They are highly precise and don’t get influenced due to temperature changes. The characteristics of strain gauges are as follows: The equivalent strain output and voltage output always have a relation of 2:1. If you replace the R4 resistor with a strain gauge, even a minor change in its resistance will change the output voltage Vex which is a function of strain. Any change in the values of R1, R2, R3, and R4 will, therefore, change the output voltage. Whenever R1/ R2 = R4 / R3, the output voltage Vo is zero and the bridge is said to be balanced. The output of the Wheatstone circuit is given by: The Wheatstone Bridge is the electrical equivalent of two parallel voltage divider circuits with R1 and R2 as one of them and R3 and R4 as the other one. The Wheatstone Bridge circuit is given below.Ī Wheatstone Bridge is a network of four resistors with an excitation voltage, Vex that is applied across the bridge. Now, in practice, the change in the strain of an object is a very small quantity that can only be measured using a Wheatstone Bridge. How do you measure strain with a strain gauge?Īs mentioned earlier, strain gauges work on the principle of the conductor’s resistance which gives you the value of Gauge Factor by the formula: The reason behind aligning them in a zig-zag fashion is that they don’t increase the sensitivity since the percentage change in resistance for a given strain for the entire conductive strip is the same for any single trace.Īlso, a single trace is liable to overheating which would change its resistance and thus, making it difficult to measure the changes precisely. The change in the shape and size of the conductor also alters its length and the cross-sectional area which eventually affects its resistance.Īny typical strain gauge will have a long, thin conductive strip arranged in a zig-zag pattern of parallel lines. We know, resistance is directly dependent on the length and the cross-sectional area of the conductor given by: Similarly, when it is compressed, it gets shorter and broader, ultimately changing its resistance. Whenever a conductor is stretched within the limits of its elasticity, it doesn’t break but, gets narrower and longer. What is the working principle of Strain Gauge?Ī strain gauge works on the principle of electrical conductance and its dependence on the conductor’s geometry. The Gauge Factor of different materials is as given below: Material The output voltage of the Wheatstone Bridge, SV is given by the formula: The gauge factor for common metallic foil is usually a little over 2. ΔR = Change in the resistance caused due to strain The Gauge Factor is the sensitivity coefficient of strain gauges and, is given by the formula: As soon as the force is exerted, the strain gauge is deformed and, this deformation causes a change in its electrical resistance which ultimately changes the output voltage. In the case of real-life monitoring, while constructing concrete structures or monuments, the load is applied at the load application point of a load cell that consists of a strain gauge underlying it. A strain gauge measures this deformity by using the Gauge Factor formula. Now, whenever an external force changes the physical parameters of an object, its electrical resistivity also changes. Suppose L1 is the original length of wire and L2 is the new length after an external force is applied on it, the strain (ε) is given by the formula: The resistance in any conducting device is dependent on its length as well as the cross-section area. How does a strain gauge work?Ī strain gauge depends on the electrical resistivity of any conductor. This change in resistivity is measured by a Wheatstone bridge which is related to strain by a quantity called, Gauge Factor. The deformation in the object causes the foil to get distorted which ultimately changes the electrical resistivity of the foil. The gauge is attached to the object under stress using an adhesive. The stress is the internal resisting capacity of an object while a strain is the amount of deformation experienced by it.Īny basic strain gauge consists of an insulating flexible backing that supports a metallic foil pattern. Whenever an external force is applied to an object, it tends to change its shape and size thereby, altering its resistance. into a change in resistance that can be measured later on. The resistance of a strain gauge varies with applied force and, it converts parameters such as force, pressure, tension, weight, etc. It is one of the significant sensors used in the geotechnical field to measure the amount of strain on any structure (Dams, Buildings, Nuclear Plants, Tunnels, etc.). Strain Gauge or Strain Gage was invented in 1938 by Edward E. We have covered its working principles, characteristics, features, as well as, application areas. Here’s everything that you need to know about Strain Gauges.
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