Campbell AM416, Instruction Manual

AM416 RELAY MULTIPLEXER

14

FIGURE 14.  Full Bridge Measurement with Excitation Compensation

6.6  THERMOCOUPLE MEASUREMENT

The datalogger manuals contain thorough
discussions of thermocouple measurement and
error analysis.  These topics will not be covered
here.

6.6.1  MEASUREMENT CONSIDERATIONS

Reference Junction - As shown in Figure 15 and
16, two reference junction configurations are
possible:  reference at the datalogger or
reference at the AM416.

Datalogger Reference - The 21X and the CR7
723-T Analog Input card with RTD have built-in
temperature references. The 10TCRT
Thermocouple Reference (not standard with
CR10(X) purchase), is installed on the wiring
panel between the two analog input terminal
strips.

When the reference junction is located at the
datalogger, the signal wires between the data-
logger and the AM416 must be of the same wire
type as the thermocouple (Figure 15).  The
"polarity" of the thermocouple wire must be
maintained on either side of the multiplexer
(e.g. if constantan wire is input to a L1 terminal,
then a constantan wire should run between the

multiplexer's COM L1 terminal and the
datalogger measurement terminal).  Figures 15
& 16 depict type T thermocouple applications,
but other thermocouple types (e.g. E, J, and K)
may also be measured and linearized by the
dataloggers.

If thermocouples are measured with respect to
the datalogger reference, then concurrent
measurement of any other sensor type through
the AM416 is not recommended.  Two problems
will be encountered if this is done.  Both
problems result from the compositional
differences of the thermocouple wires.

1.  An extraneous thermocouple voltage will be

added to the non-thermocouple signal at
the junction of dissimilar metals (e.g. the
multiplexer COM terminals).  The
magnitude of this signal will vary with the
temperature difference between the
datalogger and the AM416.

2.  Some thermocouple wires have a greater

resistance than copper, which adds
resistance to the non-thermocouple sensor
circuit.  For example, constantan is
approximately 26 times more resistive than
copper.