Lake Shore Model 340 Temperature Controller User’s Manual
2-4
Cooling System Design
2.2.3 Standard
Curves
Some types of sensors behave in a very predictable manner and a standard temperature response curve can
be created for them. Standard curves are a convenient and inexpensive way to get reasonable temperature
accuracy. Sensors that have a standard curve are often used when interchangeability is important. Some
individual sensors are selected for their ability to match a published standard curve and sold at a premium,
but in general these sensors do not provide the accuracy of a calibrated sensor. For convenience, the Model
340 has several standard curves included in firmware.
2.2.4 CalCurve™
The CalCurve service provides the user with a convenient way get the temperature response curve from Lake
Shore calibrated sensors into instruments like the Model 340. 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 340, the curve is loaded into a non-volatile
memory card which can be used for transfer into the instrument. The field installed version is a Model 3405-
128F-CALCRV and it should be ordered with the calibrated sensor. Customers that have a PC compatible
computer with an RS-232C or IEEE-488 interface have another option. The Model 8000 can also be ordered
with the calibrated sensor.
2.3 SENSOR
INSTALLATION
This section highlights some of the important elements of proper sensor installation. For more detailed
information, Lake Shore sensors are shipped 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. To further help users properly install sensors, Lake Shore offers a line of Cryogenic
Accessories. Many of the materials discussed are available through Lake Shore and can be ordered with
sensors or instruments.
Mounting materials is discussed in Paragraph 2.3.1. Sensor location is discussed in Paragraph 2.3.2. Thermal
conductivity is discussed in Paragraph 2.3.3. Contact area is discussed in Paragraph 2.3.4. Contact pressure
is discussed in Paragraph 2.3.5. Lead wire is discussed in Paragraph 2.3.6. Lead soldering is discussed in
Paragraph 2.3.7. Heat sinking leads is discussed in Paragraph 2.3.8. Finally, thermal radiation is discussed in
Paragraph 2.3.9.
2.3.1 Mounting
Materials
Choosing appropriate mounting materials is very important in a cryogenic environment. The high vacuum
used to insulate cryostats is one source of problems. Materials used in these applications should have 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 source of problems is the wide
extremes in temperature most sensors are exposed to. The linear expansion coefficient of a materials
becomes important when temperature changes are so large. Never try to permanently bond materials with
linear expansion coefficients that differ by more than three. A flexible mounting scheme should be used 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. Thermal conductivity is
a property of materials that can change with temperature. Do not assume that a heat sink grease that works
well at room temperature and above will do the same job at low temperatures.
2.3.2 Sensor
Location
Finding a good place to mount a sensor in an already crowded cryostat is never easy. There are less
problems 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 conducting through mounting structures can cause gradients. For best
accuracy, sensors should be positioned near the sample, so that little or no heat flows between the sample
and sensor. This may not, however, be the best location for temperature control as discussed below.