By supplying a precise and known voltage to a resistive-bridge circuit and
measuring the returning voltage, resistance can be calculated.
CRBasic instructions for measuring resistance include:
BrHalf() — half-bridge
BrHalf3W() — three-wire half-bridge
BrHalf4W() — four-wire half-bridge
BrFull() — four-wire full-bridge
BrFull6W() — six-wire full-bridge
Read More Available resistive-bridge completion modules are listed in the
appendix Signal Conditioners (p. 647).
The CR1000 has five CRBasic bridge-measurement instructions. Table Resistive-
Bridge Circuits with Voltage Excitation
(p. 338) shows ideal circuits and related
equations. In the diagrams, resistors labeled R
s
are normally the sensors and those
labeled R
f
are normally precision fixed (static) resistors. CRBasic example Four-
Wire Full-Bridge Measurement
(p. 340) lists CRBasic code that measures and
processes four-wire full-bridge circuits.
Offset voltages compensation applies to bridge measurements. In addition to
RevDiff and MeasOff parameters discussed in the section Offset Voltage
Compensation
(p. 323), CRBasic bridge measurement instructions include the
RevEx parameter that provides the option to program a second set of
measurements with the excitation polarity reversed. Much of the offset error
inherent in bridge measurements is canceled out by setting RevDiff, MeasOff, and
RevEx to True.
Measurement speed can be slowed when using RevDiff, MeasOff, and RevEx.
When more than one measurement per sensor are necessary, such as occur with
the BrHalf3W(), BrHalf4W(), and BrFull6W instructions, input and excitation
reversal are applied separately to each measurement. For example, in the four-
wire half-bridge (BrHalf4W()), when excitation is reversed, the differential
measurement of the voltage drop across the sensor is made with excitation at both
polarities and then excitation is again applied and reversed for the measurement of
the voltage drop across the fixed resistor. Further, the results of measurement
instructions (X) must be processed further to obtain the resistance value. This
processing requires additional program execution time.
338
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