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Chapter 2: Cooling System Design
and Temperature Control
2.1 General
This chapter provides background information related to the Model 372 AC Resis-
tance Bridge and bridge measurements in general. It is intended to give the user
insight into the benefits and limitations of the instrument and help apply the features
of the Model 372 to a variety of experimental challenges. For information on how to
install the Model 372 refer to Chapter 3. Instrument operation information is con-
tained in Chapter 4.
2.2 Useful
Temperature
Range
The Model 372 is not an ordinary temperature controller. As described throughout
this manual, every effort has been made to measure resistance with as little power as
possible entering the resistor. The need for such careful attention is most evident at
low temperatures, particularly below 1 K. The instrument is not limited to those
applications and is useful in applications above 1 K when excitation power is critical.
When excitation power is not critical, however, alternatives may be available that
provide faster readings or lower cost.
The lowest temperature at which the Model 372 can make useful resistance mea-
surements is difficult to define. In a practical sense, the instrument is limited by its
need for four electrically conductive leads to be attached to a resistor and that resis-
tor attached to a cold surface. Heat conduction and noise pick-up in the resistor leads
limit the lowest temperature that can be achieved in this environment, even before
an instrument is attached. The diminishing thermal contact between the resistor and
cold surface at lower and lower temperatures only exaggerate the problem. In gen-
eral, resistance bridge measurements are considered routine down to 100 mK in a
capable cryogenic cooling system. With careful installation of a high quality bridge
like the Model 372 it is not difficult to achieve 50 mK. Below that, each detail of the
overall system has to be optimized to a low end of between 5 and 20 mK.
2.3 Temperature
Scales
The goal of a temperature scale is to provide a measured value that is thermodynami-
cally consistent with the physical laws of nature. As technology has advanced requir-
ing better accuracy and consistency, so has the definition of the temperature scale.
The currently internationally accepted temperature scale is the International Tem-
perature Scale of 1990 (ITS-90). ITS-90 is defined from 0.65 K to the highest tempera-
ture realizable in terms of the Planck radiation law. The detailed definition relies on
primary thermometers, fixed points, and interpolating instruments. The full realiza-
tion of the temperature scale as defined is generally only done by national metrology
institutes such as the U.S. National Institute of Standards (NIST). The practical propa-
gation of the scale is typically done using secondary standards calibrated by a
national metrology institute.
For temperatures below the range of ITS-90, the Comite International des Poids et
Mesures (CIPM) adopted the Provisional Low Temperature Scale from 0.9 mK to 1 K
(PLTS-2000). This temperature scale is based on noise and magnetic thermometry
and defined using a
3
He melting curve thermometer.
Lake Shore calibrations are traceable to the above two scales, ITS-90 and PLTS-2000.
Further details on these scales can be found in the information provided on the
website for the Bureau International des Poids et Mesures (BIPM).