Campbell Scientific AM16/32A Multiplexer - Manual (Page 36)

AM16/32A Relay Analog Multiplexer
Thermal Gradients - Thermal gradients between the AM16/32A's
sensor input terminals and COM terminals can cause errors in
thermocouple readings. For example, with type T thermocouples, a
one degree gradient between the input terminals and the COM
terminals will result in an approximate one degree measurement error.
Installing the aluminum cover plate (included with AM16/32A) helps
to minimize gradients. For best results the AM16/32A should be
shielded and insulated from all radiant and conducted thermal sources.
When an enclosure is used, gradients resulting from heat conducted
along the thermocouple wire can be minimized by coiling some wire
inside the enclosure. This technique allows heat to largely dissipate
before it reaches the terminals. If the AM16/32A is housed in a field
enclosure, the enclosure should be shielded from solar radiation.
FIGURE 19. AM16/32A Aluminum Cover Plate
6.6.2 Single-ended Thermocouple Measurement
elds
In single-ended thermocouple measurements, the following
precautions must be taken to ensure accurate measurement:
Only shielded thermocouple wire should be used; the sensor shi
should be tied to multiplexer input shield (“
”) terminals.
Exposed ends of thermocouples measuring soil temperature shoul
electrically insulated to prevent differences in ground potential among
the thermocouples from causing errors in the measured temperatures.
AM16/32A panel switch set to “4x16” mode.
Sensor to Multiplexer wiring - up to three thermocouples per SET
d be
; the
high side of each thermocouple is input into terminals ODD H,
ODD L, and EVEN H. The low sides of each thermocouple are
multiplexed in common through terminal EVEN L.
30