Lake Shore 350, User Manual

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2.8.5 Sensor Package 29
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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 2.10. 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.
2.8.5 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.
2.9 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 350 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.
Sensor diode sensor calibrations
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
2.9.1 Precision
Calibration To calibrate, Lake Shore compares a sensor with an unknown temperature response
to an accepted standard. Lake Shore temperature standards are traceable to the
U.S. National Institute of Standards and Testing (NIST) or the National Physical Labo-
ratory in Great Britain. These standards allow Lake Shore to calibrate sensors from
20 mK to above room temperature. Calibrated sensors are more expensive than
uncalibrated sensors of the same type because of the labor, cryogen use, and capitol
equipment used in the process.
Precision calibration provides the most accurate temperature sensors available from
Lake Shore. Uncertainty from sensor calibration is almost always smaller than the
error contributed by the Model 350. The Lake Shore Temperature Measurement and
Control Catalog has complete accuracy specifications for calibrated sensors.
Calibrated sensors include the measured test data printed and plotted, the coeffi-
cients of a Chebychev polynomial that have been fitted to the data, and two tables of
data points to be used as interpolation tables. Both interpolation tables are opti-
mized to allow accurate temperature conversion. The smaller table, called a break-
point interpolation table, is sized to fit into instruments like the Model 350 where it is
called a temperature response curve.
TABLE 2-2