Campbell CR850, Operator'S Manual, Page: 48 / 566

Section 4. Quickstart Tutorial
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Figure 15: Short Cut thermocouple wiring
4.2.4.3 Procedure: (Short Cut Steps 10 to 11)
Historical Note In the space-race era, measuring thermocouples in the field was
a complicated and cumbersome process incorporating a thermocouple wire with
three junctions, a micro-voltmeter, a vacuum flask filled with an ice slurry, and a
thick reference book. One thermocouple junction was connected to the micro-
voltmeter. Another sat in the vacuum flask. The third was inserted into the
location of the temperature of interest. When the temperature settled out, the
micro-voltmeter was read. This value was then looked up on the appropriate table
in the reference book to determine the temperature.
Then along came Eric and Evan Campbell. Campbell Scientific designed the first
CR7 datalogger to make thermocouple measurements without the need for
vacuum flasks, third junctions, or reference books. Now, there's an idea!
Nowadays, a thermocouple consists of two wires of dissimilar metals, such as
copper and constantan, joined at one end. The joined end is the measurement
junction; the junction that is created when the thermocouple is wired to the CR800
is the reference junction.
When the two junctions are at different temperatures, a voltage proportional to the
temperature difference is induced into the wires. The thermocouple measurement
requires the reference junction temperature to calculate the measurement junction
temperature using proprietary algorithms in the CR800 operating system.
10. Click 3. Outputs to advance to the next step.