Lakeshore 336, User Manual

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2.6.1 Thermal Conductivity 23
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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 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 336 have multiple sensor inputs for this reason.
2.6.4 Thermal Mass Cryogenic designers understandably want to keep the thermal mass of the load as
small as possible so the system can cool quickly and improve cycle time. Small mass
can also have the advantage of reduced thermal gradients. Controlling a very small
mass is difficult because there is no buffer to adsorb small changes in the system.
Without buffering, small disturbances can very quickly create large temperature
changes. In some systems it is necessary to add a small amount of thermal mass such
as a copper block in order to improve control stability.
2.6.5 System
Non-Linearity Because of nonlinearities, a system controlling well at one temperature may not con-
trol well at another temperature. While nonlinearities exist in all temperature control
systems, they are most evident at cryogenic temperatures. When the operating tem-
perature changes the behavior of the control loop, the controller must be retuned. As
an example, a thermal mass acts differently at different temperatures. The specific
heat of the load material is a major factor in thermal mass. The specific heat of
materials like copper change as much as three orders of magnitude when cooled from
100 K to 10 K. Changes in cooling power and sensor sensitivity are also sources
of nonlinearity.