Lakeshore 336, User Manual

Page: 28 / 192

16 c
HAPTER
2: Cooling System Design and Temperature Control
Model 336 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 when the sensor is compared to a calibration standard
and the temperature response of a sensor will shift with time and with repeated ther-
mal cycling (from very cold temperatures to room temperature). Instrument and sen-
sor manufacturers specify these errors, but there are things you can do to maintain
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. Calibrate the
sensor and instrument periodically, 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 There are different packages for the various types of sensors. 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 the leads will be thermally
anchored when choosing.
2.3 Sensor
Calibrations It can sometimes be confusing to choose the right sensor, get it calibrated, translate
the calibration data into a temperature response curve that the Model 336 can
understand, and then load the curve into the instrument. Lake Shore provides a vari-
ety of calibration services to fit different accuracy 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 avail-
able 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