Campbell Scientific CR9000X Measurement and Control System - Manual (Page 233)

Section 7. Measurement Instructions
7-9
7.3 Resistive Bridge Measurements
7.3.1 Electrical Bridge Circuits
Electrical bridge circuits are used to determine the electrical resistance of a
sensor. Bridge measurements combine an excitation with voltage
measurements and are used to measure sensors that change resistance in
response to the phenomenon being measured.
There are various standard bridge measurement instructions that the CR9000X
supports. These instructions include three half bridge and two full bridge
(Wheatstone Bridge) measurements. Through the use of these circuits,
multiple sensor types are supported. For instance, a short list of the sensors
that the full bridge instructions are used for include RTDs, thermistors,
potentiometers, resistive accelerometers, load cells, scales, pressure
transducers, and multiple types of strain gage measurement circuits (1/4 Bridge
strain, half bridge Strain, and Full bridge strain circuits).
Electrical bridge sensors require either regulated current or voltage excitation,
and the means to read the analogue voltage output from the bridge circuit.
This section covers measurements using the CR9060 to supply the regulated
voltage excitation and the CR9050(E) or CR9051E to measure the output from
the bridge circuit. Bridge measurements can also be performed using the
CR9052DC Filter module. The CR9052DC has a dedicated, regulated, voltage
and current excitation source for each differential analogue input channel.
See Section 7.8 CR9052DC and CR9052IEPE Filter Module for more
information on making measurements using the CR9052DC.
See Section 3.1.5 Bridge Resistance Measurements for more information on
Bridge Circuits.
7.3.2 Bridge Excitation
Bridge measurements require excitation. The CR9060 module supplies this for
the CR9000X bridge measurements. Each CR9060 module has 10 Switched
excitation channels and 6 Continuous Excitation Outputs (CAOs). Each of
these can source up to 50 milliamperes. Care should be taken not to exceed the
drive capabilities of the excitation channels.
The current required for a specific sensor can be determined by dividing the
excitation voltage by the sensor's smallest expected resistance value. For
example, if a sensor's lowest resistance would be 200 ohms, and the sensor is
excited with 5 Volts, then the current would be 5/200 = 0.025 amperes or 25
milliamperes. So 1 excitation channel could be used to excite two of these
sensors.
The Bridge measurement instructions all include a Measurement per Excitation
(MesPEx) parameter. This is used to set the number of sensors to excite with
the same excitation channel before automatically advancing to the next
excitation channel when using a single Bridge Instruction with multiple
repetitions. Care should be taken that the total current requirement for all of
the sensors hooked to each individual excitation channel does not exceed 50
mA. This can be accomplished through limiting the number of sensors
hooked to an individual excitation channel, or through limiting the excitation