The three types of strain gage configurations, quarter-, half-, and full-bridge, are determined by the number of active elements in the Wheatstone bridge, the orientation of the strain gages, and the type of strain being measured.

 

Quarter-Bridge Strain Gage

Configuration Type I

• Measures axial or bending strain
• Requires a passive quarter-bridge completion resistor known as a dummy resistor
• Requires half-bridge completion resistors to complete the Wheatstone bridge
• R₄ is an active strain gage measuring the tensile strain (+ε)

 

 

Configuration Type II

Ideally, the resistance of the strain gage should change only in response to applied strain. However, strain gage material, as well as the specimen material to which the gage is applied, also responds to changes in temperature. The quarter-bridge strain gage configuration type II helps further minimize the effect of temperature by using two strain gages in the bridge. As shown in Figure 6, typically one strain gage (R₄) is active and a second strain gage(R₃) is mounted in close thermal contact, but not bonded to the specimen and placed transverse to the principal axis of strain. Therefore the strain has little effect on this dummy gage, but any temperature changes affect both gages in the same way. Because the temperature changes are identical in the two strain gages, the ratio of their resistance does not change, the output voltage (Vo) does not change, and the effects of temperature are minimized.

 

 

Half-Bridge Strain Gage

You can double the bridge’s sensitivity to strain by making both strain gages active in a half-bridge configuration.

 

 

Configuration Type I

• Measures axial or bending strain
• Requires half-bridge completion resistors to complete the Wheatstone bridge
• R₄ is an active strain gage measuring the tensile strain (+ε)
• R₃ is an active strain gage compensating for Poisson’s effect (-νε)

This configuration is commonly confused with the quarter-bridge type II configuration, but type I has an active R₃ element that is bonded to the strain specimen.

Configuration Type II

• Measures bending strain only
• Requires half-bridge completion resistors to complete the Wheatstone bridge
• R₄ is an active strain gage measuring the tensile strain (+ε)
• R₃ is an active strain gage measuring the compressive strain (-ε)

 

Full-Bridge Strain Gage

A full-bridge strain gage configuration has four active strain gages and is available in three different types. Types 1 and 2 measure bending strain and type 3 measures axial strain. Only types 2 and 3 compensate for the Poisson effect, but all three types minimize the effects of temperature.

 

 

Configuration Type I

• Highly sensitive to bending strain only
• R₁ and R₃ are active strain gages measuring compressive strain (–e)
• R₂ and R₄ are active strain gages measuring tensile strain (+e)

Configuration Type II

• Sensitive to bending strain only
• R₁ is an active strain gage measuring the compressive Poisson effect (–νe)
• R₂ is an active strain gage measuring the tensile Poisson effect (+νe)
• R₃ is an active strain gage measuring the compressive strain (–e)
• R₄ is an active strain gage measuring the tensile strain (+e)

Configuration Type III

• Measures axial strain
• R₁ and R₃ are active strain gages measuring the compressive Poisson effect (–νe)
• R₂ and R₄ are active strain gages measuring the tensile strain (+e)