Campbell AM25T, Instruction Manual

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AM25T Solid-State Thermocouple Multiplexer
7
TABLE 7-1. AM25T to Datalogger Wiring
Function AM25T Datalogger
12 Vdc Power 12V 12V
Power Ground G G
Analog Ground 1
⏚ ⏚
Clock CLK U or Control Port
Reset RES U or Control Port
PRT Excitation VX VX , U , or Control Port
Common High HI Diff H , U , or Control Port
Common Low LO Diff L , U , or Control Port
1 Analog Ground is the ⏚ terminal located between the VX and HI terminals on the
AM25T.
7.3 Sensor to AM25T Wiring
This section and the examples describe differential voltage measurements of
thermocouples. It is possible to make single-ended measurements with the
AM25T; however, they are more likely to have problems.
Shield wires are connected to the ground terminal next to the measurement
channel and left unattached at the sensor.
7.3.1 Thermocouple Measurement
An internal reference PRT is located in the AM25T. This reference temperature
does not require an additional datalogger input to measure the reference.
Thermal gradients between the AM25T sensor input terminals and the PRT
cause errors in thermocouple readings. For example, a one-degree gradient
between input terminals and the PRT will result in a one-degree measurement
error. The thermal bar in the AM25T, which includes the strain relief, and the
AM25T cover are designed to reduce gradients. The PRT is attached to the
thermal bar.
Heat conduction along the thermocouple wire, into the terminal strips, can be
reduced by coiling some excess lead wire inside the enclosure.
The datalogger manual contains a thorough discussion on thermocouple
measurements and error analysis. Consult the datalogger manual for more
details.
For a differential voltage measurement of a thermocouple, wire the high side of
the thermocouple to the high side of a differential input channel on the AM25T
and the low side of the thermocouple to the low side of the channel.
Thermocouples that follow the U.S. industry standards use red insulation on
the low side of the thermocouple. Wire one thermocouple per differential input
channel (FIGURE 7-2 ).