2.3.1 Precision Calibration
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2.3.1 Precision
Calibration
Calibration is done by comparing 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 capital
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 335. 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 335, where it
is called a temperature response curve.
It is important to look at instrument specifications before ordering calibrated sen-
sors. A calibrated sensor is required when a sensor does not follow a standard curve
and you wish to display in temperature. Otherwise the Model 335 will operate in sen-
sor units like ohms or volts. The Model 335 may not work over the full temperature
range of some sensors. The standard inputs are limited to operation above 300 mK
even with sensors that can be calibrated to 20 mK.
2.3.2 SoftCal™
SoftCal™ is a good solution for applications that do not require the accuracy of a pre-
cision calibration. The SoftCal™ algorithm uses the well-behaved nature of sensors
that follow a standard curve to improve the accuracy of individual sensors. A few
known temperature points are required to perform SoftCal™. The Model 335 can also
perform a SoftCal™ calibration. You must provide one, two, or three known tempera-
ture reference points. The range and accuracy of the calibration is based on these
points (section 5.9).
Lake Shore offers two or three-point SoftCal™ calibrated sensors that include both
the large interpolation table and the smaller breakpoint interpolation table for 400
Series diodes and platinum sensors.
2.3.3 Sensors Using
Standard Curves
Some types of sensors behave in a very predictable manner and a standard tempera-
ture response curve can be created for them. Standard curves are a convenient and
inexpensive way to get reasonable temperature accuracy. Sensors that have a stan-
dard curve are often used when interchangeability is important. Some individual sen-
sors are selected for their ability to match a published standard curve, but in general
these sensors do not provide the accuracy of a calibrated sensor. For convenience, the
Model 335 has several standard curves included in firmware.
2.3.4 Curve Handler™
Lake Shore provides a software application called Curve Handler™, which makes
loading temperature curves into the Model 335 a very simple process. The program
can copy curves from properly formatted files into the Model 335 user curve loca-
tions. You can use it to read curves from the Model 335 and save them to files. A CD is
provided with Lake Shore calibrated sensors that contains all the proper formats to
load curves using the Curve Handler™ software program.