We will discuss about the Difference between bonded and Unbonded Strain Gauge in this session before that lets look in to the working principle of strain gauges
Principle of Measurement
Several types of strain gauges are available, all of them based on the principle that any material changes its resistance when it is stretched. Strain gauges are the most commonly used type of pressure-sensing element for pressure transmitters. Strain gauges also are used in weight measurement and strain measurement in concrete and metal structures. In strain gauges, a displacement is caused by the increasing or decreasing pressure. This displacement causes a change in the length of the element, which is part of a Wheatstone bridge circuit. The change in resistance within the bridge is converted, through electronics, into a pressure value. In addition, the circuitry can easily adjust the zero output level, the span of measurement, and the ambient temperature effects (through automatic temperature compensation).
Strain Gauge: Unbonded
Principle of Measurement
An unbonded strain gauge has a frame that consists of stationary and movable parts. A wire (about 0.4 mil in diameter) is located on both parts and is wrapped around nonconductive posts. Wire tension increases and decreases with changes in pressure. When the movable part is displaced, this strains the wire(s) and increases or decreases the resistance accordingly. The electronics convert this resistance measurement (through the Wheatstone bridge) into a pressure output. Sometimes four wires are used, two in tension and two in compression.
Application Notes Unbonded strain gauges can accommodate overtravel stop limits. This provides mechanical overpressure protection. They have a low mass and long-term stability. However, they are sensitive to shock.
Strain Gauge: Bonded
Principle of Measurement
In a bonded strain gauge , a foil (or wire) is bonded to a diaphragm. Changes in pressure cause the diaphragm to flex, which in turn is sensed by the foil (or wire). Sometimes four strain gauges are used as a set: two near the center of the diaphragm, where they encounter maximum tangential strain, and two near the circumference, where they encounter maximum radial strain. This is an improvement over the unbonded type since it eliminates the posts and frame.
Application Notes Bonded strain gauges offer a rugged assembly and good accuracy that is not degraded by shock and vibration. However, bonded strain gauges are limited in their pressure and temperature ranges
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