Quarter Bridge, Half Bridge and Full Wheatstone Bridge Strain Gauge Load Cell configurations.
All straingauge configurations are based on the concept of a Wheatstone bridge.
A Wheatstone bridge is a network of four resistive legs. One or more of these legs can be active sensing elements. Figure 11 shows a Wheatstone bridge circuit diagram.
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Figure 11. Basic Wheatstone Bridge Circuit Diagram
The Wheatstone bridge is the electrical equivalent of two parallel voltage divider circuits. R1 and R2 compose one voltage divider circuit, and R4 and R3 compose the second voltage divider circuit. The output of a Wheatstone bridge is measured between the middle nodes of the two voltage dividers.
A physical phenomena, such as a change in strain applied to a specimen or a temperature shift, changes the resistance of the sensing elements in the Wheatstone bridge. The Wheatstone bridge configuration is used to help measure the small variations in resistance that the sensing elements produce corresponding to a physical change in the specimen.
Straingauge configurations are arranged as Wheatstone bridges. The gauge is the collection of all of the active elements of the Wheatstone bridge. There are three types of straingauge configurations: quarterbridge, halfbridge, and fullbridge. The number of active element legs in the Wheatstone bridge determines the kind of bridge configuration. Refer to Table 11 to see how many active elements are in each configuration.
Each of these three configurations is subdivided into multiple configuration types. The orientation of the active elements and the kind of strain measured determines the configuration type
Acronyms, Formulas, and Variable Definitions
In the figures and equations in this document, the acronyms, formulas, and variables are defined as:
e is the measured strain (+e is tensile strain and e is compressive strain).
e_{S} is the simulated strain.
GF is the Gauge Factor, which should be specified by the gauge manufacturer.
R_{g} is the nominal gauge resistance, which should be specified by the gauge manufacturer.
R_{L} is the lead resistance. If lead lengths are long, RL can significantly impact measurement accuracy.
Rs is the shunt calibration resistor value.
U is the ratio of expected signal voltage to excitation voltage with the shunt calibration circuit engaged. Parameter U appears in the equations for simulated strain and is defined by the following equation:
n is the Poisson’s ratio, defined as the negative ratio of transverse strain to axial strain (longitudinal) strain.
V_{CH} is the measured signal’s voltage.
V_{EX} is the excitation voltage.
V_{r} is the voltage ratio that is used in the voltage to strain conversion equations and is defined by the following equation:
QuarterBridge Type I
This section provides information for the quarterbridge straingauge configuration type I. The quarterbridge type I measures either axial or bending strain.
Figure 12. QuarterBridge Type I Measuring Axial and Bending Strain
A quarterbridge type I has the following characteristics:
 A single active straingauge element is mounted in the principle direction of axial or bending strain.
 A passive quarterbridge completion resistor (dummy resistor) is required in addition to halfbridge completion.
 Temperature variation in specimen decreases the accuracy of the measurements.
 Sensitivity at 1000 me is ~ 0.5 mV_{out}/ V_{EX} input.
Figure 13. QuarterBridge Type I Circuit Diagram
The following symbols apply to the circuit diagram and equations:
 R1 and R2 are halfbridge completion resistors.
 R3 is the quarterbridge completion resistor (dummy resistor).
 R4 is the active straingauge element measuring tensile strain (+e).
To convert voltage readings to strain units use the following equation:
To simulate the effect on strain of applying a shunt resistor across R_{3}, use the following equation:
QuarterBridge Type II
This section provides information for the quarterbridge straingauge configuration type II.
The quarterbridge type II measures either axial or bending strain.
Figure 14. QuarterBridge Type II Measuring Axial and Bending Strain
A quarterbridge type II has the following characteristics:
 One active straingauge element and one passive, temperaturesensing quarterbridge element (dummy gauge). The active element is mounted in the direction of axial or bending strain. The dummy gauge is mounted in close thermal contact with the strain specimen but not bonded to the specimen, and is usually mounted transverse (perpendicular) to the principle axis of strain.
 This configuration is often confused with the halfbridge type I configuration, with the difference being that in the halfbridge type I configuration the R3 element is active and bonded to the strain specimen to measure the effect of Poisson’s ratio.
 Completion resistors provide half bridge completion.
 Compensates for temperature.
 Sensitivity at 1000 me is ~ 0.5 mV_{out}/ V_{EX} input.
Figure 15. QuarterBridge Type II Circuit Diagram
The following symbols apply to the circuit diagram and equations:
 R_{1} and R_{2} are a halfbridge completion resistors.
 R_{3} is the quarterbridge temperaturesensing element (dummy gauge).
 R_{4} is the active straingauge element measuring tensile strain (+e).
To convert voltage readings to strain units use the following equation:
To simulate the effect on strain of applying a shunt resistor across R_{3}, use the following equation:
HalfBridge Type I
This section provides information for the halfbridge straingauge configuration type I. The halfbridge type I measures either axial or bending strain.
Figure 16. Halfbridge Type I Measuring Axial and Bending Strain
A halfbridge type I has the following characteristics:
 Two active straingauge elements. One is mounted in the direction of axial strain, the other acts as a Poisson gauge and is mounted transverse (perpendicular) to the principal axis of strain.
 Completion resistors provide half bridge completion.
 Sensitive to both axial and bending strain.
 Compensates for temperature
 Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.
 Sensitivity at 1000 me is ~ 0.65 mV_{out}/ V_{EX} input.
Figure 17. HalfBridge Type I Circuit Diagram
The following symbols apply to the circuit diagram and equations:
 R_{1} and R_{2} are halfbridge completion resistors.
 R_{3} is the active straingauge element measuring compression from Poisson effect (–ne).
 R_{4} is the active straingauge element measuring tensile strain (+e).
To convert voltage readings to strain units use the following equation:
To simulate the effect on strain of applying a shunt resistor across R_{3}, use the following equation:
HalfBridge Type II
This section provides information for the halfbridge straingauge configuration type II.
The halfbridge type II only measures bending strain.
Figure 18. HalfBridge Type II Rejecting Axial and Measuring Bending Strain
A halfbridge type II configuration has the following characteristics:
 Two active straingauge elements. One is mounted in the direction of bending strain on one side of the strain specimen (top), the other is mounted in the direction of bending strain on the opposite side (bottom).
 Completion resistors provide half bridge completion.
 Sensitive to bending strain.
 Rejects axial strain.
 Compensates for temperature.
 Sensitivity at 1000 me is ~ 1 mV_{out}/ V_{EX} input.
Figure 19. HalfBridge Type II Circuit Diagram
The following symbols apply to the circuit diagram and equations:
 R_{1} and R_{2} are halfbridge completion resistors.
 R_{3} is the active straingauge element measuring compressive strain (–e).
 R_{4} is the active straingauge element measuring tensile strain (+e).
To convert voltage readings to strain units use the following equation:
To simulate the effect on strain of applying a shunt resistor across R_{3}, use the following equation:
FullBridge I
This section provides information for the fullbridge straingauge configuration type I.
The fullbridge type I only measures bending strain.
Figure 110. FullBridge Type I Rejecting Axial and Measuring Bending Strain
A fullbridge type I configuration has the following characteristics:
 Four active straingauge elements. Two are mounted in the direction of bending strain on one side of the strain specimen (top), the other two are mounted in the direction of bending strain on the opposite side (bottom).
 Highly sensitive to bending strain.
 Rejects axial strain.
 Compensates for temperature.
 Compensates for lead resistance.
 Sensitivity at 1000 me is ~ 2.0 mV_{out} / V_{EX} input.
Figure 111. FullBridge Type I Circuit Diagram
The following symbols apply to the circuit diagram and equations:
To convert voltage readings to strain units use the following equation:
To simulate the effect on strain of applying a shunt resistor across R_{3}, use the following equation:
FullBridge Type II
This section provides information for the fullbridge type II straingauge configuration.
The fullbridge type II only measures bending strain.
Figure 112. FullBridge Type II Rejecting Axial and Measuring Bending Strain
A fullbridge type II configuration has the following characteristics:
 Four active straingauge elements. Two are mounted in the direction of bending strain with one on one side of the strain specimen (top), the other on the opposite side (bottom). The other two act together as a Poisson gauge and are mounted transverse (perpendicular) to the principal axis of strain with one on one side of the strain specimen (top), the other on the opposite side (bottom).
 Rejects axial strain.
 Compensates for temperature.
 Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.
 Compensates for lead resistance.
 Sensitivity at 1000 me is ~ 1.3 mV_{out} / V_{EX} input.
Figure 113. FullBridge Type II Circuit Diagram
The following symbols apply to the circuit diagram and equations:
 R_{1} is an active straingauge element measuring compressive Poisson effect (–ne).
 R_{2} is an active straingauge element measuring tensile Poisson effect (+ne).
 R_{3} is an active straingauge element measuring compressive strain (–e).
 R_{}4 is an active straingauge element measuring tensile strain (+e).
To convert voltage readings to strain units use the following equation:
To simulate the effect on strain of applying a shunt resistor across R_{3}, use the following equation:
FullBridge Type III
This section provides information for the fullbridge straingauge configuration type III.
The fullbridge type III only measures axial strain.
Figure 114. FullBridge Type III Measuring Axial and Rejecting Bending Strain
A fullbridge type III configuration has the following characteristics:
 Four active straingauge elements. Two are mounted in the direction of axial strain with one on one side of the strain specimen (top), the other on the opposite side (bottom). The other two act together as a Poisson gauge and are mounted transverse (perpendicular) to the principal axis of strain with one on one side of the strain specimen (top), the other on the opposite side (bottom).
 Compensates for temperature.
 Rejects bending strain.
 Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.
 Compensates for lead resistance.
 Sensitivity at 1000 me is ~ 1.3 mV_{out} / V_{EX} input.
Figure 115. FullBridge Type III Circuit Diagram
The following symbols apply to the circuit diagram and equations:
 R_{1} is an active straingauge element measuring compressive Poisson effect (–ne).
 R_{2} is an active straingauge element measuring tensile strain (+e).
 R_{3} is an active straingauge element measuring compressive Poisson effect (–ne).
 R_{4} is an active straingauge element measuring the tensile strain (+e).
To convert voltage readings to strain units use the following equation:
To simulate the effect on strain of applying a shunt resistor across R_{3}, use the following equation:
