14 cHAPTER 2: Cooling System Design and Temperature Control
Model 335 Temperature Controller
2.2.3 Environmental
Conditions
The experimental environment is also important when choosing a sensor. Environ-
mental 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 very common. Field dependence of tempera-
ture 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 1.3). 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.
2.2.4 Measurement
Accuracy
Temperature measurements have several sources of uncertainty that reduce accu-
racy. Be sure to account for errors induced by both the sensor and the instrumenta-
tion when computing accuracy. The instrument has measurement error in reading
the sensor signal, and error in calculating a temperature using a temperature
response curve. Error results from the sensor being compared to a calibration stan
-
dard and the temperature response of a sensor will shift with time and with repeated
thermal cycling (from very cold temperatures to room temperature). Instrument and
sensor manufacturers specify these errors, but there are things you can do to main
-
tain good accuracy. For example, choose a sensor that has good sensitivity in the most
critical temperature range, as sensitivity can minimize the effect of most error
sources. Install the sensor properly following guidelines in
section 2.4. Have the sen-
sor and instrument periodically recalibrated, or in some other way null the time
dependent errors. Use a sensor calibration that is appropriate for the accuracy
requirement.
2.2.5 Sensor Package
Many types of sensors can be purchased in different packages. Some types of sensors
can even be purchased as bare chips without any package. A sensor package generally
determines its size, thermal and electrical contact to the outside, and sometimes lim
-
its temperature range. When different packages are available for a sensor, you should
consider the mounting surface for the sensor and how leads will be thermally
anchored when choosing.
2.3 Sensor
Calibrations
There can sometimes be confusion in the difficult task of choosing the right sensor,
getting it calibrated, translating the calibration data into a temperature response
curve that the Model 335 can understand, and then getting the curve loaded into the
instrument. Lake Shore provides a variety of calibration services to fit different accu
-
racy requirements and budgets.
Best Precision calibration All sensors can be calibrated over various temperature ranges.
Lake Shore has defined calibration ranges available for each sensor type.
Better SoftCal™ An abbreviated calibration (2-point: 77 K and 305 K; 3-point: 4.2 K, 77 K,
and 305 K; or 3-point: 77 K, 305 K, and 480 K), which is available for 400
Series silicon diodes and platinum sensors
Good Sensors using standard curves Silicon diodes follow standard curves
Platinum resistors follow standard curves
Ruthenium oxide (Rox™) resistors follow standard curves
Thermocouples follow standard curves
GaAlAs diode, carbon-glass, Cernox™, germanium, and rhodium-iron
sensors can be purchased uncalibrated, but must be calibrated to
accurately read in temperature units
TABLE 2-1 Sensor diode sensor calibrations
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