Lake Shore Model 370 AC Resistance Bridge User’s Manual
lowest temperatures where signal to noise is worst, making them useful at lower temperatures than a linear device of the
same resistance.
2.9.1.2 Temperature
Range
Every type of sensor has an upper and lower limit to its temperature range. The upper limit of an NTC resistor is
normally the point at which sensitivity falls so low that the sensor is no longer useful. The exact temperature where this
happens is determined by the sensor itself, instrumentation capabilities and required resolution. Sensors that have a few
micro kelvin resolution at 50 mK may have a resolution of several kelvin at room temperature. The sensor may be useful
to monitor warm up and cool down but has little experimental value.
The lower limit of an NTC resistor is the temperature at which self-heating makes the measurement impractical. As
temperature decreases the thermal resistance between sensor and load increases and eventually the heat of excitation can
no longer be dissipated into the load. A nice feature of NTC resistors is that resistance increases as temperature decreases
and, if the voltage across the sensor is limited, excitation power in the sensor decreases to offset the increase in thermal
resistance. Unfortunately there is always a temperature at which self-heating wins and it is usually between 10 mK and
50 mK. The exact temperature depends on the sensor construction, mounting and instrumentation used for readout.
2.9.1.3 Temperature
Resolution
Temperature measurement resolution is controlled to a large extent by sensor sensitivity when using a resistance bridge.
The temperature example given in the instrument specifications shows the sensitivity of a typical germanium RTD
changing from 3.5 M
Ω
/K at 50 mK to 36
Ω
/K at 1 K. An associated change in resolution is also shown as 1.7
μ
K at
50 mK to 83
μ
K at 1 K. The reason that the drop in resolution is not as severe as the drop in sensitivity is that excitation
current is increased approximately 1,000 times by the Model 370 to maintain practical measurement parameters.
Sensitivity (and therefore resolution) of most sensors changes with temperature so it is important to choose a sensor with
high sensitivity in the most critical temperature range. Two or more sensors may be necessary to cover a large range of
temperature with high resolution.
2.9.1.4 Accuracy
Resistive thermometers serve well as secondary standards and are much easier to use than most primary thermometers.
Accuracy of a resistive temperature sensor depends on how closely it is compared to a primary standard, how the
temperature scale is interpreted, how much the sensor drifts over time and how well the conditions used in calibration are
duplicated when the sensor is being used. General rules to achieve best accuracy, choose a sensor with good stability and
high sensitivity in the most critical temperature range, calibrate it carefully or purchase a calibration from a reputable
company, install the sensor according to manufacture recommendations and use the same excitation that was used during
calibration. Do not forget to include instrumentation errors when determining overall accuracy and recalibrate both
sensors and instruments on a regular basis. If the pieces are put together properly, accuracy of a few millikelvin is
possible at low temperatures.
2.9.1.5 Packaging
Sensor packaging is important for two main reasons. First, the package creates sensor contact with its environment
playing a key role in the overall thermal resistance. Second, the package protects the resistive element from stress that is
normally caused by thermal contraction as the load cools. When choosing a pre-packaged sensor watch for a
specification of thermal resistance or a recommended maximum excitation to make sure it is compatible with the
experiment and measuring instruments. Also make sure the package is “strain free” or makes some accommodation to
reduce stress on the resistive element during thermal cycling. Many sensors are also offered without packaging for
applications that require smaller size or lower mass than packaged sensors provide. These must be installed with the
greatest care and all manufacture recommendations should be followed.
2.9.1.6 Environment
All cryogenic temperature sensors have to contend with high vacuum and drastic temperature changes but some
experiments create an even more hostile environment by adding high magnetic fields, ionizing radiation and more. These
additional requirements make sensor selection more difficult because they can limit the useful range of many sensors.
The Lake Shore Temperature Measurement and Control catalog includes magnetic field dependence for most
temperature sensors. Other specific data on environmental factors is included when available. Lake Shore Cernox RTDs
are an excellent choice for many of these critical applications.
Theory of Operation
2-15
