MercuryiTC
©2014 Oxford Instruments Omicron NanoScience. All rights reserved.
Page 62
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.
4.11.6 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.
The MercuryiTC accepts calibration files with .dat or .prn extensions.
• A calibration file contains a number of discrete data pairs, each mapping a sensor reading on
to a temperature value. The first column is the output (eg. temperature, pressure) and the
second column is the input (eg. resistance, voltage, current).
• By default, values are assumed to be in SI units. However, a header for each column may be
used to define the units. Both columns must have valid units or both will be ignored
completely. Other headers are also ignored.
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