Strain Gauge Calibration - Lab Report Example

STRAIN GAUGE CALLIBRATION By Introduction A strain gauge is a measuring device used to measure the strainin an object. The most common type of a strain gauge is made up of an insulating flexible backing that supports a metallic foil pattern. The strain gauge is fixed to the object using a suitable adhesive, such as cyanoacrylate. As the object is being deformed, the foil is also experiencing deformation, causing a change in its electrical resistance. This change in resistance is determined by a Wheatstone bridge. A strain gauge utilizes the advantage of its physical property (electrical conductance) and the fact that it depends on the conductors. When an electrical conductor is stretched to the limits of its elasticity ensuring that it does not break or deform on a permanent basis, it becomes longer and narrower, changes that increase its electrical resistance end-to-end. Alternatively, when a conductor is compressed, but to an extent that it does not buckle, the effect will be only be broadening and shortening. This effect only decreases the electrical resistance end-to-end. Out of the measured electrical resistance of the strain gauge, the amount of applied stress may be calculated. An ordinary strain gauge arranges a long, thin conductive strip in a zigzag pattern of parallel lines in a way that a small amount of stress in the direction of the orientation of the parallel lines results to a multiplicatively larger strain determination over the total effective length of the conductor surfaces and this is with respect to the conductive lines and therefore a multiplicatively more significant change in the resistance is noted compared to when observed with a single straight-line conductive wire. The strain gage is among the most widely used strain measurement sensors. The strain gauge is a resistive elastic unit whose change in resistance is termed as a function of applied strain (Northrop, 2005). Among the several strain gauges, an electric resistance wire strain gage stands the privilege of lower cost and being an established product. This makes it commonly used the type of device. Other types of strain gages include the acoustic, the capacitive, the inductive, the mechanical, and the semi-conductive. A wire strain gage is constructed by a resistor, made in the metal foil form, and commonly bonded to an elastic backing. The principle of a strain gauge is based on the fact that the resistance of a wire increases with an increase in strain and decreases with a reduction in stress. The composition of the wire is a wire is a uniform conductor with parameters electric resistivity r, length l and cross-section area A. the resistance R is a function of the geometry that is given by the formula. When an external pressure is supplied to an elastic material, under normal circumstances, stress is generated. As a result, the material is deformed. At this time if the applied force is a tensile force, the length L of the material extends to the level given by L+DL. The ratio of DL to L, which is DL/L, is what is referred to as the strain. Additionally, if a compressive force is applied, the length L is reduced by a factor given by L- DL. Strain here then is given by (-DL)/L. Strain is usually represented by e. Assuming the cross-sectional area A for the material and an applied force of P, the stress s will, therefore, be given by be P/A, since stress is defined as the force acting over a given cross sectional area. A strain gauge is commonly constructed by bonding excellent electric resistance wires to an electrically insulated backing, and attaching the wire leads. The strain gage is afterwards used for strain measurement by joining it to the surface of the specimen containing a unique adhesive (Northrop, 2005). The proper calibration of strain gage based on transducers sometimes is a tricky exercise. Before the calibration process, considerations of the gage factor of the transducer need to be looked into. The gage factor of the amplifier module and the excitation voltage requirements need also to be considered. With this information at hand, psi, micro strain, lbs or any other quantity can accurately be measured A strain gage based transducer performs the operation of converting the physical quantity, for example, the pressure, into a voltage output signal measured in millivolts. The output signal changes in accordance with the amount of the physical quantity that is being measured. As the load is placed upon the gage changes, the electrical resistance is developed as a result of flexing of the gage under load. Similarly the same happens when a wire is bends, and the cross-sectional area alters, and this leads to a change in resistance. The output signal of the gage is picked up by an amplifier, amplified and later transmitted to the computer for the purpose of both display and record. Some of the most common strain gage based transducers include the pressure transducers, strain gages, and other load cells. The sensitivity specification of the transducer portrays what output voltage the sensor will give a supplied amplified voltage. The output of the strain gauge, any given moment is always directly proportional to the physical quantity that is the input to the module. Dealing with the gage factors is often considered as a relatively easy exercise when kept in mind that there will automatically be a voltage output resulting from the strain gage or the strain gage based transducer that will change according to the quantities being measured. Experiment apparatus and equipment 1. Strain gauge 2. Bending beam 3. Strain gauge meter 4. Weights Experiment procedures The circuit was connected by as shown below. The connecting cables were marked with their connection terminals and included the D, ¼ and F. 1. The meter was adjusted to zero with no weight on the arm. 2. Weights were added and the measurements recorded at each step. 3. Produce a graph of the load against the deflection of the meter was later plotted. 4. The process was repeated for other three times for certainty purposes. Results and observations After the setting up of the apparatus, the final working project was as below: The results recorded after the reading was as follows BOTTOM TOP 5N (0.9 - 1) 10N (1.8 - 2) 15N (2.9 - 3) 20N (3.8 - 4) C Rod 7 After plotting a graph, the shape obtained resembled the figure below Result analysis From the graph plotted, two characteristics can be derived. First, there was a noted significant difference in the curve slope. The marked variations explain this difference as a result of a change in resistance due to the microstructure of the strain gauge. Secondly, at certain instances of stress the slope of the curve changes. The initial slope changed from the value of 2. This change resulted from the different levels of strain and the transition from the elastic to the plastic deformation condition. The strains recorded with the strain gages are usually critically small. As a result, these changes of resistance are small and can never be measured directly with another apparatus. This explains in details the reason as to why the strain gage was included in the measurement system where the precise search for the strain gages change of resistance is termed to be possible. Dimensional ratios contain the same unit in the numerator as in the denominator of the ratio (Northrop, 2005). Conclusion The calibration relationship for the instrument verification is based on the different reasoning than it is for device scaling. For the purpose of applications, the calibration resistor is determined in order to come up with the same bridge output voltage that would be brought about when a strain gauge of the specified gage factor is exposed to a given strain. When calibrating for instrument, it is noted that the device gage factor is present to some close value. The audit relationship is as a result expressed as the registered strain. From the results obtained, it is noted that the gage factor of the strain gage itself does not get involved in the entire calibration process (Northrop, 2005). References Northrop, R. B. (2005). Introduction to instrumentation and measurements. CRC Press. p 56-89 Read More