Lake Shore Model 340 Temperature Controller User’s Manual
2-6
Cooling System Design
2.3.6 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 heat sinking and connecting to a bulk head
connector at the vacuum boundary. The lead wire must be a good electrical conductor, but a poor
thermal
conductor, or heat will transfer down the leads and change the temperature reading of the sensor. Small 30 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.
2.3.7 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. Leads 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.
2.3.8 Heat Sinking Leads
Sensor leads can be a significant source of error if they are not properly heat sinked. 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 contact 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 cigarette 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.
2.3.9 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
surround 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.
2.4 HEATER SELECTION AND INSTALLATION
There is a variety of resistive heaters that can be used as the controlled heating source for temperature
control. The mostly metal alloys like nichrome are usually wire or foil. Shapes and sizes vary to permit
installation into different systems.
Heater Resistance and power are discussed in Paragraph 2.4.1. Heater location is discussed in Paragraph
2.4.2. Heater types are discussed in Paragraph 2.4.3. Finally, heater wiring is discussed in Paragraph 2.4.4.
2.4.1 Heater Resistance and Power
Cryogenic cooling systems have a wide range of cooling power. The resistive heater must be able to provide
sufficient heating power to warm the system. The Model 340 can supply up to 100 W of power to a heater
(if the heater resistance is appropriate). The Model 340 heater output current source has a maximum output
of 2 A, limiting maximum power to:
Max Power (watts) = (2 A)
2
× Resistance (ohms).
Even though the Model 340 output is a current source, it has a voltage limit (called the compliance voltage)
of 50 V, which also limits maximum power:
Max Power (watts)
(50 volts)
Resistance (ohms)
2
=
.