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2.10.3 Sensor Installation 47
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Small 30 to 40 AWG wire made of an alloy like Manganin or phosphor bronze is much
better than copper wire. Super conducting wire is a good choice if wire temperature
remains below the transition temperature. Thin wire insulation is preferred for better
heat sinking. Twisted wire is recommended if electrical noise is present. The wire used
on the room temperature side of the vacuum boundary is not critical so shielded cop-
per cable is normally used.
2.10.3.7 Lead Soldering
When additional wire is soldered to short sensor leads, care must be taken not to
overheat the sensor. A heat sink such as a metal wire clamp or alligator clip will heat
sink the leads and protect the sensor. Lead wires should be tinned before bonding to
reduce the time that heat is applied to the sensor lead. Solder flux should be cleaned
after soldering to prevent corrosion.
2.10.3.8 Heat Sinking Leads
Sensor leads can be a significant source of error if they are not properly heat sunk.
Heat will transfer down even small leads and alter the sensor reading. The goal of
heat sinking is to cool the leads to a temperature as close to the sensor as possible.
This can be accomplished by putting a significant length of lead wire in thermal con-
tact with every cooled surface between room temperature and the sensor. Lead wires
can be adhered to cold surfaces with varnish over a thin electrical insulator like ciga-
rette paper. They can also be wound onto a bobbin that is firmly attached to the cold
surface. Some sensor packages include a heat sink bobbin and wrapped lead wires to
simplify heat sinking.
2.10.3.9 Thermal Radiation
Thermal (black body) radiation is one of the ways heat is transferred. Warm surfaces
radiate heat to cold surfaces even through a vacuum. The difference in temperature
between the surfaces is one thing that determines how much heat is transferred.
Thermal radiation causes thermal gradients and reduces measurement accuracy.
Many cooling systems include a radiation shield. The purpose of the shield is to sur-
round the load, sample, and sensor with a surface that is at or near their temperature
to minimize radiation. The shield is exposed to the room temperature surface of the
vacuum shroud on its outer surface, so some cooling power must be directed to the
shield to keep it near the load temperature. If the cooling system does not include an
integrated radiation shield (or one cannot be easily made), one alternative is to wrap
several layers of super-insulation (aluminized Mylar) loosely between the vacuum
shroud and load. This reduces radiation transfer to the sample space.