Campbell Scientific CR23X Micrologger - Manual (Page 220)

SECTION 13. CR23X MEASUREMENTS
13-20
CR23X
FIGURE 13.4-3. Diagram of Junction Box
An external reference junction box must be
constructed so that the entire terminal area is
very close to the same temperature. This is
necessary so that a valid reference temperature
can be measured, and to avoid a thermoelectric
offset voltage which will be induced if the
terminals at which the thermocouple leads are
connected (points A and B in Figure 13.4-3) are
at different temperatures. The box should
contain elements of high thermal conductivity,
which will act to rapidly remove any thermal
gradients to which the box is subjected. It is not
necessary to design a constant temperature
box; it is desirable that the box respond slowly
to external temperature fluctuations. Radiation
shielding must be provided when a junction box
is installed in the field. Care must also be taken
that a thermal gradient is not induced by
conduction through the incoming wires. The
CR23X can be used to measure the
temperature gradients within the junction box.
13.5 BRIDGE RESISTANCE
MEASUREMENTS
There are 6 bridge measurement instructions
included in the standard CR23X software.
Figure 13.5-1 shows the circuits that would
typically be measured with these instructions.
In the diagrams, the resistors labeled R
s
would
normally be the sensors and those labeled R
f
would normally be fixed resistors. Circuits other
than those diagrammed could be measured,
provided the excitation and type of
measurements were appropriate.
With the exception of Instructions 4 and 8,
which apply an excitation voltage then wait a
specified time before making a measurement,
all of the bridge measurements make one set of
measurements with the excitation as
programmed and another set of measurements
with the excitation polarity reversed. The error
in the two measurements due to thermal emfs
is then accounted for in the processing of the
measurement instruction. The excitation is
switched on 450µs before the integration portion
of the measurement starts and is grounded as
soon as the integration is completed. Figure
13.5-2 shows the excitation and measurement
sequence for Instruction 6, a 4 wire full bridge.
When more than one measurement per sensor
is necessary (Instructions 7 and 9), excitation is
applied separately for each measurement. (For
example, in Instruction 9 (6 wire full bridge), the
differential measurement of the voltage drop
across the sensor is made with the excitation at
both polarities; excitation is again applied and
reversed for the measurement of the output
voltage.)
Instruction 8 applies an excitation voltage,
delays a specified time, and makes a differential
voltage measurement. If a delay of 0 is
specified, the inputs for the differential
measurement are not switched for a second
integration as is normally the case (Section
13.2). The result stored is the voltage
measured. Instruction 8 does not have as good
resolution or common mode rejection as the
ratiometric bridge measurement instructions. It
does provide a very rapid means of making
bridge measurements as well as supplying
excitation to circuitry requiring differential
measurements. This instruction does not
reverse excitation. A 1 before the excitation
channel number (1X) causes the channel to be
incremented with each repetition. The output of
Instruction 8 is simply the voltage
measurement. When 8 is used to measure a
full bridge (same connections as Instruction 6 in
Figure 13.5-1), the result is V
1
which equals V
x
(R
3
/(R
3
+R
4
) R
2
/(R
1
+R
2
)). (In other words, to
make the output the same as Instruction 6, use
a factor of 1000/V
x
in the multiplier.)