Lake Shore Model 340 Temperature Controller User’s Manual
2-2
Cooling System Design
2.1.3 Environmental
Conditions
Environmental factors such as high vacuum, magnetic field, corrosive chemicals, or even radiation may limit
the effectiveness of some sensors. Lake Shore has devoted much time to develop sensor packages that
withstand the environmental factors found in typical cryogenic cooling systems.
Magnetic field experiments are very common. Field dependence is an important selection criteria for
temperature sensors used in these experiments. Table 1-1 states the field dependence of most common
sensors. The Lake Shore Temperature Measurement and Control Catalog includes detailed field dependence
tables along with specific data on other environmental factors when available.
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 Paragraph 2.3. Have the sensor and instrument periodically recalibrated, or in some
other way null the time dependent errors. 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 leads will be heat sinked.
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 340, 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 340. The Lake Shore Temperature
Measurement and Control Catalog has complete accuracy specifications 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 340 where it is called a temperature response curve. Install a curve into a Model 340 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 340
operates in sensor units like ohms or volts. The Model 340 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 50 mK.