Electrical resistance of any conductor is proportional to the ratio of length over cross-sectional area (R ∝ { l / A } ), which means that tensile deformation (stretching) will increase electrical resistance by simultaneously increasing length and decreasing cross-sectional area while compressive deformation (squishing) will decrease electrical resistance by simultaneously decreasing length and increasing cross-sectional area. or. Strain Gage: Materials material gage factor, G TCR (10-5) Ni80 Cr20 2.1 - 2.6 10 Pt92 W8 3.6 – 4.4 24 Silicon (n type) -100 to -140 70 to 700Germanium (p type) 102TCR = temperature coefficient of resistivity (ºC-1) • Note: • G for semiconductor materials ~ 50-70 x that of metals Earlier wire types of strain gauges were used commonly, which are now being replaced by the metal foil types of gauges as shown in the figure below. 2.1. One of the most popular materials used for the strain gauges is the copper-nickel-manganese alloy, which is known by the trade name ‘Advance.’ Some semiconductor materials can also be used for making the strain gauges. BRIEF HISTORY The electrical resistance strain gage in the basic form known today was first used in 1936. Tie Bar Sensors Clamping Force The majority of strain gauges are foil types, available in a wide choice Measuring System of shapes and … The following formula is valid: = ×ε Δ K R R Types of Strain Gages Types of strain gages are classified into foil strain gages, wire strain gages, and semiconductor strain gages, etc. If L1 is the initial length of the wire and L2 is the final length after application of the force, the strain is given as: Further, as the length of the stretched wire increases, its diameter decreases. In principle, there are two sensor types: Piezoelectric sensors and strain gauges. discussed in later chapters. Stress, Strain, and Strain Gages, Page 2 Strain gage The principle discussed above, namely that a wire’s resistance increases with strain, is key to understanding how a strain gage works. A strain gauge is a long length of conductor arranged in a zigzag pattern on a membrane.. Book: Measurement and Instrumentation Principles by Alan S. Morris, Pro Engineer Surface Modeling Tutorial -The Simplest Command to Start ProE Surface Modeling. Strain Gauge Working Principle.jpg 603×243 19.8 KB. When force is applied to any metallic wire its length increases due to the strain. 22 Full PDFs related to this paper. When compression applied area thickness and resistance decreases. Measurement of other quantities: The principle of change in resistance due to applied force can also be calibrated to measure a number of other quantities like force, pressure, displacement, acceleration etc since all these parameters are related to each other. There are some materials whose resistance changes when strain is applied to them or when they are stretched and this change in resistance can be measured easily. Two or one strain gauge can work for a half bridge. Metals, however, are subject to fatigue over repeated cycles of strain (tension and compression), and they will begin to “flow” if strained beyond their elastic limit. This is a common source of error in metallic piezoresistive pressure instruments: if overpressured, they tend to lose accuracy due to damage of the spring and strain gauge elements. In some designs, a single silicon wafer serves as both the diaphragm and the strain gauge so as to fully exploit the excellent mechanical properties of silicon (high linearity and low fatigue). Strain gauge 1.1. As a result of … As mentioned earlier, strain gauges work on the principle of the conductor’s resistance which gives you the value of Gauge Factor by the formula: GF = … Now, we know that resistance of the conductor is the inverse function of the length. Download Free PDF. In this way, the silicon sensor experiences the same pressure that it would if it were directly exposed to the process fluid, without having to contact the process fluid. Download with Google Download with Facebook. For applying the strain you need force, thus the change in resistance of the material can be calibrated to measure the applied force. Thus the devices whose resistance changes due to applied strain or applied force are called as the strain gauges. This also affects the application areas of both methods. When force is applied to any metallic wire its length increases due to the strain. However, in instrumental term, it is generally taken to mean the electrical resistance strain gauge, and as the name implies, the strain gauge is an electrical conductor whose resistance varies in proportion to the amount of strain in the device. Strain Gauges (source: omega.com) Contents hide 1. • The principle of operation of bonded gauges is as follows. Strain gauge transducers usually employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. This paper. Measure a strain down to 10microstrain over a 50mm gauge length. When force is applied to any metallic wire its length increases due to the strain. When a silicon strain gauge is over-stressed, it fails completely rather than “flows” as is the case with metal strain gauges. Strain Gauge Working Principle. It converts mechanical strain to electrical signal. strain gauge transducers usually employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. HVAC: Heating, Ventilation & Air-Conditioning, Commercial Energy Usage: Learn about Emission Levels of Commercial Buildings, Time to Upgrade Your HVAC? A short summary of this paper. It is often easy to measure the parameters like length, displacement, weight etc that can be felt easily by some senses. This change in resistance of the conductor can be measured easily and calibrated against the applied force. Which measuring principle is being used? This deformation in the shape is both compressive or tensile is called strain, and it is measured by the strain gauge. The more is the applied force, more is the strain and more is the increase in length of the wire. As mentioned earlier, strain gauges work on the principle of the conductor’s resistance which gives you the value of Gauge Factor by the formula: GF = [ΔR / (RG * ε)] Now, in practice, the change in the strain of an object is a very small quantity which can only be measured using a Wheatstone Bridge. For such cases special devices called strain gauges are very useful. If L1 is the initial length of the wire and L2 is the final length after application of the force, the strain is … This is generally considered a better result, as it clearly indicates the need for sensor replacement (whereas a metallic strain sensor may give the false impression of continued function following an over-stress event). Optical type 3. When an object deforms within the limit of elasticity, either it becomes narrower and longer or it become shorter and broadens. Download Free PDF. Piezoelectric load cells work on the same principle of deformation as the strain gauge load cells, but a voltage output is generated by the basic piezoelectric material – proportional to the deformation of load cell. Copyright © 2020 Bright Hub PM. Pressure forces the diaphragm to deform, which in turn causes the strain gauge to change resistance. General-purpose strain gages use copper-nickel or nickel-chrome alloy for the resistive elements, and the gage factor provided by these alloys is approximately 2. However, it is very difficult to measure the dimensions like force, stress and strain that cannot be really sensed directly by any instrument. Credits : Tony R. Kuphaldt – Creative Commons Attribution 4.0 License. The Figure 1 shows a typical strain gauge diagram. To efficiently detect the strain, strain gauges are bonded to the position on the spring material where the strain will be the largest. 13. A metal isolating diaphragm transfers process fluid pressure to the fill fluid, which in turn transfers pressure to the silicon wafer. This change in resistance imbalances the bridge circuit, causing a voltage (Vout) proportional to the amount of applied pressure. Thus strain gauges can be used to measure force and related parameters like displacement and stress. A Wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical resistance. gauge factor G f = (∆R/R)/ ( ∆l/l) where, R = nominal gauge resistance, ∆R = change in resistance, l = length of the specimen in an unstressed condition, ∆l = change in specimen length. Measuring strain gauge circuits In order to measure strain with a bonded resistance strain gauge, it must be connected to an electric circuit that is capable of measuring the minute changes in resistance corresponding to strain. All Rights Reserved. Strain gauges work on the principle of the conductor’s resistance which gives you the value of Gauge Factor by the formula: GF = [∆R / (RG * ε)] In practice, the change in the strain of an object is a very small quantity which can only be measured using a Wheatstone Bridge. A strain gauge (also spelled strain gage) is a device used to measure strain on an object. The discovery of the principle upon which electrical resistance strain gage is based was made in 1856 by Lord Kelvin, who loaded copper They can be used for measurement of force, strain, stress, pressure, displacement, acceleration etc. Modern manufacturing techniques have made possible the construction of strain gauges made of silicon instead of metal. The strain gauges are used for two main purposes: Measurement of strain: Whenever any material is subjected to high loads, they come under strain, which can be measured easily with the strain gauges. As the diaphragm bows outward with applied fluid pressure, the strain gauge stretches to a greater length, causing its resistance to increase. There is a linear relationship between the strain of the strain gauge and the change in its resistance. Demountable Mechanical Strain Gauge Different types of Mechanical strain Gauge:a) Berry strain gauge Lever arm and dial gauge. The strain can also be used to carry out stress analysis of the member. Working principle of strain gauge : Gauge Factor: It is the ration of per unit change in resistance to per unit change in length. Another simplified illustration shows how this works: The isolating diaphragm is designed to be much more flexible (less rigid) than the silicon diaphragm, because its purpose is to seamlessly transfer fluid pressure from the process fluid to the fill fluid, not to act as a spring element. The change in resistance is converted to an output signal There are three separate effects that contribute to the change in resistance of a conductor. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. Gauge factor is given as the ratio of change in electrical resistance R to the mechanical strain ε Mechanical strain is defined as the ratio of change in dimension to the original dimension in which the strain is applied, in this case length. They are usually connected to the mechanical transducers like bellows for measuring pressure and displacement and other quantities. What is Strain? In this article there has been also presented construction and principle operation of a strain gauge measurement system. 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