What Is A Strain Gauge; Quarter Bridge Circuit - Microedge Instruments iLog Series User Manual

iBG logger has one external strain gauge/bridge differential voltage and one excitation sense
channel to measure the excitation power supply.
iBG provide seven models for different input ranges:
Model Input Range
iBG-8 ± 8mV
iBG-16 ± 16mV
iBG-32 ± 32mV

What is a Strain Gauge

A strain gauge (also strain gage) is a device used to measure the strain of an object.
The gauge is attached to the object by a suitable adhesive. As the object is deformed, the foil is
deformed, causing its electrical resistance to change. This resistance change, usually measured
using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.
The gauge factor GF is defined as:
where
is the change in resistance caused by strain,
is the resistance of the undeformed gauge, and
is strain.
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by
balancing two legs of a bridge circuit. The Wheatstone bridge consists of four resistances (R1,
R2, R3 and R4), an excitation voltage and an output voltage. Generally, one or more of the
resistances are variable and change in accordance with some physical phenomenon, such as
strain in this case. The Wheatstone bridge then converts this change in resistance to a change in
voltage.
There are 3 configurations are used - quarter bridge, half bridge or full bridge.

Quarter Bridge Circuit:

Only one active strain gauge (Rg) is used as shown below. R3 is the inactive gauge, which is
identical to the active gauge but does not encounter any mechanical strains and is used for
compensating the temperature effect. The other two arms contain fixed resistors.
iLog Data Loggers User's Manual Page 18/63