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Lake Shore Model 325 Temperature Controller User’s Manual
Cooling System Design 2-1
CHAPTER 2
COOLING SYSTEM DESIGN
2.0 GENERAL
Selecting the proper cryostat or cooling source is probably the most important decision in designing a temperature
control system. The cooling source defines minimum temperature, cool-down time, and cooling power. (Information on
choosing a cooling source is beyond the scope of this manual.) This chapter provides information on how to get the best
temperature measurement and control from cooling sources with proper setup including sensor and heater installation.
2.1 TEMPERATURE SENSOR SELECTION
This section attempts to answer some of the basic questions concerning temperature sensor selection. Additional useful
information on temperature sensor selection is available in the Lake Shore Temperature Measurement and Control
Catalog. The catalog has a large reference section that includes sensor characteristics and sensor selection criteria.
2.1.1 Temperature Range
Several important sensor parameters must be considered when choosing a sensor. The first is temperature range. The
experimental temperature range must be known when choosing a sensor. Some sensors can be damaged by temperatures
that are either too high or too low. Manufacturer recommendations should always be followed.
Sensor sensitivity is also dependent on temperature and can limit the useful range of a sensor. It is important not to
specify a range larger than necessary. If an experiment is being done at liquid helium temperature, a very high sensitivity
is needed for good measurement resolution at that temperature. That same resolution may not be required to monitor
warm up to room temperature. Two different sensors may be required to tightly cover the range from helium to room
temperature, but lowering the resolution requirement on warm up may allow a less expensive, one sensor solution.
Another thing to consider when choosing a temperature sensor is that instruments like the Model 325 are not able to read
some sensors over their entire temperature range. Lake Shore sells calibrated sensors that operate down to 50 millikelvin
(mK), but the Model 325 is limited to above 1 kelvin (K) in its standard configuration.
2.1.2 Sensor Sensitivity
Temperature sensor sensitivity is a measure of how much a sensor signal changes when the temperature changes. It is an
important sensor characteristic because so many measurement parameters are related to it. Resolution, accuracy, noise
floor, and even control stability depend on sensitivity. Many sensors have different sensitivities at different temperatures.
For example, a platinum sensor has good sensitivity at higher temperatures but has limited use below 30 K because its
sensitivity drops sharply. It is difficult to determine if a sensor has adequate sensitivity over the experimental
temperature range. This manual has specifications (Section 1.2) that include sensor sensitivity translated into temperature
resolution and accuracy at different points. This is typical sensor response and can be used as a guide when choosing a
sensor to be used with the Model 325.
2.1.3 Environmental Conditions
The experimental environment is also important when choosing a sensor. Environmental factors such as high vacuum,
magnetic field, corrosive chemicals, or even radiation can limit the use of some types of sensors. Lake Shore has devoted
much time to developing sensor packages that withstand the temperatures, vacuum levels, and bonding materials found
in typical cryogenic cooling systems.
Experiments done in magnetic fields are becoming very common. Field dependence of temperature sensors is an
important selection criteria for sensors used in these experiments. This manual briefly qualifies the field dependence of
most common sensors in the specifications (Section 1.2). Detailed field dependence tables are included in the Lake Shore
Temperature Measurement and Control Catalog. When available, specific data on other environmental factors is also
included in the catalog.