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Lake Shore Model 325 Temperature Controller User’s Manual
2-2 Cooling System Design
2.1.4 Measurement Accuracy
Temperature measurements have several sources of error that reduce accuracy. Be sure to account for errors induced by
both the sensor and the instrumentation 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 standard 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 makers specify these errors
but there are things a user 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 2.3. Have the sensor 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.
2.1.5 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 limits temperature range. When different packages are available for a sensor, the user should consider the
mounting surface for the sensor and how leads will be heat sinked when choosing.
2.2 CALIBRATED SENSORS
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 325 can understand, then getting the curve loaded into
the instrument. Lake Shore provides a variety of calibration and curve loading services to fit different accuracy
requirements and budgets.
2.2.1 Traditional 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 Laboratory in Great Britain. These standards allow Lake Shore to calibrate sensors from 50 mK to above room
temperature. Calibrated sensors are more expensive than uncalibrated sensors of the same type because of the labor and
capitol equipment used in the process.
This type of calibration provides the most accurate temperature sensors available from Lake Shore. Errors from sensor
calibration are almost always smaller than the error contributed by the Model 325. 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 coefficients of a Chebychev polynomial that
has been fitted to the data, and two tables of data points to be used as interpolation tables. Both interpolation tables are
optimized to allow accurate temperature conversion. The smaller table, called a breakpoint interpolation table, is sized to
fit into instruments like the Model 325 where it is called a temperature response curve. Getting a curve into a Model 325
may require a CalCurve™ described below or hand entering through the instrument front panel.
It is important to look at instrument specifications before ordering calibrated sensors. A calibrated sensor is required
when a sensor does not follow a standard curve
if the user wishes to display in temperature. Otherwise the Model 325
will operate in sensor units like ohms or volts. The Model 325 may not work over the full temperature range of some
sensors. The standard inputs in are limited to operation above 1 K even with sensors that can be calibrated to 50 mK.
2.2.2
SoftCal™
SoftCal is a good solution for applications that do not require the accuracy of a traditional 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.
Lake Shore sells SoftCal calibrated sensors that include both the large interpolation table and the smaller breakpoint
interpolation table. A CalCurve may be required to get the breakpoint table into a Model 325 where it is called a
temperature response curve. Refer to Section 2.2.4.
The Model 325 can also perform a SoftCal calibration. The user must provide one, two, or three known temperature
reference points. The range and accuracy of the calibration is based on these points. Refer to Section 5.3.