrence G. Rubin
• MIT National Magnet Laboratory
575 McCorkle Blvd. • Westerville, OH 43082
170 Albany St. • Cambridge, MA 02139
I INTRODUCTION
Cryogenic temperature controllers have been available for years, but users often have an incomplete understanding of
their operating principles and of the closed-loop interactions between the controller and the controlled low temperature
environment. The object of this primer is to address this problem by presenting some fundamental and practical concepts
of 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 emphasis
will be placed on analog control systems, the advantages and disadvantages of digital versus analog control will also be
presented.
II CHARACTERISTICS OF CRYOGENIC TEMPERATURE CONTROL SYSTEMS
The adjective "cryogenic" as applied to temperature control systems defines a set of conditions that distinguishes such
systems from those for which the great majority of applications exist, i.e., industrial processes in which temperatures are
above—and often well above—room temperature. There are at least five factors which crucially affect temperature control
performance when one compares a cryogenic system with that existing inside a furnace, for example:
1.
The values of heat capacity (lower, C
p
, and thermal conductivity (often higher,
κ
, are such that much shorter thermal
time constants (
τ
α
C
p
/
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 the
10-100uV/°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 level
signals, and hence require a low noise background. For that reason, ripple-free direct current, usually controlled by a
series 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 quite
large
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 input
required to maintain a set point temperature is approximately constant. This is because the heat loss through a fixed
thermal conductance to the room temperature environment outside the furnace is also constant. However, there are
cryogenic systems where the low temperature environment provided by, e.g., a surrounding cryogen such as a liquid
helium or liquid nitrogen bath, may vary drastically as the level of the cryogen changes. In addition, the thermal
conductance to the outside world is highly dependent on the gas pressure (vacuum) maintained in the cryostat. The
resulting variations in "cooling power" will cause the heat input requirements to be anything
but
constant. A few
cryogenic 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 parameters
are involved.
