Lake Shore Model 218 Temperature Monitor User’s Manual
2-2
Sensor Considerations
2.1.4 Measurement
Accuracy
Temperature measurements have several sources of error. Account for errors induced by both the
sensor and the instrumentation when computing accuracy. The instrument has measurement error in
both reading the sensor signal and calculating a temperature using a temperature response curve.
Error results from the sensor comparison to a calibration standard; the sensor temperature response
shifts with time and repeated thermal cycling. Instrument and sensor makers specify these errors, but
some things help maintain good accuracy. For example, choose a sensor with good sensitivity in the
most critical temperature range, as sensitivity minimizes the effect of most error sources. Install the
sensor properly (Paragraph 2.3). Recalibrate the sensor and instrument periodically. Use a sensor
calibration appropriate for the accuracy requirement.
2.1.5 Sensor
Package
There are many types of sensor packages which generally determine sensor size, thermal and
electrical contact to the outside, and sometimes limit temperature range. Some sensors may be
purchased as bare chips without a package. When different packages are available for a sensor,
consider the sensor mounting surface and how to heat sink the leads.
2.2 CALIBRATED
SENSORS
It can be difficult to choose the right sensor, calibrate it, translate calibration data into a temperature
response curve understandable to the Model 218, and load the curve into the instrument. Lake Shore
offers a variety of calibration and curve loading services to fit different accuracy requirements and
budgets: Traditional Calibration in Paragraph 2.2.1, SoftCal™ in Paragraph 2.2.2, Standard Curves in
Paragraph 2.2.3, and the Lake Shore CalCurve™ Service in Paragraph 2.2.4.
2.2.1 Traditional
Calibration
Calibration 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 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.
Calibrated temperature sensors are the most accurate available from Lake Shore. Errors from sensor
calibration are almost always smaller than error contributed by the Model 218. The Lake Shore
Temperature Measurement and Control Catalog has complete accuracy specs for calibrated sensors.
Calibrated sensors include measured test data printed and plotted, coefficients of a Chebychev
polynomial fitted to the data, and two tables of data points used as interpolation tables optimized for
accurate temperature conversion. The smaller table, called a breakpoint interpolation table, fits into
instruments like the Model 218 where it is called a temperature response curve. Install a curve into a
Model 218 through a CalCurve™ (Paragraph 2.2.4) or manually through the instrument front panel.
Note 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 218 operates in sensor units like ohms or volts. The Model 218 may not work
over the full temperature range of some sensors. The Model 218 is limited to operation above 1 K or
more 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 predictability of sensors that follow a standard curve to
improve individual sensor accuracy. 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™ (refer to Paragraph 2.2.4) or front panel curve
entry (refer to Paragraph 5.2) may be required to get the breakpoint table into a Model 218 where it is
called a temperature response curve.
