Campbell Scientific AM416 Multiplexer - Manual (Page 22)

AM416 RELAY MULTIPLEXER
16
6.6.2 SINGLE-ENDED THERMOCOUPLE
MEASUREMENT
In single-ended thermocouple measurement,
the following precautions must be taken to
ensure accurate measurement:
1. Only shielded thermocouple wire should be
used; the sensor shields should be tied to
datalogger earth ground through the
multiplexer shield terminals.
2. The exposed end of the thermocouple
should be electrically insulated to prevent
differences in ground potential from causing
an error in the measured temperature.
Sensor to Multiplexer wiring - up to three
thermocouples per SET; the high side of each
thermocouple is input into terminals H1, L1, and
H2. The low sides of each thermocouple are
multiplexed in common through terminal L2.
Multiplexer to Datalogger wiring - If the
reference junction is at the datalogger, then the
wire that connects the COM H1, COM L1, and
COM H2 terminals to the datalogger should be
the same composition as the high side of the
thermocouples. Also, the wire that connects
COM L2 to datalogger ground should be the
same composition as the low side of the
thermocouples.
If the reference junction is at the AM416 (CSI
107 thermistor, RTD, etc.), then copper wire is
used to connect all COM terminals to the
datalogger.
6.6.3 DIFFERENTIAL THERMOCOUPLE
MEASUREMENT
Sensor to Multiplexer wiring - up to two
thermocouples per input SET.
Multiplexer to Datalogger wiring - The wires
here can be handled in one of two ways. If a
reference junction (107 thermistor, RTD, etc.) is
at the AM416, then two pair of copper wires
may be run between the COM terminals of the
multiplexer and two differential input channels.
If the reference junction is at the datalogger,
then two pairs of thermocouple wire should be
run between the COM terminals of the
multiplexer and two differential input channels.
6.7 MIXED SENSOR TYPES
In applications where sensors types are mixed,
multiple hook-up configurations and
programming sequences are possible. Please
consult CSI for application assistance if you
intend to multiplex markedly different sensor
types in your application.
6.7.1 MIXED SENSOR EXAMPLE: SOIL
MOISTURE BLOCKS AND
THERMOCOUPLES
In this example, 16 thermocouples and 16 soil
moisture blocks will be multiplexed through the
AM416. One thermocouple and one soil
moisture block are input into each SET.
FIGURE 17. Thermocouple and Soil Block Measurement