Lakeshore 336, User Manual

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2.4.4 Contact Area 19
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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).
2.4.4 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 like indium
foil and cryogenic grease. 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 environ-
mental extremes. Indium foil and cryogenic grease are good examples.
2.4.5 Contact Pressure When sensors are permanently mounted, the solder or epoxy used to hold the sensor
act as both gasket and adhesive. Permanent mounting is not a good solution for
everyone because it limits flexibility and can potentially damage sensors. Much care
should be taken not to over heat or mechanically stress sensor packages. Less perma-
nent mountings require some pressure to hold the sensor to its mounting surface.
Pressure greatly improves the action of gasket material to increase thermal conduc-
tivity and reduce thermal gradients. A spring clamp is recommended so that different
rates of thermal expansion do not increase or decrease pressure with temperature
change.
2.4.6 Lead Wire Different types of sensors come with different types and lengths of electrical leads. In
general a significant length of lead wire must be added to the sensor for proper ther-
mal anchoring and connecting to a bulk head connector at the vacuum boundary. The
lead wire must be a good electrical conductor, but should not be a good thermal con-
ductor, or heat will transfer down the leads and change the temperature reading of
the sensor. Small 30 AWG to 40 AWG wire made of an alloy like phosphor bronze is
much better than copper wire. Thin wire insulation is preferred, and twisted wire
should be used to reduce the effect of RF noise if it is present. The wire used on the
room temperature side of the vacuum boundary is not critical, so copper cable is
normally used.