2.5.4 Heater Wiring
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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 car-
tridge is 6.35 mm (0.25 in) in diameter and 25.4 mm (1 in) long. The cartridge should
be snugly held in a hole in the load or clamped to a flat surface. Thermal anchoring for
good thermal contact is again important.
Foil heaters are thin layers of resistive material adhered to, or screened onto, electri-
cally insulating sheets. There are a variety of shapes and sizes. The proper size heater
can evenly heat a flat surface or 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 overheating and burning out.
2.5.4 Heater Wiring
For wiring inside a vacuum shroud, we recommend using 30 AWG copper wire for
heater leads. Too much heat can transfer in when larger wire is used. Thermal anchor-
ing, similar to that used for the sensor leads, should be included so that any heat
transfer 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 gauge copper cable can
be used, and twisting is still recommended.
2.6 Consideration
for Good Control
Most of the techniques discussed in section 2.4 and section 2.5 to improve cryogenic
temperature accuracy apply to control as well. There is an obvious exception in sen-
sor location. A compromise is suggested below in section 2.6.3.
2.6.1 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 over-
looked. In order for control to work well, junctions between the elements of the con-
trol loop must be in close thermal contact and have good thermal conductivity.
Gasket materials should always be used along with reasonable pressure (section
2.4.4 and section 2.4.5).
2.6.2 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 in their junctions.
2.6.3 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 con-
trol 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 tempera-
ture controllers including the Model 335 have multiple sensor inputs for this reason.
