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Handbook
November 2011
/
59-UMC0071
/
Issue
01
Mercury
iTC
Page 68
FUNCTIONAL DESCRIPTION
Original Instructions
4.5.1
Thermocouples
The iTC measures the voltage produced by the thermocouple and converts it to a
corresponding temperature. A 2-wire connection is used for a thermocouple.
4.5.2
Resistance and diode sensors
Resistance and diode sensors require an excitation current. A 4-wire connection is used
for these sensors.
When the circuit is configured to measure positive temperature coefficient (PTC)
sensors, such as PT100 platinum resistance thermometers, the excitation current is
generated by a high-stability current source.
When the circuit is configured to measure negative temperature coefficient (NTC)
sensors, such as CERNOX ceramic oxi-nitride or RuO
2
ruthenium oxide resistance
thermometers, the excitation is generated by a true voltage source.
The magnitude of the excitation current can be programmed to suit the application.
Three types of excitation are available:
Unipolar
The excitation current is always positive.
Bipolar
The excitation changes polarity between each successive measurement. The
displayed sensor value is an average of the results from the last two measurements.
Thus every measurement is an average of values measured with positive and
negative excitation currents.
Soft edge
This is similar to bipolar excitation, except the current ramps between the positive
and negative values. The current is maintained at a stable value between the ramps
to allow a measurement to be taken. Soft-edge excitation might be used to reduce
electrical interference produced by fast-changing currents.
4.5.3
Calibration for different sensors
The iTC uses calibration tables to store calibration data for different sensor types. The
user can also create a new calibration file for a specific sensor. Calibration tables can be
saved as files.
A calibration file contains a number of discrete data pairs, each mapping a sensor
reading onto a temperature value. Linear interpolation is used to provide calibration at
intermediate values.
The iTC calculates setpoint limits and sensor limits from the chosen calibration file.
4. This is for a platinum element that is ballasted to BS1904/
DIN43760. This element is more readily available than a
pure platinum element, but its performance is unspecified
below 73K (the data between 50K and 70K is based on
BS1904:964 rather than BS1904:1984).