Lakeshore 336, User Manual

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18 c
HAPTER
2: Cooling System Design and Temperature Control
Model 336 Temperature Controller
There are two versions of the Curve Handler™ application. The fully featured version
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 336, and allows you to manipulate the temperature curves directly in the pro-
gram window. This version will also work with all existing Lake Shore temperature
controller and temperature monitor instruments. The Windows™ version of the
Curve Handler™ application is available, free of charge, from the Lake Shore website
at www.lakeshore.com.
The second version of Curve Handler™ is written in the Java™ programming language
and is available through the Ethernet web interface on the Model 336. This version
allows you to copy curves from files to the Model 336, and vice versa, but it does not
allow manipulation of curve data and only works using the Ethernet interface. Refer
to section 6.4.4 for details on connecting to the web interface and opening the
embedded Curve Handler™ application.
2.4 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 Tem-
perature Measurement and Control Catalog includes an installation section as well.
To further help you 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.
2.4.1 Mounting
Materials 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 large.
Never try to permanently bond materials with linear expansion coefficients that dif-
fer 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.
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.
2.4.2 Sensor Location 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, position sensors 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.
2.4.3 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 con-