All Rights Reserved. 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 = … Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. A Wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical resistance. 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. Fig. Strain gauge and rosettes Introduction A strain gauge is a device which is used to measure strain (deformation) on an object subjected to forces. Demountable Mechanical Strain Gauge Different types of Mechanical strain Gauge:a) Berry strain gauge Lever arm and dial gauge. Download with Google Download with Facebook. Half end is twisted in one direction, while other half in other direction. This change in resistance of the conductor can be measured easily and calibrated against the applied force. 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. Mechanical type, 2. 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 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. PRINCIPLES OF SENSORS & TRANSDUCERS • Strain gauge • The strain gauge can be considered as an electromechanical transducer used for measuring strain in a structure. For applying the strain you need force, thus the change in resistance of the material can be calibrated to measure the applied force. They are usually connected to the mechanical transducers like bellows for measuring pressure and displacement and other quantities. 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. 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. Silicon exhibits very linear spring characteristics over its narrow range of motion, and a high resistance to fatigue. This also affects the application areas of both methods. or. When an external force is applied on an object, due to which there is a deformation occurs in the shape of the object. When a silicon strain gauge is over-stressed, it fails completely rather than “flows” as is the case with metal strain gauges. 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. The strain gauge can be attached to a diaphragm that recognises a change in resistance when the sensor element is deformed. discussed in later chapters. Download Free PDF. Create a free account to download. In the case of a bridge system, the strain gauges can work independently in the number of four, two or one. B) Johansson Extensometer Torsion tape stretched between knife edges. 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. Erick C. Ortiz. Types of Strain Gauges 3.1. 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 gage consists of a small diameter wire (actually an etched metal foil) that is attached to a backing material (usually made of plastic) as 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. The classic strain gauge system represented in the previous illustration is made of metal (both the test specimen and the strain gauge itself). However, it is very difficult to measure the dimensions like force, stress and strain that cannot be really sensed directly by any instrument. Book: Measurement and Instrumentation Principles by Alan S. Morris, Pro Engineer Surface Modeling Tutorial -The Simplest Command to Start ProE Surface Modeling. Thus the devices whose resistance changes due to applied strain or applied force are called as the strain gauges. Related posts: Strain gauge A strain gauge is an electrical transducer which is used for measuring mechanical surface strain. 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. BRIEF HISTORY The electrical resistance strain gage in the basic form known today was first used in 1936. • The principle of operation of bonded gauges is as follows. In principle, there are two sensor types: Piezoelectric sensors and strain gauges. 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. The Wheatstone Bridge circuit is given below. 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. This change in resistance imbalances the bridge circuit, causing a voltage (Vout) proportional to the amount of applied pressure. Working principle of strain gauge : Gauge Factor: It is the ration of per unit change in resistance to per unit change in length. Poisson’s Ratio (ν) 2.2. Although both measure the same physical quantity, they differ fundamentally in their mode of operation. Strain Gauge Sensors or Piezoresistive sensors. Strain Gauge Sensors or Piezoresistive sensors. As the length of the conductor increases its resistance decreases. 14/02/2010 how sensors work - strain gauge. 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 This video explains in just 2 minutes the working principle of an electrical strain gauge. Which measuring principle is being used? 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. Strain Gauge Working Principle.jpg 603×243 19.8 KB. Optical type 3. For such cases special devices called strain gauges are very useful. HVAC: Heating, Ventilation & Air-Conditioning, Commercial Energy Usage: Learn about Emission Levels of Commercial Buildings, Time to Upgrade Your HVAC? Credits : Tony R. Kuphaldt – Creative Commons Attribution 4.0 License. They can be used for measurement of force, strain, stress, pressure, displacement, acceleration etc. Strain gauge: Principle of Working, Materials Used, Applications Strain gauges are devices whose resistance changes under the application of force or strain. Some of them are as follows: 1. 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. where v is poisson’s ratio and ρ is resistivity. 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. The strain gage was invented by Ed Simmons at Caltech in 1936. It is often easy to measure the parameters like length, displacement, weight etc that can be felt easily by some senses. The more is the applied force, more is the strain and more is the increase in length of the wire. As a result of … The gauge factor of strain gauge is defined as the unit change in resistance per unit change in length. WORKING PRINCIPLE An electronic pressure sensor relies on a physical reaction to applied pressure, and then measuring the resulting proportional change electronically. The strain gauges can sense the displacements as small as 5 µm. A strain gauge is a long length of conductor arranged in a zigzag pattern on a membrane.. 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. 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. As the diaphragm bows outward with applied fluid pressure, the strain gauge stretches to a greater length, causing its resistance to increase. Fig. Strain gauge transducers usually employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. When an object deforms within the limit of elasticity, either it becomes narrower and longer or it become shorter and broadens. This deformation in the shape is both compressive or tensile is called strain, and it is measured by the strain gauge. Thus strain gauges can be used to measure force and related parameters like displacement and stress. Download Free PDF. 22 Full PDFs related to this paper. The strain can also be used to carry out stress analysis of the member. Tie Bar Sensors Clamping Force The majority of strain gauges are foil types, available in a wide choice Measuring System of shapes and … A strain gauge (also spelled strain gage) is a device used to measure strain on an object. 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 READ PAPER. 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. 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. Strain gauges are mounted in the same direction as the strain and often in fours to form a full ‘Wheatstone Bridge’.. 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