User`s manual

2.2.1 Temperature Range 13
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Chapter 2: Cooling System Design
and Temperature Control
2.1 General
Selecting the proper cryostat or cooling source is probably the most important deci-
sion in designing a temperature control system. The cooling source defines the mini-
mum temperature, cool-down time, and cooling power. Information on choosing a
coo
ling 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.
2.2 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 cat-
alog has a large reference section that includes sensor characteristics and sensor
sel
ection criteria.
2.2.1 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 changes with temperature and can limit t
he useful range of a sen-
sor. It is important not to specify a range lar
ger 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 base temperature 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
lik
e the Model 335 are not able to read some sensors over their entire temperature
range. Lake Shore sells calibrated sensors that operate down to 20 millikelvin (mK),
but the Model 335 is limited to above 300 mK in its standard configuration.
2.2.2 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 sensitiv-
ities 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 experi-
mental temperature range. This manual has specifications (section 1.3) that include
sensor sensitivity translated into temperature resolution and accu
racy 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 335.