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2.7.7 Instrument Operation 39
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2.7.7 Instrument
Operation
There are several features of the Model 372 intended to improve accuracy and
repeatability that can have the opposite effect when used improperly. This paragraph
provides a brief summary of operating features that may contribute to measurement
uncertainty.
Use of the Model 372 with the 3726 scanner can improve or degrade measurement
performance depending on application. The scanner can be calibrated in the field to
eliminate gain error, but the addition of a large number of lead wires is always
unpredictable. More information on the 3726 scanner is given in section 2.9.
Measurement uncertainty increases significantly if the instrument hardware and
firmware filters are not allowed to settle completely. Information on settling time is
given in section 2.5.7.
The common mode reduction feature can significantly reduce common mode voltage
on the measurement input leads when used as described in section 2.5.6.5. Be sure
that all four measurement leads remain isolated from measurement common
(shield) when using the feature.
If one lead is grounded the instrument is able to make resistance measurements as
long as the common mode reduction feature is turned off. This is not a recommended
operating mode and accuracy is reduced. Increase in measurement error will not
exceed 0.1% in a typical installation.
The Model 372 is calibrated and specified at its default measurement frequency of
13.7 Hz. An additional calibration is done at each of the four alternate measurement
frequencies to compensate for the frequency response of the hardware filters. This
calibration eliminates instrument errors only and will not account for attenuation
caused by lead capacitance that is different at each frequency.
The autorange algorithm includes a hysteresis band to prevent instability when the
measured resistor is at a range boundary. It is possible for the instrument to choose a
different excitation current when approaching a value from below than it chooses
when approaching from above. Autorange should not be used when a specific exci-
tation current is critical.
2.8 Operating
Tradeoffs Any measurement must be optimized to provide the best practical result, and trade-
offs must be made to favor the most important measurement characteristics. Some
of the common trade-offs of low temperature resistance measurement are described
below.
2.8.1 Resistor Self-
heating vs. Excitation Even after leads are properly heat sunk and noise is effectively shielded, there can be a
temperature error in resistance measurements that comes from within. Self-heating
will occur when the power from excitation current is large enough to warm the resis-
tor above the temperature of its surroundings. The effect is most noticeable at low
temperatures because the thermal conduction at material boundaries (thermal
boundary resistance) becomes very poor. Proper attachment of the resistor to the
cooling source is critical but cannot eliminate the problem entirely.
If noise is not suspected as a potential problem, it may be easier to measure the effect
of self-heating using the many current ranges available in the Model 372 than to try
to calculate thermal resistance.
1. Begin by cooling the resistor to the desired temperature and measure its value
using current excitation; select a current in the middle of the available choices.