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42 c
HAPTER
2: Cooling System Design and Temperature Control
Model 372 AC Resistance Bridge and Temperature Controller
2.9.1 Scanner
Performance
Up to 16 resistors with up to 64 leads can be attached to the 3726 scanner. Up to 8
resistors with up to 32 leads can be attached to the 3708 scanner. The large number
and potential length of lead wires provide an excellent path for noise to couple into
the measurement system. Proper installation combined with the noise reduction
features built into both the Model 372, and 3726 and 3708 scanners can minimize
the effects, but a fully loaded scanner may never be as quiet as a single guarded and
shielded resistance input. A measurable increase in noise is most likely when
measuring large resistance of more than 100 k
)
. Large resistances pose an additional
problem because the guards available for a single resistance input end at the scanner
enclosure. Scanner and lead capacitance after that point can attenuate
measurement signals for large resistors.
On the other hand, the times 100 amplifier in the scanner can actually reduce
measurement noise in installations that require the Model 372 to be a significant
distance from the test Dewar. The built-in amplifier in the scanner may improve
measurements any time long input leads are required, even if only one channel is
being used.
Accuracy is not as big a concern as noise when using the 3726 and 3708 scanners. The
Model 372 AC measurement eliminates any voltage offsets that may be present in
the scanner’s relays or amplifier. The amplifier gain is calibrated at the factory and
can be recalibrated in the field to keep scanner gain error well below overall
measurement uncertainty.
Settling time is a concern any time multiple inputs are multiplexed and must be
scanned. The channel change settling time of the 3726 and 3708 scanners are virtu-
ally the same as the range change settling time of the Model 372. Additional time
may be required for the instrument to autorange if that feature is enabled and the
resistance on the new channel changes significantly between scans.
2.10 Temperature
Measurement Temperature measurement is a popular use of resistance bridges because most of the
temperature sensors that can operate at low temperatures are negative temperature
coefficient resistive temperature detectors (NTC RTDs) and need the flexibility of a
bridge. Resistive temperature sensors must be treated with the same care as any
other resistor being measured at low temperatures. The Model 372 is well equipped
to measure and control temperature using these devices.
2.10.1 Sensor Selection Lake Shore offers a full line of cryogenic temperature sensors including several mod-
els that are well suited for low temperature work to 50 mK and below. Two of the
most popular are germanium and ruthenium oxide RTDs. The Lake Shore Tempera-
ture Measurement and Control catalog gives detailed specifications of these sensors
and should be consulted as a reference when choosing temperature sensors. The
Model 372 will work with resistive temperature sensors from Lake Shore or any other
manufacture. A few important selection criteria are discussed below.
2.10.1.1 Sensitivity
Sensitivity is often the most important criteria when selecting a temperature sensor
because it dictates many of the other specifications including range, resolution and
accuracy. Many NTC resistive materials like germanium have a beneficial characteris-
tic of temperature sensitivity that makes them particularly useful at low tempera-
tures. The resistance and sensitivity of these materials increase exponentially as
temperature decreases. The sensitivity of the sensors is best at the lowest tempera-
tures where signal to noise is worst, making them more useful at lower temperatures
than a linear device of the same resistance.