Lake Shore Model 331 Temperature Controller User’s Manual
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.
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.
2.3.3 Thermal
Conductivity
The ability of heat to flow through a material is called thermal conductivity. Good thermal conductivity
is important in any part of a cryogenic system that is intended to be the same temperature. Copper
and aluminum are examples of metals that 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, the connections between thermally conductive mounting surfaces often have very poor
thermal conductivity.
2.3.4 Contact
Area
Thermal contact area greatly affects thermal conduction because a larger area has more opportunity
to transfer heat. Even when the size of a sensor package is fixed, thermal contact area can be
improved with the use of a gasket material. A soft gasket material forms into the rough mating surface
to increase the area of the two surfaces that are in contact. Good gasket materials are soft, thin, and
have good thermal conductivity. They must also withstand the environmental extremes. Indium foil
and cryogenic grease are good examples.
Cooling System Design
2-5
