PRINCIPLES OF SENSORS & TRANSDUCERS • Strain gauge • The strain gauge can be considered as an electromechanical transducer used for measuring strain in a structure. Download Free PDF. The metals can be easily cut into the zigzag foils for the formation of the strain gauges. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. The following formula is valid: = ×ε Δ K R R 13. Types of Strain Gages Types of strain gages are classified into foil strain gages, wire strain gages, and semiconductor strain gages, etc. In the case of a bridge system, the strain gauges can work independently in the number of four, two or one. We'll assume you're ok with this, but you can opt-out if you wish. Types of Strain Gauges 3.1. These are: 1. Strain gauge: Principle of Working, Materials Used, Applications Strain gauges are devices whose resistance changes under the application of force or strain. In principle, there are two sensor types: Piezoelectric sensors and strain gauges. 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. A strain gauge is a resistor used to measure strain on an object. The strain gauge can be attached to a diaphragm that recognises a change in resistance when the sensor element is deformed. 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. Book: Mechanical Measurement by Thomas G. Beckwith and N. Lewis Buck. They are usually connected to the mechanical transducers like bellows for measuring pressure and displacement and other quantities. 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. By measuring this change in resistance, we can infer the amount of pressure applied to the diaphragm. Two or one strain gauge can work for a half bridge. 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 = … When it is stretched, its resistance increases and Vice Versa. Download Full PDF Package. 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 discovery of the principle upon which electrical resistance strain gage is based was made in 1856 by Lord Kelvin, who loaded copper 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). 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. For such cases special devices called strain gauges are very useful. 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. A strain gauge (also spelled strain gage) is a device used to measure strain on an object. The strain gage is used universally by stress analysts in the experimental deter-mination of stresses. Silicon exhibits very linear spring characteristics over its narrow range of motion, and a high resistance to fatigue. Strain gauge transducers usually employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. The gauge factor of strain gauge is defined as the unit change in resistance per unit change in length. A metal isolating diaphragm transfers process fluid pressure to the fill fluid, which in turn transfers pressure to the silicon wafer. Optical type 3. 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. What is Strain? This deformation in the shape is both compressive or tensile is called strain, and it is measured by the strain gauge. The Figure 1 shows a typical strain gauge diagram. 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. strain gauge transducers usually employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. Strain Gauge Sensors or Piezoresistive sensors. 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. Figure 13.5. This change in resistance of the conductor can be measured easily and calibrated against the applied force. Fig. 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. Gauge factor = GF = (∆R/R)/ (∆L/L). The classic strain gauge system represented in the previous illustration is made of metal (both the test specimen and the strain gauge itself). Which measuring principle is being used? The resistance of a conductor is proportional to its length so … 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. Although both measure the same physical quantity, they differ fundamentally in their mode of operation. Within its elastic limits, many metals exhibit good spring characteristics. Related posts: Strain gauge A strain gauge is an electrical transducer which is used for measuring mechanical surface strain. 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. Strain Gauge Working Principle. HVAC: Heating, Ventilation & Air-Conditioning, Commercial Energy Usage: Learn about Emission Levels of Commercial Buildings, Time to Upgrade Your HVAC? When force is applied to any metallic wire its length increases due to the 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). • The principle of operation of bonded gauges is as follows. Copyright © 2020 Bright Hub PM. 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. When compression applied area thickness and resistance decreases. 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. 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. Now, we know that resistance of the conductor is the inverse function of the length. A load cell is made by bonding strain gauges to a spring material. B) Johansson Extensometer Torsion tape stretched between knife edges. As the diaphragm bows outward with applied fluid pressure, the strain gauge stretches to a greater length, causing its resistance to increase. This video explains in just 2 minutes the working principle of an electrical strain gauge. Half end is twisted in one direction, while other half in other direction. 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 Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. The more is the applied force, more is the strain and more is the increase in length of the wire. 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). WORKING PRINCIPLE An electronic pressure sensor relies on a physical reaction to applied pressure, and then measuring the resulting proportional change electronically. 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 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. 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. where v is poisson’s ratio and ρ is resistivity. 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. 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. 22 Full PDFs related to this paper. A strain gauge, in mechanical term, is a device for measuring mechanical strain. working of strain gauge. Principle of Working of Strain Gauges. When force is applied to any metallic wire its length increases due to the strain. The strain gauges can sense the displacements as small as 5 µm. Demountable Mechanical Strain Gauge Different types of Mechanical strain Gauge:a) Berry strain gauge Lever arm and dial gauge. 14/02/2010 how sensors work - strain gauge. • A thin piece of conductive material is … An Introduction to Measurements using Strain Gages. i.e. Download Free PDF. 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. Thus strain gauges can be used to measure force and related parameters like displacement and stress. 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. Modern manufacturing techniques have made possible the construction of strain gauges made of silicon instead of metal. Strain Gauge Sensors or Piezoresistive sensors. Erick C. Ortiz. There is a linear relationship between the strain of the strain gauge and the change in its resistance. Download with Google Download with Facebook. The Wheatstone Bridge circuit is given below. 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. Mechanical type, 2. Poisson’s Ratio (ν) 2.2. Commonly used phenomena include changes in capacitance, or changes in ohmic resistance of a strain gauge … Useful for dynamic/frequent measurements of force. How straing gauge works.jpg 846×317 28.2 KB. 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. A STRAIN GAUGE? Strain gauges are mounted in the same direction as the strain and often in fours to form a full ‘Wheatstone Bridge’.. Save my name, email, and website in this browser for the next time I comment. This paper. If L1 is the initial length of the wire and L2 is the final length after application of the force, the strain is … Working principle of strain gauge : Gauge Factor: It is the ration of per unit change in resistance to per unit change in length. Fig. The more is the applied force, more is the strain and more is the increase in length of the wire. To efficiently detect the strain, strain gauges are bonded to the position on the spring material where the strain will be the largest. The strain gage was invented by Ed Simmons at Caltech in 1936. When force is applied to any metallic wire its length increases due to the strain. The animation represents what might happen if a strip of metal were fitted with four gauges. Some of them are as follows: 1. 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.jpg 603×243 19.8 KB. or. They can be used for measurement of force, strain, stress, pressure, displacement, acceleration etc. 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. The strain can also be used to carry out stress analysis of the member. Pressure forces the diaphragm to deform, which in turn causes the strain gauge to change resistance. The change in resistance of device is reasonably linear function on its formation. A short summary of this paper. A strain gauge is a long length of conductor arranged in a zigzag pattern on a membrane.. When an object deforms within the limit of elasticity, either it becomes narrower and longer or it become shorter and broadens. This also affects the application areas of both methods. For applying the strain you need force, thus the change in resistance of the material can be calibrated to measure the applied force. 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. BRIEF HISTORY The electrical resistance strain gage in the basic form known today was first used in 1936. Basic Principle of Strain Gauge 2. It is often easy to measure the parameters like length, displacement, weight etc that can be felt easily by some senses. Strain gauge 1.1. However, it is very difficult to measure the dimensions like force, stress and strain that cannot be really sensed directly by any instrument. 2.1. It converts mechanical strain to electrical signal. As the length of the conductor increases its resistance decreases. Tie Bar Sensors Clamping Force The majority of strain gauges are foil types, available in a wide choice Measuring System of shapes and … Strain gauge and rosettes Introduction A strain gauge is a device which is used to measure strain (deformation) on an object subjected to forces. Strain Gauges (source: omega.com) Contents hide 1. 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. This change in resistance imbalances the bridge circuit, causing a voltage (Vout) proportional to the amount of applied pressure. 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