3 resistance measurements – Campbell Scientific CR3000 Micrologger User Manual

Page 300

Section 8. Operation

300

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. 563).

Six bridge measurement instructions are features of the CR3000. Table Resistive
Bridge Circuits -- Voltage Excitation (p. 300) and table Resistive Bridge Circuits -
- Current Excitation

(p. 302)

show circuits that are typically measured with these

instructions. In the diagrams, resistors labeled R

are normally the sensors and

those labeled R

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. 302)

shows CR3000 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
tables Resistive-Bridge Circuits with Voltage Excitation

(p. 300)

and Resistive

Bridge Circuits with Current Excitation

(p. 302)

list the schematics of bridge

configurations and related resistance equations.

Table 63. Resistive-Bridge Circuits with Voltage Excitation

Resistive-Bridge Type and

Circuit Diagram

CRBasic Instruction and

Fundamental Relationship

Relationships

Half-Bridge

CRBasic Instruction: BrHalf()

Fundamental Relationship