Since strain always accompanies vibration, the strain gage or the principle by which it works is broadly applicable in the field of shock and vibra-tion measurement.Here it serves to determine not only the magnitude of the strains Principle of Working of Strain Gauges. As the length of the conductor increases its resistance decreases. Although both measure the same physical quantity, they differ fundamentally in their mode of operation. 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. strain gauge transducers usually employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. Thus strain gauges can be used to measure force and related parameters like displacement and stress. A strain gage consists of a small diameter wire (actually an etched metal foil) that is attached to a backing material (usually made of plastic) as This also affects the application areas of both methods. Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. Strain Gauge Working Principle.jpg 603×243 19.8 KB. Tie Bar Sensors Clamping Force The majority of strain gauges are foil types, available in a wide choice Measuring System of shapes and … The basic principle of the piezoresistive pressure sensor is to use a strain gauge made from a conductive material that changes its electrical resistance when it is stretched. A strain gauge, in mechanical term, is a device for measuring mechanical strain. i.e. 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. Demountable Mechanical Strain Gauge Different types of Mechanical strain Gauge:a) Berry strain gauge Lever arm and dial gauge. Save my name, email, and website in this browser for the next time I comment. When an external force is applied on an object, due to which there is a deformation occurs in the shape of the object. Fig. Strain gauges are mounted in the same direction as the strain and often in fours to form a full ‘Wheatstone Bridge’.. They are usually connected to the mechanical transducers like bellows for measuring pressure and displacement and other quantities. Download Full PDF Package. Frequently Asked Questions. When force is applied to any metallic wire its length increases due to the strain. The main principle of the bridge’s balance is that in strain gauge in the judged arms they must change the values of their resistance in opposite directions. Gauge factor (GF) 3. Fig. The Wheatstone Bridge circuit is given below. The more is the applied force, more is the strain and more is the increase in length of the wire. As the diaphragm bows outward with applied fluid pressure, the strain gauge stretches to a greater length, causing its resistance to increase. WORKING PRINCIPLE An electronic pressure sensor relies on a physical reaction to applied pressure, and then measuring the resulting proportional change electronically. Commonly used phenomena include changes in capacitance, or changes in ohmic resistance of a strain gauge … 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. To efficiently detect the strain, strain gauges are bonded to the position on the spring material where the strain will be the largest. BRIEF HISTORY The electrical resistance strain gage in the basic form known today was first used in 1936. 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. Thus, the strain gauge works to convert an applied pressure into a measurable voltage signal which may be amplified and converted into a 4-20 mA loop current signal (or into a digital “fieldbus” signal). working of strain gauge. Strain gauge 1.1. THE STRAIN GAUGE The strain gauge has been in use for many years and is the fundamental sensing element for many types of sensors, including pressure sensors, load cells, torque sensors, position sensors, etc. Erick C. Ortiz. Some of them are as follows: 1. A strain gauge based displacement transducer for measurement of the displacement in the range of 0 to 10 mm is reported. Modern manufacturing techniques have made possible the construction of strain gauges made of silicon instead of metal. Attaching a strain gauge to a diaphragm results in a device that changes resistance with applied pressure. This paper. 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 = … or. Strain can be measured using various types of devices classified depending upon their principle of operation. These are: 1. 13. 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. Pressure forces the diaphragm to deform, which in turn causes the strain gauge to change resistance. Which measuring principle is being used? If L1 is the initial length of the wire and L2 is the final length after application of the force, the strain is … READ PAPER. This change in resistance imbalances the bridge circuit, causing a voltage (Vout) proportional to the amount of applied pressure. When an object deforms within the limit of elasticity, either it becomes narrower and longer or it become shorter and broadens. We'll assume you're ok with this, but you can opt-out if you wish. 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. Measure a strain down to 10microstrain over a 50mm gauge length. The change in resistance of device is reasonably linear function on its formation. In this article there has been also presented construction and principle operation of a strain gauge measurement system. What is Strain? However, silicon is not chemically compatible with many process fluids, and so pressure must be transferred to the silicon diaphragm/sensor via a non-reactive fill fluid (commonly a silicone-based or fluorocarbon-based liquid). discussed in later chapters. Gauge factor = GF = (∆R/R)/ (∆L/L). This use of a fill fluid to transfer pressure from an isolating diaphragm to a sensing diaphragm inside the transmitter is used in most if not all modern pressure transmitter designs, even those that are not piezoresistive. 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. The classic strain gauge system represented in the previous illustration is made of metal (both the test specimen and the strain gauge itself). 2.1. Two or one strain gauge can work for a half bridge. This website uses cookies to improve your experience. The strain gage is used universally by stress analysts in the experimental deter-mination of stresses. Credits : Tony R. Kuphaldt – Creative Commons Attribution 4.0 License. where v is poisson’s ratio and ρ is resistivity. This change in resistance of the conductor can be measured easily and calibrated against the applied force. 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 Book: Mechanical Measurement by Thomas G. Beckwith and N. Lewis Buck. When force is applied to any metallic wire its length increases due to the strain. A Wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical resistance. The more is the applied force, more is the strain and more is the increase in length of the wire. The animation represents what might happen if a strip of metal were fitted with four gauges. The electrical resistance strain gages very closely meet the requirements stated above. Types of Strain Gages Types of strain gages are classified into foil strain gages, wire strain gages, and semiconductor strain gages, etc. Accept Read More, Differential Pressure Transmitter Working Principle, Pressure Gauges with Capsule Sensor Principle, Programmable Logic Controllers Multiple Choice Questions, Three-valve Manifold on Remote Seal DP Transmitter, Pressure Temperature Compensation Flow Measurement, Communicating Delta PLC Software to Simulator. Basic Principle of Strain Gauge 2. When a silicon strain gauge is over-stressed, it fails completely rather than “flows” as is the case with metal strain gauges. Mechanical type, 2. This deformation in the shape is both compressive or tensile is called strain, and it is measured by the strain gauge. The resistance of a conductor is proportional to its length so … 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. When it is stretched, its resistance increases and Vice Versa. Strain Gauge Sensors or Piezoresistive sensors. 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. Types of Strain Gauges 3.1. It is often easy to measure the parameters like length, displacement, weight etc that can be felt easily by some senses. The discovery of the principle upon which electrical resistance strain gage is based was made in 1856 by Lord Kelvin, who loaded copper Strain gauge: Principle of Working, Materials Used, Applications Strain gauges are devices whose resistance changes under the application of force or strain. 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. All Rights Reserved. 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. Create a free account to download. When force is applied to any metallic wire its length increases due to the strain. Resistive sensors rely on the variation of the resistance of a material when the measured variable is applied to it. 22 Full PDFs related to this paper. When compression applied area thickness and resistance decreases. The strain gauge is a classic example of a piezoresistive element, a typical strain gauge element shown here on the tip of my finger: In order to be practical, a strain gauge must be glued (bonded) on to a larger specimen capable of withstanding an applied force (stress): As the test specimen is stretched or compressed by the application of force, the conductors of the strain gauge are similarly deformed. The metals can be easily cut into the zigzag foils for the formation of the strain gauges. The strain gage was invented by Ed Simmons at Caltech in 1936. They can be used for measurement of force, strain, stress, pressure, displacement, acceleration etc. This video explains in just 2 minutes the working principle of an electrical strain gauge. Copyright © 2020 Bright Hub PM. Figure 13.5. 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. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. 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. It converts mechanical strain to electrical signal. A strain gauge is a resistor used to measure strain on an object. Silicon exhibits very linear spring characteristics over its narrow range of motion, and a high resistance to fatigue. • A thin piece of conductive material is … Useful for dynamic/frequent measurements of force. Now, we know that resistance of the conductor is the inverse function of the length. 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. Strain Gauges (source: omega.com) Contents hide 1. A strain gauge is a long length of conductor arranged in a zigzag pattern on a membrane.. Within its elastic limits, many metals exhibit good spring characteristics. 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. 14/02/2010 how sensors work - strain gauge. A metal isolating diaphragm transfers process fluid pressure to the fill fluid, which in turn transfers pressure to the silicon wafer. The gauge factor of strain gauge is defined as the unit change in resistance per unit change in length. How straing gauge works.jpg 846×317 28.2 KB. The strain can also be used to carry out stress analysis of the member. Poisson’s Ratio (ν) 2.2. 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. Download with Google Download with Facebook. 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). Here's What You Need to Know, 4 Most Common HVAC Issues & How to Fix Them, Commercial Applications & Electrical Projects, Fluid Mechanics & How it Relates to Mechanical Engineering, Hobbyist & DIY Electronic Devices & Circuits, Naval Architecture & Ship Design for Marine Engineers, Book: Measurement and Instrumentation Principles by Alan S. Morris. Strain gauge and rosettes Introduction A strain gauge is a device which is used to measure strain (deformation) on an object subjected to forces. 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. However, it is very difficult to measure the dimensions like force, stress and strain that cannot be really sensed directly by any instrument. For such cases special devices called strain gauges are very useful. Strain gauge transducers usually employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. Thus the devices whose resistance changes due to applied strain or applied force are called as the strain gauges. In principle, there are two sensor types: Piezoelectric sensors and strain gauges. Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. As a result of … Optical type 3. PRINCIPLES OF SENSORS & TRANSDUCERS • Strain gauge • The strain gauge can be considered as an electromechanical transducer used for measuring strain in a structure. Strain Gauge Working Principle. In the case of a bridge system, the strain gauges can work independently in the number of four, two or one. The strain gauges can sense the displacements as small as 5 µm. A short summary of this paper. Strain Gauge Sensors or Piezoresistive sensors. B) Johansson Extensometer Torsion tape stretched between knife edges. The flexibility of the metal isolating diaphragm also means it experiences much less stress than the silicon sensing diaphragm, which avoiding the problems of metal fatigue experienced by transmitter designs using metal as the sensing (spring) element. 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. 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. 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. • The principle of operation of bonded gauges is as follows. The input and output relationship of the strain gauges can be expressed by the term gauge factor or gauge gradient, which is defined as the change in resistance R for the given value of applied strain ε. A STRAIN GAUGE? Download Free PDF. Related posts: Strain gauge A strain gauge is an electrical transducer which is used for measuring mechanical surface strain. 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). A load cell is made by bonding strain gauges to a spring material. For applying the strain you need force, thus the change in resistance of the material can be calibrated to measure the applied force. An Introduction to Measurements using Strain Gages. By measuring this change in resistance, we can infer the amount of pressure applied to the diaphragm. 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. Half end is twisted in one direction, while other half in other direction. Book: Measurement and Instrumentation Principles by Alan S. Morris, Pro Engineer Surface Modeling Tutorial -The Simplest Command to Start ProE Surface Modeling. The Figure 1 shows a typical strain gauge diagram. The strain gauge can be attached to a diaphragm that recognises a change in resistance when the sensor element is deformed. Working principle of strain gauge : Gauge Factor: It is the ration of per unit change in resistance to per unit change in length. Download Free PDF. 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. The following formula is valid: = ×ε Δ K R R A strain gauge (also spelled strain gage) is a device used to measure strain on an object. There is a linear relationship between the strain of the strain gauge and the change in its resistance. 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