Sensor Installation; Mounting Materials; Sensor Location; Thermal Conductivity - Lakeshore 335 User Manual

16 c 2: Cooling System Design and Temperature Control
HAPTER
2.4 Sensor
Installation
2.4.1 Mounting
Materials

2.4.2 Sensor Location

2.4.3 Thermal
Conductivity
Model 335 Temperature Controller
The Curve Handler™ application is a 32-bit Microsoft® Windows® application that
must be installed on a Windows® PC. This version works with the IEEE-488 and USB
computer interfaces on the Model 335, and allows the temperature curves to be
manipulated directly in the program window. This version will also work with all
existing Lake Shore temperature controller and temperature monitor instruments.
Curve Handler™ is available free of charge from the Lake Shore website at
www.lakeshore.com.
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 Temper-
ature Measurement and Control Catalog includes an installation section as well. To
further help you properly install sensors, Lake Shore offers a line of cryogenic accesso-
ries. Many of the materials discussed are available through Lake Shore and can be
ordered with sensors or instruments.
Choosing appropriate mounting materials is very important in a cryogenic environ-
ment. The high vacuum used to insulate cryostats is one consideration. 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 consideration is the
wide extremes in temperature most sensors are exposed to. The linear expansion
coefficient of materials becomes important when temperature changes are so large.
Never try to permanently bond materials with linear expansion coefficients that dif-
fer 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 thermal anchor grease that works well at
room temperature and above will do the same job at low temperatures.
Finding a good place to mount a sensor in an already crowded cryostat is never easy.
There are fewer problems if the entire load and sample holder are at the same tem-
perature. 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 in section 2.4.3.
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 con-
ductivity, while G-10 epoxy-impregnated fiberglass does not. Sensor packages, cool-
ing 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 (refer to section 2.4.4
and section 2.4.5).