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In some measurement situations, however, it would be nice to remove
the need for an ice bath (or any other fixed external reference). To do
this, an isothermal block is used to make the connections. An isothermal
block is an electrical insulator, but a good heat conductor. The additional
thermocouples created at J1 and J2 are now held at the same temperature
by the isothermal block.
Once the temperature of the isothermal block is known, accurate
temperature measurements can be made. A temperature sensor is
mounted to the isothermal block to measure its temperature.
Thermocouples are available in a variety of types. The type is specified
by a single letter. The table on the following page shows the most
commonly used thermocouple types and some key characteristics of each.
Note: The thermocouple conversion routines used by the 34970A are
compatible with the International Temperature Scale of 1990 (ITS-90).
Measurement
Thermocouple
Reference
Sensor
Isothermal Block
(Internal or External Reference)
Reference
Temperature
Internal DMM
Chapter 8 Tutorial
Measurement Fundamentals
350