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TC-08 User's Guide44
3.6
Glossary
CJC. A method of compensating for ambient temperature variations in thermocouple
circuits.
Cold junction compensation. See CJC.
Common mode range. The voltage range, relative to the ground of the data logger,
within which both inputs of a differential measurement must lie in order to achieve an
accurate measurement.
DLL. Dynamic Link Library. Files with this file extension contain a collection of
Windows functions designed to perform a specific class of operations.
Input impedance. The resistance measured between the input terminals of a
circuit.
NFR. Noise-Free Resolution. The effective number of bits of resolution that can be
considered noise-free.
Overvoltage protection. The maximum input voltage that can be applied without
damaging the unit.
QNaN. Quiet Not a Number. In the context of the USB TC-08, QNaNs are numbers
created artificially to fill in gaps in sampling. These gaps are interruptions caused by
lack of available PC or laptop processor time, normally caused by too many
applications being open simultaneously. QNaNs are defined in the IEEE 754-1985
ISO standard and are indeterminate, meaning no two QNaNs have the same value.
In C/C++, the int _isnan(double) function in the <float.h> header can be used to
identify QNaN float representations, cast to a double first. QNaNs will not cause an
error if arithmetic operations are performed on them - however, the results will
remain indeterminate.
Resolution. A value in bits, related to the number of increments of an analog input
signal that can be detected by a digital measurement system. A high-resolution
measurement system detects smaller signal increments than a low-resolution
measurement system.
Thermocouple. A device consisting of two dissimilar metals joined together. The
thermoelectric voltage developed between the two junctions is proportional to the
temperature difference between the junctions.
Type B thermocouple. Type B thermocouples are made from platinum and rhodium
and are suitable for high temperature measurements of up to 1820°C. Unusually, due
to the shape of their temperature / voltage curve, type B thermocouples give the
same output at 0°C as at 42°C.
Type E thermocouple. Type E thermocouples are made from chromel and
constantan. They have a high output (68 uV/°C), making them well suited to
low-temperature (cryogenic) use. They are non-magnetic.
Type J thermocouple. Type J thermocouples are made from iron and constantan.
They measure temperatures in the range -210 to +1200°C. The main application is
with old equipment that can not accept the more modern thermocouple. J types
should not be used above 760°C, as an abrupt magnetic transformation will cause
permanent decalibration.