Campbell AM32, Instruction Manual

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AM32 MULTIPLEXER
10
channels 5 and 7, respectively. To reduce
reference temperature errors, measure the
thermocouples on these channels when using
the datalogger reference. Figures 6-1 and 6-2
depict type T thermocouple applications, but
other thermocouple types (e.g. E, J, and K) may
also be measured and linearized by the
dataloggers. Also, type R, S, and B
thermocouples can be read with Library Special
software installed in the datalogger.
FIGURE 6-1. Differential thermocouple
measurement with reference junction at the
datalogger.
FIGURE 6-2. Differential thermocouple
measurement with reference junction at the
AM32.
If the thermocouple reference is at the
datalogger (i.e. thermocouple lead is used to
connect the datalogger and multiplexer),
concurrent measurement of other low level
signals 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 AM32.
2. Some thermocouple wires have a greater
resistance than copper, thus adding
resistance to the non-thermocouple sensor
circuit. For example, constantan is
approximately 26 times more resistive than
copper.
If a mix of TC's and other low level signals are
multiplexed through the AM32, it is generally
best to locate the reference junction on the
AM32, as shown in Figure 6-2. So that copper
wire, instead of thermocouple wire, can be run
between the AM32 and datalogger. If the other
sensors have larger outputs (e.g. soil moisture
blocks, Figure 7-3) these problems are
negligible.
AM32 REFERENCE - An external reference,
usually a 107 Temperature Probe, may be
located at the AM32 as shown in Figure 6-2.
This approach requires an additional single-
ended analog input to measure the reference.
Locate the reference between the terminal
strips, near the COM terminals and, when
practical, measure the thermocouples on
channels that are in close proximity to the COM
terminals in order to minimize thermal
gradients. For best results, the AM32 should be
shielded and insulated from thermal sources in
an AM-ENCT.
6.2 THERMAL GRADIENTS
Thermal gradients between the AM32's sensor
input terminals and its COM terminals can
cause errors in thermocouple readings. For
example, with type T thermocouples, a one
degree gradient between input terminals and
the COM terminals will result in a one degree
measurement error (approximately). The
aluminum cover plate helps to minimize
gradients.
When an enclosure is used, gradients induced
from heat conducted along the thermocouple