Campbell Scientific AM416 Multiplexer - Manual (Page 21)

AM416 RELAY MULTIPLEXER
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FIGURE 15. Differential Thermocouple Measurement with Reference Junction at the Datalogger.
FIGURE 16. Differential Thermocouple Measurement with Reference Junction at the AM416.
If a mix of TC's and other sensor types are
multiplexed through the AM416, it is generally
best to locate the reference junction on the
AM416, as shown in Figure 16.
AM416 Reference - An external reference,
usually a thermistor, may be located at the
AM416, as shown in Figure 16. This approach
requires an additional single-ended datalogger
input to measure the reference. Locate the
reference between the COM terminals and,
when practical, measure the thermocouples on
SETs that are in close proximity to the COM
terminals in order to minimize thermal
gradients.
Thermal Gradients - Thermal gradients between
the AM416's sensor input terminals and 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, but for best results,
the AM416 should be shielded and insulated
from thermal sources.
When an enclosure is used, gradients induced
from heat conducted along the thermocouple
wire can be minimized by coiling some wire
inside the enclosure. This procedure allows the
heat to dissipate before it reaches the terminal.
If the AM416 is housed in a field enclosure, the
enclosure should be shielded from solar
radiation.