2.10.1 Sensor Selection
43
|
www.lakeshore.com
2.10.1.2 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 kel-
vin 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 lon-
ger 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 usu-
ally between 10 mK and 50 mK. The exact temperature depends on the sensor con-
struction, mounting and instrumentation used for readout.
2.10.1.3 Temperature Resolution
Temperature measurement resolution is controlled to a large extent by sensor sensi-
tivity when using a resistance bridge. The temperature example given in the instru-
ment specifications shows the sensitivity of a typical germanium RTD changing from
3.5 M
)
/K at 50 mK to 36
)
/K at 1 K. An associated change in resolution is also shown
as 1.7 µK at 50 mK to 83 µK 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 372 to maintain practical measurement parameters. Sensi-
tivity (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.
2.10.1.4 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 condi-
tions used in calibration are duplicated when the sensor is being used. General rules
to achieve best accuracy are to choose a sensor with good stability and high sensitiv-
ity in the most critical temperature range, calibrate it carefully or purchase a calibra-
tion from a reputable company, install the sensor according to manufacturer
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.
2.10.1.5 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. Sec-
ond, 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