Lake Shore Model 340 Temperature Controller User’s Manual
Cooling System Design
2-7
Heater Resistance and Power (Continued)
Both limits are in place at the same time, so the smallest of the two computations gives the maximum power
available to the heater. A heater of 25
Ω
allows the instrument to provide its maximum power of 100 watts.
A typical smaller resistance of 10
Ω
allows 40 watts of power, while a typical larger resistance of 50
Ω
allows
50 watts. The resistor chosen as a heater must be able to withstand the power being dissipated in it.
Pre-packaged resistors have a power specification that is usually given for the resistor in free air. This power
may need to be derated if used in a vacuum where convection cooling can not take place and it is not
adequately heat sinked to a cooled surface.
2.4.2 Heater
Location
For best temperature measurement accuracy the heater should be located so that heat flow between the
cooling power and heater is minimized. For best control the heater should be in close thermal contact with the
cooling power. Geometry of the load can make one or both of these difficult to achieve. That is why there are
several heater shapes and sizes.
2.4.3 Heater
Types
Resistive wire like nichrome is the most flexible type of heater available. The wire can be purchased with
electrical insulation and has a predictable resistance per given length. This type of heater wire can be
wrapped around a cooling load to give balanced, even heating of the area. Similar to sensor lead wire, the
entire length of the heater wire should be in good thermal contact with the load to allow for thermal transfer.
Heat sinking also protects the wire from over heating and burning out.
Resistive heater wire is also wound into cartridge heaters. Cartridge heater are more convenient but are bulky
and more difficult to place on small loads. A typical cartridge is ¼ inch in diameter and 1 inch long. The
cartridge should be snugly held in a hole in the load or clamped to a flat surface. Heat sinking for good
thermal contact is again important.
Foil heaters are thin layers of resistive material adhered to, or screened on to, electrically insulating sheets.
There are a variety of shapes and sizes. The proper size heater can evenly heat a flat surface or around a
round load. The entire active area should be in good thermal contact with the load, not only for maximum
heating effect, but to keep spots in the heater from over heating and burning out.
2.4.4 Heater
Wiring
Small 30 AWG copper wire is recommended for heater leads inside of a vacuum shroud. Larger wire causes
too much heat to leak. These leads should be heat sunk similar to sensor leads so that heat leak does not
warm the load when the heater is not running. The lead wires should be twisted to minimize noise coupling
between the heater and other leads in the system. Outside the vacuum shroud larger copper cable should be
used but twisting is still recommended.
2.5 CONSIDERATIONS FOR GOOD CONTROL
Most of the techniques discussed above to improve cryogenic temperature accuracy apply to control as well.
There is an obvious exception in sensor location. A compromise is suggested below in the Two Sensor
Approach.
Thermal conductivity is discussed in Paragraph 2.5.1. Thermal lag is discussed in Paragraph 2.5.2. The two-
sensor approach is discussed in Paragraph 2.5.3. Thermal mass is discussed in Paragraph 2.5.4. Finally,
system nonlinearity is discussed in Paragraph 2.5.5.
2.5.1 Thermal
Conductivity
Good thermal conductivity is important in any part of a cryogenic system that is intended to be at the same
temperature. Most systems begin with materials that have good conductivity themselves, but as sensors,
heaters, sample holders, etc., are added to an ever more crowded space, the junctions between parts are
often overlooked. In order for control to work well, junctions between the elements of the control loop must be
in close thermal contact and have good thermal conductivity. Gasket materials should always be used along
with reasonable pressure.