AM32 MULTIPLEXER
16
7.3 CR10(X) PROGRAMMING EXAMPLE: 223
MOISTURE BLOCKS AND TYPE T
THERMOCOUPLES WITH REFERENCE AT
THE MULTIPLEXER
The AM32 is used to switch the inputs from 16
soil moisture blocks into the hi side of a
differential channel to make a single-ended
measurement, it then switches thermocouples
to be measured differentially .
The signal leads are connected to a differential
input channel to make differential
measurements on thermocouples. The low
side of the differential channel is jumped to
ground so single-ended soil moisture block
measurements can be made between hi and
ground.
A 1k ohm 1% resistor must be placed between
the input and excitation channel used to
complete the resistance measurement.
It is assumed that the AM32 is isothermal so
that any thermocouple effects between the
thermocouple wire and the AM32 circuitry are
negligible. The thermocouple reference junction
is at the AM32; it is measured using the 107
temperature probe with Instruction 11.
A thermocouple reference temperature could
also be made at the datalogger. Thermocouple
wire would have to be run from the AM32 to the
datalogger. The use of constantan wire as the
low common lead has no effect on the moisture
block measurement.
The impedance of the switched excitation port
is great enough that when off the thermocouple
measurement is not affected by the connection
to the excitation port. Because the circuit
between a gypsum block and ground is closed
only when measurements are being made,
capacitors are not necessary to block DC
current (created by galvanic action between
block electrode and datalogger ground) which, if
allowed to flow, would degrade the block. In the
example program, the reference temperature
measurement is made with a 107 temperature
probe. AC resistance of 16 gypsum moisture
blocks are measured on the first 16 AM32
channels, thermocouple temperature
measurements are made on channels 17-32.
The soil moisture readings (Bars) are stored in
Input Storage locations 1-16 and the
temperatures (°C) are in locations 17-32.
FIGURE 7-3. Wiring Diagram for example of
CR10(X) measuring soil moisture blocks and
thermocouples with reference temperature
at the AM32.
Содержание AM32
Страница 27: ...This is a blank page ...