Isotech TTI-7-R, Operator'S Handbook Manual

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Measuring Temperature
The ITS-90 scale has much improved continuity, precision and
reproducibility compared with IPTS68. The implementation of the
ITS-90 scale according to its definition calls for changes in
equipment and procedure compared with IPTS68, but lower
uncertainties of calibration are achievable in all parts of the range.
However, the instruments and equipment needed to implement the
ITS-90 scale in calibration laboratories will be substantially the
same.
4.3 Thermocouple measurement introduction
Very broadly the thermoelectric effect occurs when an electrical
circuit consisting of dissimilar metal conductors is subjected to a
temperature gradient. An electric potential or voltage is developed
along the conductors. This voltage potential varies proportionally with
temperature and provides a means by which to measure
temperature.
There are two categories of thermocouple:
• Rare metal, Platinum based types
• Base metal, Nickel based
Rare metal, platinum types are mostly used for high temperature
precision thermometry. Maximum temperatures of 1700 °C and
measurement uncertainties of up to 0.3
°C are possible. The
sensitivity of platinum based thermocouples is usually in the region of
10µV/
°C, which means that high accuracy, high resolution
measurements require sensitive instruments such as the TTI-7-R .
Base metal thermocouples easily account for the bulk of temperature
sensors used today, and offer the advantages of being easy to
package into a variety of sensor configurations and relatively low
cost. Base metal thermocouples operate over a wide temperature
range with high temperature types designed for use up to 1600
° C.
Temperatures above 2300 °C are possible with new high temperature
tungsten rhenium types. Typical sensitivity figures of
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