Campbell Scientific CR510 Basic Datalogger - Manual (Page 156)

SECTION 13. CR510 MEASUREMENTS
13-12
CR510
FIGURE 13.3-8. Measuring Input Settling Error with the CR510
CR510
FIGURE 13.3-9. Incorrect Lead Wire Extension on Model 107 Temperature Sensor
13.4 BRIDGE RESISTANCE
MEASUREMENTS
There are 6 bridge measurement instructions
included in the CR510 software. Figure 13.4-1
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
can then be accounted for in the processing of
the measurement instruction. The excitation is
switched on 450 µs before the integration
portion of the measurement starts and is
grounded as soon as the integration is
completed. Figure 13.4-2 shows the excitation
and measurement sequence for Instruction 6, a
4 wire full bridge. Excitation is applied
separately for each phase of a bridge
measurement. For example, in Instruction 6, as
shown in Figure 13.4-2, excitation is switched
on for the 4 integration periods and switched off
between integrations.
Instruction 8 measurement sequence consists
of applying a single excitation voltage, delaying
a specified time, and making a differential
voltage measurement. If a delay of 0 is
specified, the inputs for the differential
measurement are not switched for a second