Lake Shore Model 218 Temperature Monitor User’s Manual
2-4
Sensor Considerations
2.2.4 CalCurve™
The CalCurve™ service provides the user with a convenient way to get the temperature response
curve from Lake Shore’s calibrated sensors into instruments like the Model 218. It can be performed
at the factory when calibrated sensors and instruments are ordered together. The factory installed
CalCurve™ option is Model 8001 and should be ordered with the calibrated sensor.
A CalCurve™ can be done in the field when additional or replacement sensors are installed. Curve
data is loaded into some type of non-volatile memory that is installed into the instrument by the user.
In the case of the Model 218, the curve is loaded into a non-volatile memory chip which can be
installed into the instrument. The field installed version is a Model 8002 and it should be ordered with
the calibrated sensor.
Customers that have an RS-232C or IEEE-488 interface have another option in loading curves from
calibrated sensors. A Model 8000 includes the curve and a download program on a disk. The down
load program is a good idea if sensors are changed often. The Model 8000 should also be ordered
with the calibrated sensor.
2.3 SENSOR
INSTALLATION
This section covers Mounting Materials in Paragraph 2.3.1, Sensor Location in Paragraph 2.3.2,
Thermal Conductivity in Paragraph 2.3.3, Contact Area in Paragraph 2.3.4, Contact Pressure in
Paragraph 2.3.5, Lead Wire in Paragraph 2.3.6, Lead Soldering in Paragraph 2.3.7, Heat Sinking Leads
in Paragraph 2.3.8, Thermal Radiation in Paragraph 2.3.9, and Thermal EMF Compensation with
Voltage Excitation in Paragraph 2.3.10.
For more detailed information, Lake Shore sensors ship with installation instructions that cover that
specific sensor type and package. The Lake Shore Temperature Measurement and Control Catalog
includes an installation section as well. Lake Shore also offers a line of cryogenic accessories. Many of
the materials discussed are available through Lake Shore and can be ordered with sensors or
instruments.
2.3.1 Mounting
Materials
The high vacuum used to insulate cryostats is one consideration in choosing sensor mounting
materials. Choose materials with a low vapor pressure so they do not evaporate or out-gas and spoil
the vacuum insulation. Metals and ceramics do not have this problem, but greases and varnishes
must be checked. Another consideration is temperature extremes most sensors are exposed to. The
linear expansion coefficient of a material becomes important when temperature changes are so large.
Never try to permanently bond materials with linear expansion coefficients that differ by more than
three. Use a flexible mounting scheme or the parts will break apart, potentially damaging them. The
thermal expansion or contraction of rigid clamps or holders could crush fragile samples or sensors
that do not have the same coefficient.
2.3.2 Sensor
Location
Positioning a sensor is less problematic if the entire load and sample holder are at the same
temperature. Unfortunately, this not the case in many systems. Temperature gradients (differences in
temperature) exist because there is seldom perfect balance between the cooling source and heat
sources. Even in a well-controlled system, unwanted heat sources like thermal radiation and heat
conduction through mounting structures can cause gradients. For best accuracy, position sensors
near the sample, so that little or no heat flows between the sample and sensor.
2.3.3 Thermal
Conductivity
Thermal conductivity is the ability of heat to flow through a material. Copper and aluminum have good
thermal conductivity, while stainless steel does not. Non-metallic, electrically-insulating materials like
alumina oxide and similar ceramics have good thermal conductivity, while G-10 epoxy-impregnated
fiberglass does not. Sensor packages, cooling loads, and sample holders should have good thermal
conductivity to reduce temperature gradients. Surprisingly, connections between thermally conductive
mounting surfaces often have very poor thermal conductivity. Thermal conductivity can change with
temperature. Do not assume a heat sink grease that works well at room temperature and above will
do the same job at low temperatures.
