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Lake Shore Model 325 Temperature Controller User’s Manual
Cooling System Design 2-7
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 heaters are more convenient but are bulky and more
difficult to place on small loads. A typical cartridge is 0.25 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
When wiring inside a vacuum shroud, we recommend using 30 AWG copper wire for heater leads. Too much heat can
leak in when larger wire is used. Heat sinking, similar to that used for the sensor leads, should be included so that any
heat leaking in 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. When wiring outside the vacuum shroud, larger gage
copper cable can be used, and twisting is still recommended.
2.5 CONSIDERATION 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 Section 2.5.3 – Two Sensor Approach.
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.
2.5.2 Thermal Lag
Poor thermal conductivity causes thermal gradients that reduce accuracy and also cause thermal lag that make it difficult
for controllers to do their job. Thermal lag is the time it takes for a change in heating or cooling power to propagate
through the load and get to the feedback sensor. Because the feedback sensor is the only thing that lets the controller
know what is happening in the system, slow information to the sensor slows the response time. For example, if the
temperature at the load drops slightly below the setpoint, the controller gradually increases heating power. If the
feedback information is slow, the controller puts too much heat into the system before it is told to reduce heat. The
excess heat causes a temperature overshoot, which degrades control stability. The best way to improve thermal lag is to
pay close attention to thermal conductivity both in the parts used and their junctions.
2.5.3 Two-Sensor Approach
There is a conflict between the best sensor location for measurement accuracy and the best sensor location for control.
For measurement accuracy the sensor should be very near the sample being measured, which is away from the heating
and cooling sources to reduce heat flow across the sample and thermal gradients. The best control stability is achieved
when the feedback sensor is near both the heater and cooling source to reduce thermal lag. If both control stability and
measurement accuracy are critical it may be necessary to use two sensors, one for each function. Many temperature
controllers including the Model 325 have two sensor inputs for this reason.