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Lake Shore Model 325 Temperature Controller User’s Manual
2-4 Cooling System Design
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 325. It can be performed at the factory when calibrated sensors and
instruments are ordered together. The factory installed CalCurve option is Model 8001-325 and should be ordered with
the calibrated sensor. A CalCurve can be done in the field when additional or replacement sensors are installed.
Customers that have a PC-compatible computer with an RS-232C or IEEE-488 interface can load the curve into the
instrument using one of the computer interfaces. The Model 8000 CalCurve is offered on CD or via e-mail free of charge
at time of order to any customer who orders a calibrated sensor. See Section 7.2 for details.
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 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 fewer 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.