Campbell Scientific CR800 and CR850 Measurement and Control Systems - Manual (Page 291)

Section 8. Operation
291
1
A/D (analog-to-digital) conversion time = 15 µs
2 Reps/No Reps -- If Reps > 1 (i.e., multiple measurements by a single instruction), no additional time
is required. If Reps = 1 in consecutive voltage instructions, add 15 µs per instruction.
8.1.3 Resistance Measurements
Many sensors detect phenomena by way of change in a resistive circuit.
Thermistors, strain gages, and position potentiometers are examples. Resistance
measurements are special-case voltage measurements. By supplying a precise,
known voltage to a resistive circuit, and then measuring the returning voltage,
resistance can be calculated.
Read More! Available resistive bridge completion modules are listed in the
appendix Signal Conditioners
(p. 539).
Five bridge measurement instructions are features of the CR800. Table Resistive
Bridge Circuits -- Voltage Excitation (p. 292) show circuits that are typically
measured with these instructions. In the diagrams, resistors labeled R
s
are
normally the sensors and those labeled R
f
are normally precision fixed (static)
resistors. Circuits other than those diagrammed can be measured, provided the
excitation and type of measurements are appropriate. CRBasic example Four-wire
Full-bridge Measurement
(p. 293)
shows CR800 code for measuring and processing
four-wire full-bridge circuits.
All bridge measurements have the parameter RevEx , which has an option to make
one set of measurements with the excitation as programmed and another set of
measurements with the excitation polarity reversed. The offset error in the two
measurements due to thermal EMFs can then be accounted for in the processing
of the measurement instruction. The excitation channel maintains the excitation
voltage or current until the hold for the analog to digital conversion is completed.
When more than one measurement per sensor is necessary (four-wire half-bridge,
three-wire half-bridge, six-wire full-bridge), excitation is applied separately for
each measurement. For example, in the four-wire half-bridge, when the excitation
is reversed, the differential measurement of the voltage drop across the sensor is
made with the excitation at both polarities and then excitation is again applied and
reversed for the measurement of the voltage drop across the fixed resistor.
Calculating the resistance of a sensor that is one of the legs of a resistive bridge
requires additional processing following the bridge measurement instruction. The
table Resistive-Bridge Circuits with Voltage Excitation
(p. 292)
lists the schematics
of bridge configurations and related resistance equations.