Campbell Scientific AM16/32B Relay Multiplexer - Manual (Page 42)

AM16/32B Relay Analog Multiplexer
Thermal Gradients — Thermal gradients between the AM16/32B’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 (FIGURE
6-10) helps to minimize gradients. For best results the AM16/32B 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 AM16/32B is housed in a field enclosure, the enclosure
should be shielded from solar radiation.
FIGURE 6-10. AM16/32B aluminum cover plate
6.6.2 Single-ended Thermocouple Measurement
In single-ended thermocouple measurements, the following precautions must
be taken
• Onl should be used; the sensor shields
to ensure accurate measurement:
y shielded thermocouple wire
should be tied to multiplexer input shield (“ ”) terminals.
Exposed ends of thermocouples measuring soil temperature should be
electrically insula •
ted to prevent differences in ground potential among
•
•
of each thermocouple are
multiplexed in common through terminal EVEN L.
the thermocouples from causing errors in the measured temperatures.
AM16/32B panel switch set to “4x16” mode.
Sensor to Multiplexer Wiring — Up to three thermocouples per SET;
the high side of each thermocouple is input into terminals ODD H,
ODD L, and EVEN H. The low sides
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