Unit III Permanent magnet moving coil instruments (PMMC) Construction The permanent magnet moving coil instruments are most accurate type for direct current measurements. This consists of a coil on a metal or insulated former which is suspended in a permanent magnetic field. When the current flows through the coil a torque will be set up. The coil then gets displaced against a torque provided by a pair of hair spring which provides the restraining torque. Generally the shape of the coil is rectangular & is placed in a radial magnetic field. The springs are generally of phosphors bronze & are provided at each end of the spindle. The springs which are wound in the opposite directions provide torque which is proportional to the deflection. The hair spring further act as carrier of a current, supplying the coil. Working principle Instrument coil current I ampere & deflection of it produces Ɵ due to current I ampere, the torque produced by the spring that restrain the coil movement= KƟ Nm the spring factor K is constant at steady deflection of the instrument. Torque due to the coil current= restraining torque of the spring= KƟ. Deflection is proportional to current. Hence the scale is uniform. For indicating instruments the former is made of a metallic aluminum former. In this former eddy current develops a damping torque which opposes the movement of the disk about the final position & prevents oscillations of the pointer by critical damping action. The PMMC instrument is shown in the below images . Advantages of PMMC It has uniform scale. REVA University With a powerful magnet, its torque to weight ratio is very high. So operating current of PMMC is small. The sensitivity is high. The eddy currents induced in the metallic former over which coil is wound, provide effective damping. It consumes low power, of the order of 25 W to 200 mW. It has high accuracy. Instrument is free from hysteresis error. Extension of instrument range is possible. Not affected by external magnetic fields called stray magnetic fields. Disadvantages of PMMC PMMC is Suitable for direct current measurement only. Ageing of permanent magnet and the control springs introduces the errors. The cost is high due to delicate construction and accurate machining. The friction is due to jewel-pivot suspension. Moving Iron or MI Instrument Definition: The instrument in which the moving iron is used for measuring the flow of current or voltage is known as the moving iron instrument. It works on the principle that the iron place near the magnet attracts towards it. The force of attraction depends on the strength of the magnet field. Construction and working of Moving Iron Instrument The plate or vane of soft iron is used as the moving element of the instrument. The vane is so placed that it can freely move in the magnetic field of the stationary coil. The conductor makes the stationary coil, and it is excited by the voltage or current whose magnitude is used to be measured. The moving iron instrument uses the stationary coil as an electromagnet. The electromagnet is the temporary magnet whose magnetic field strength increases or decreases with the magnitude of the current passes through it. REVA University Working of the Moving Iron Instrument The moving iron instruments use the stationary coil of copper or aluminium wire which acts as an electromagnet when an electric current passes through it. The strength of the magnetic field induces by the electromagnet is directly proportional to the current passes through it. The plates or vane of the iron pass through the coil increases the inductance of the stationary coil (the inductance is the property of the conductor which increases their electromotive force when the varying current passes through it). The electromagnet attracts the iron vane. The vane passes through the coil tries to occupy the minimum reluctance path (the reluctance is the property of the magnet which opposes the flow of electric current). The vane passes through the coil experience a force of repulsion caused by the electromagnet. The repulsion force increases the strength of the coil inductance. This happens because the inductance and reluctances are inversely proportional to each other. Advantages of the MI Instruments The following are the advantages of the moving iron instruments. Universal use – The MI instrument is independent of the direction of current and hence used for both AC and DC. Less Friction Error – The friction error is very less in the moving iron instrument because their torque weight ratio is high. The torque weight ratio is high because their current carrying part is stationary and the moving parts are lighter in weight. Cheapness – The MI instruments require less number of turns as compared to PMMC instrument. Thus, it is cheaper. Robustness – The instrument is robust because of their simple construction. And also because their current carrying part is stationary. Disadvantages of Moving Iron Instruments. The following are the disadvantages of Moving Iron Instrument. 1. Accuracy – The scale of the moving iron instruments is not uniform, and hence the accurate result is not possible. 2. Errors – Some serious error occurs in the instruments because of the hysteresis, frequency and stray magnetic field. 3. Waveform Error – In MI instrument the deflection torque is not directly proportional to the square of the current. Because of which the waveforms error occurs in the instrument. 4. Difference between AC and DC calibration – The calibration of the AC and DC are differed because of the effect of the inductance of meter and the eddy current which is used on AC. The AC is calibrated on the frequency at which they use. REVA University Tariff The cost of electrical generation of electrical energy consists of fixed cost and running cost. Since the electricity generated is to be supplied to the consumers, the total cost of generation has to be recovered from the consumers. • Tariffs or energy rates are the different methods of charging the consumers for the consumption of energy. It is desirable to charge according to the maximum demand (KW) and the energy consumed (KWh). Objectives of Tariff : (i) Recovery of cost of capital investment in generating equipment, transmission and distribution system. (ii) Recovery of the cost of operation, supplies and maintenance. (iii) Recovery of the cost material, equipment, billing and collection, cost as well as for miscellaneous services. (iv) A net return on the total capital investment must be ensured. (v) Requirements of Tariff: 1. It should be easier to understand. 2. It should provide low rates for high consumption. 3. It should be uniform over large population. 4. It should encourage the consumers having high load factors. 5. It should provide incentives for using power during off peak hours. 6. It should have a provision of penalty for low PF REVA University 38 4.1 Tariff Types of Tariff (i) Simple Tariff: When there is a fixed rate per unit of energy consumed, it is known as simple tariff (Uniform Rate Tariff). This is the most simplest of all tariff. In this type, the price charged per unit is constant. It means, the price will not vary with increase or decrease in number of units used. Disadvantages: • The cost per unit delivered is high. • There is no discrimination among various types of consumers. Flat Rate Tariff: When different types of consumers are charged at different uniform per unit rates, it is said to be (ii) Flat rate Tariff: In this type, the consumers are grouped into different classes. Each class is charged at different uniform rate, the different classes of consumers may be taken into account of their diversity and load factors. Since this type of tariff varies according to the way of supply used, separate meters are required for lighting load, power load etc. (iii) Block rate tariff: When a given block of energy is charged at a specified rate and the succeeding blocks of energy are charged at progressively reduced rates is called as block rate tariff. In this type, the energy consumption is divided into many blocks and price per unit is fixed in each block. (iv) Two Part tariff: When the rate of electrical energy is charged on the basis of maxi- mum demand of the consumer and the units consumed it is called twopart tariff. In this type, the total charge to be made from the consumer is split into two components. i.e., fixed charges and running charges. The fixed charges depend upon the number of units consumed by the customer. Thus the consumer is charged at a certain amount per kW of maximum demand + a certain amount per kWh of energy consumed. Total charges = Rs(X kW + Y kWh) • It is easily understood by the consumer. • It recovers fixed charges which depend upon the maximum demand of the consumer independent of the units consumed. • This form of tariff is generally used for industrial customers. Disadvantages: • Consumer has to pay the fixed charges irrespective of the fact whether he has consumed or not the electrical energy. • There is always error in assessing the maximum demand of the consumer. (v) Three part Tariff: When the total charges to be made from the consumer is split into three parts, fixed charge, semi-fixed charge and running charge, it is known as three-part tariff. This type of tariff is applied to big consumers. The principle objection of this type of tariff is the charges are split into three components (fixed charge, charge per kW of maximum demand, charge per kWh of energy consumed). (vi) Maximum demand tariff: It is similar to two-part tariff. The only difference is the maximum demand of the consumer is calculated by installing a maximum demand meter at his premises. This type of tariff is mostly applied to the bulk consumers. (vii) Time-Of-Day tariffs (TOD, peak-load): Rates vary depending on when the service is being used. For example, the operator would charge higher prices REVA University during peak use hours and lower prices during off-peak hours to reflect the cost of generation. This structure requires sophisticated measurement of customer usage, such as metering technologies. It encourages consumers to use less power during peak hours. With decreasing costs of TOD meters, use of the TOD tariff structure is becoming more common. (viii) Seasonal tariffs: These rates allow higher charges for electricity in summer and winter when demand for cooling or heating is higher. Typically they are used in climates where utilities experience significant seasonal cost differences. With traditional regulation, sea- sonal rates reduce net revenue stability for utilities by concentrating revenue into the weather-sensitive seasons. (ix) Power factor tariff: The tariff in which the power factor of the consumers is taken into account is known as power factor tariff. Domestic Wiring Introduction A network of wires drawn connecting the meter board to the various energy consuming loads (lamps, fans, motors etc) through control and protective devices for efficient distribution of power is known as electrical wiring. Electrical wiring done in residential and commercial buildings to provide power for lights, fans, pumps and other domestic appliances is known as domestic wiring. There are several wiring systems in practice. They can be classified into: Tree system: In this system branches are tapped from the main circuit at required points. This involves many joints making the location of the fault point difficult. Though the method is economical it is visually unappealing with scattered fuses and is affected by large voltage drops. Distribution system: This system is more organized in the sense that the main circuit is drawn to several distribution centers and connected to the distribution boards. Branches are tapped from these distribution boards. This system of wiring has an aesthetic appeal, as they are without joints and also makes the location of the fault point easier. All the points are maintained almost at the same potential. Each circuit is provided with an independent fuse. Provides flexibility for repair and maintenance. This system is widely preferred for indoor wiring though expensive. Types of Wiring: (i) Cleat wiring (ii) CTS wiring or TRS wiring or batten wiring (iii) Metal sheathed wiring or lead sheathed wiring (iv) Casing and capping (v) Conduit wiring Cleat Wiring: (i) In this type of wiring, insulated conductors (usually VIR, Vulcanized Indian REVA University Rubber) are supported on porcelain or wooden cleats. (ii) The cleats have two halves one base and the other cap. (iii) The cables are placed in the grooves provided in the base and then the cap is placed. (iv) Both are fixed securely on the walls by 40mm long screws. (v) The cleats are easy to erect and are fixed 4.5 –15 cms apart. (vi) This wiring is suitable for temporary installations where cost is the main criteria but not the appearance. Figure: Cleat Wiring Advantages: (i) Easy installation (ii) Materials can be retrieved for reuse (iii) Flexibility provided for inspection (iv) Relatively economical Disadvantages: (i) Appearance is not good (ii) Higher risk of mechanical injury. Casing and Capping Figure: Casing and capping REVA University (i) It consists of insulated conductors laid inside rectangular, teakwood or PVC boxes having grooves inside it. (ii) A rectangular strip of wood called capping having same width as that of casing is fixed over it. (iii) Both the casing and the capping are screwed together at every 15 cms. (iv) Casing is attached to the wall. Two or more wires of same polarity are drawn through different grooves. (v) The system is suitable for indoor and domestic installations. Advantages: (i) Cheaper than lead sheathed and conduit wiring (ii) Provides good isolation as the conductors are placed apart reducing the risk of short circuit (iii) Easily accessible for inspection and repairs (iv) Since, the wires are not exposed to atmosphere; insulation is less affected by dust, dirt and climatic variations. Disadvantages: (i) Highly inflammable (ii) Usage of unseasoned wood gets damaged by termites (iii) Skilled workmanship is required. Conduit wiring (i) In this system PVC (polyvinyl chloride) or VIR cables are run through metallic or PVC pipes providing good protection against mechanical injury and fire due to short circuit. (ii) They are either embedded inside the walls or supported over the walls, and are known as concealed wiring or surface conduit wiring (open conduit) respectively. (iii) The conduits are buried inside the walls on wooden gutties and the wires are drawn through them with fish (steel) wires. (iv) The system is best suited for public buildings, industries and workshops Figure: Conduit wiring REVA University Advantages: i. ii. iii. iv. v. vi. vii. No risk of fire and has good protection against mechanical injury The lead and return wires can be carried in the same tube Earthing and continuity is assured Waterproof and troubleshooting is easy Shock proof with proper earthing and bonding Durable and maintenance free Aesthetic in appearance Disadvantages: (i) (ii) (iii) (iv) (v) Disadvantages: Very expensive system of wiring Requires good skilled workmanship Erection is quite complicated and is time consuming Risk of short circuit under wet conditions (due to condensation of water in tubes). Factors affecting the choice of Wiring System: The choice of wiring system for a particular installation depends on technical factors and economic viability. 1. Durability: Type of wiring selected should conform to standard specifications, so that it is durable i.e. without being affected by the weather conditions, fumes etc. 2. Safety: The wiring must provide safety against leakage, shock and fire hazards for the operating personnel. 3. Appearance: Electrical wiring should give an aesthetic appeal to the interiors 4. Cost: It should not be prohibitively expensive. 5. Accessibility: The switches and plug points provided should be easily accessible. There must be provision for further extension of the wiring system, if necessary. 6. Maintenance Cost: The maintenance cost should be a minimum 7. Mechanical safety: The wiring must be protected against any mechanical damage PROTECTIVE DEVICES 1. Protection for electrical installation must be provided in the event of faults such as short circuit, overload and earth faults. 2. The protective circuit or device must be fast acting and isolate the faulty part of the circuit immediately. 3. It also helps in isolating only required part of the circuit without affecting the remaining circuit during maintenance. REVA University 4. The following devices are usually used to provide the necessary protection: Fuses, Relays, Miniature circuit breakers (MCB), Earth leakage circuit breakers (ELCB) Fuse 1. The electrical equipment’s are designed to carry a particular rated value of current under normal circumstances. 2. Under abnormal conditions such as short circuit, overload or any fault the current raises above this value, damaging the equipment and sometimes resulting in fire hazard. 3. Fuses are pressed into operation under such situations. 4. Fuse is a safety device used in any electrical installation, which forms the weakest link between the supply and the load. 5. It is a short length of wire made of lead / tin /alloy of lead and tin/ zinc having a low melting point and low ohmic losses. 6. Under normal operating conditions it is designed to carry the full load current. If the current increases beyond this designed value due any of the reasons mentioned above, the fuse melts (said to be blown) isolating the power supply from the load. Characteristics of Fuse Material The material used for fuse wires must have the following characteristics: 1. Low melting point 2. Low ohmic losses 3. High conductivity 4. Lower rate of deterioration REVA University Different types of fuses: 1. Re-wireable or Kit -Kat fuses: These fuses are simple in construction, cheap and available up-to a current rating of 200A. They are erratic in operation and their performance deteriorates with time. 2. Plug fuse: The fuse carrier is provided with a glass window for visual inspection of the fuse wire. 3. Cartridge fuse: Fuse wire usually an alloy of lead is enclosed in a strong fibre casing. The fuse element is fastened to copper caps at the ends of the casing. They are available up-to a voltage rating of 25kV. They are used for protection in lighting installations and power lines. Miniature Cartridge fuses: These are the miniature version of the higher rating cartridge fuses, which are extensively used in automobiles, TV sets, and other electronic equipments. 4. Transformer fuse blocks: These porcelain housed fuses are placed on secondary of the distribution transformers for protection against short circuits and overloads. 5. Expulsion fuses: These consist of fuse wire placed in hollow tube of fibre lined with asbestos. These are suited only for outdoor use for example, protection of high voltage circuits. 6. Semi-enclosed re-wireable fuses: These have limited use because of low breaking capacity. 7. Time-delay fuse: These are specially designed to withstand a current overload for a limited time and find application in motor circuits. MCB (Miniature Circuit Breaker): Figure: MCB REVA University 1. MCB’s are electromechanical devices which protect an electrical circuit from an over current. 2. The over current, in an electrical circuit, may result from short circuit, overload or faulty design. 3. An MCB is a better alternative to a Fuse since it does not require replacement once an overload is detected. 4. Unlike fuse, an MCB can be easily reset and thus offers improved operational safety and greater convenience without incurring large operating cost. 5. The principal of operation is simple. 6. An MCB functions by interrupting the continuity of electrical flow through the circuit once a fault is detected. 7. In simple terms MCB is a switch which automatically turns off when the current flowing through it passes the maximum allowable limit. 8. Generally MCB are designed to protect against over current and over temperature faults (over heating). 9. There are two contacts one is fixed and the other moveable. 10. When the current exceeds the predefined limit a solenoid forces the moveable contact to open (i.e., disconnect from the fixed contact) and the MCB turns off thereby stopping the current to flow in the circuit. 11. In order to restart the flow of current the MCB is manually turned on. 12. This mechanism is used to protect from the faults arising due to over current or over load. 13. To protect against fault arising due to over heating or increase in temperature a bi-metallic strip is used. 14. MCBs are generally designed to trip within 2.5 millisecond when an over current fault arises. 15. In case of temperature rise or over heating it may take 2 seconds to 2 minutes for the MCB to trip. 16. The following image shows the different internal parts of an MCB with top casing removed. EARTHING 1. The potential of the earth is considered to be at zero for all practical purposes as the generator (supply) neutral is always earthed. 2. The body of any electrical equipment is connected to the earth by means of a wire of negligible resistance to safely discharge electric energy, which may be due to failure of the insulation, line coming in contact with the casing etc. 3. Earthing brings the potential of the body of the equipment to ZERO i.e. to the REVA University earth’s potential, thus protecting the operating personnel against electrical shock. 4. The body of the electrical equipment is not connected to the supply neutral because due to long transmission lines and intermediate substations, the same neutral wire of the generator will not be available at the load end. Even if the same neutral wire is running it will have a self-resistance, which is higher than the human body resistance. Hence, the body of the electrical equipment is connected to earth only. 5. Thus earthing is to connect any electrical equipment to earth with a very low resistance wire, making it to attain earth’s potential. 6. The wire is usually connected to a copper plate placed at a depth of 2.5 to 3meters from the ground level. The earth resistance is affected by the following factors: 1. Material properties of the earth wire and the electrode 2. Temperature and moisture content of the soil 3. Depth of the pit 4. Quantity of the charcoal used Necessity of Earthing: 1. To protect the operating personnel from danger of shock in case they come in contact with the charged frame due to defective insulation. 2. To maintain the line voltage constant under unbalanced load condition. 3. Protection of the equipments 4. Protection of large buildings and all machines fed from overhead lines against lightning. Methods of Earthing: 1. The important methods of earthing are the plate earthing and the pipe earthing. 2. The earth resistance for copper wire is 1 ohm and that of G I wire less than 3 ohms. 3. The earth resistance should be kept as low as possible so that the neutral of any electrical system, which is earthed, is maintained almost at the earth potential. 4. The typical value of the earth resistance at powerhouse is 0. 5 ohm and that at substation is 1 ohm. Types of Earthing 1. Plate earthing 2. Pipe earthing PLATE EARTHING • In this method a copper plate of 60cm x 60cm x 3.18cm or a GI plate of the size 60cm x 60cm x 6.35cm is used for earthing. 1. The plate is placed vertically down inside the ground at a depth of 3m and is REVA University embedded in alternate layers of coal and salt for a thickness of 15 cm. 2. In addition, water is poured for keeping the earth electrode resistance value well below a maximum of 5 ohms. 3. The earth wire is securely bolted to the earth plate. 4. A cement masonry chamber is built with a cast iron cover for easy regular maintenance. 5. Excavation on earth for a normal earth Pit size is 1.5M X 1.5M X 3.0 M. 6. Use 500 mm X 500 mm X 10 mm GI Plate or Bigger Size for more Contact of Earth and reduce Earth Resistance. 7. Make a mixture of Wood Coal Powder Salt & Sand all in equal part 8. Wood Coal Powder use as good conductor of electricity, anti corrosive, rust proves for GI Plate for long life. 9. The purpose of coal and salt is to keep wet the soil permanently. 10. The salt percolates and coal absorbs water keeping the soil wet. 11. Care should always be taken by watering the earth pits in summer so that the pit soil will be wet. 12. Coal is made of carbon which is good conductor minimizing the earth resistant. Salt use as electrolyte to form conductivity between GI Plate Coal and Earth with humidity. 13. Sand has used to form porosity to cycle water & humidity around the mixture. Put GI Plate (EARTH PLATE) of size 500 mm X 500 mm X 10 mm in the mid of mixture. REVA University PIPE EARTHING 1. Earth electrode made of a GI (galvanized) iron pipe of 38mm in diameter and length of 2m (depending on the current) with 12mm holes on the surface is placed upright at a depth of 4.75m in a permanently wet ground. 2. To keep the value of the earth resistance at the desired level, the area (15 cm) surrounding the GI pipe is filled with a mixture of salt and coal. 3. The efficiency of the earthing system is improved by pouring water through the funnel periodically. REVA University 4. A cement masonry chamber is built with a cast iron cover for easy regular maintenance. 5. The GI earth wires of sufficient cross- sectional area are run through a 12.7mm diameter pipe (at 60cms below) from the 19mm diameter pipe and secured tightly at the top as shown in the following figure. 6. When compared to the plate earth system the pipe earth system can carry larger leakage currents as a much larger surface area is in contact with the soil for a given electrode size. 7. The system also enables easy maintenance as the earth wire connection is housed at the ground level. Figure: Pipe Earthing Sensors A device which detects or measures a physical property and records, indicates, or otherwise responds to it. REVA University Types of Sensors: (i) Acoustic & sound sensors e.g.: Microphone, Hydrophone. (ii) Automotive sensors e.g.: Speedometer, Radar gun, Speedometer, fuel ratio meter. (iii) Chemical Sensors e.g.: Ph sensor, Sensors to detect presences of different gases or liquids. (iv) Electric & Magnetic Sensors e.g.: Galvanometer, Hall sensor (measures flux density), Metal detector. (v) Environmental Sensors e.g.: Rain gauge, snow gauge, moisture sensor. (vi) Optical Sensors e.g.: Photodiode, Phototransistor, Wave front sensor. (vii) Mechanical Sensors e.g.: Strain Gauge, Potentiometer (measures displacement). (viii) Thermal & Temperature sensors. e.g.: Calorimeter, Thermocouple, Thermistor, Gardon gauge. (ix) Proximity & Presences sensors: A proximity or presences sensor is the one which is able to detect the presences of nearby objects without any physical contact. They usually emit electromagnetic radiations and detect the changes in reflected signal if any. e.g.: Doppler radar, Motion detector. Ultrasonic / Level sensor As the name indicates, ultrasonic / level sensors measure distance by using ultrasonic waves.The sensor head emits an ultrasonic wave and receives the wave reflected back from the target. ultrasonic / level sensors measure the distance to the target by measuring the time between the emission and reception. Figure: Ultrasonic Waves An Optical sensor has a transmitter and receiver, whereas an ultrasonic / level sensor uses a single ultrasonic element for both emission and reception. In a reflective model ultrasonic / level sensor, a single oscillator emits and receives ultrasonic waves alternately. This enables miniaturisation of the sensor head. The distance can be calculated with the following formula: Distance L = 1 × T ×2C where L is the distance, T is the time between the emission and reception, and C is the sonic speed. (The value is multiplied by 1/2 because T is the time for go-andREVA University return distance.) Other Applications Ultrasonic material testing (to detect cracks, air bubbles, and other flaws in the products), Object detection, position detection, ultrasonic mouse, etc. Figure 4.7: Block Diagram Figure : Using Aurdino Displacement sensor Measurements of size, shape and position utilize displacement sensors. Ex: diameter of part under stress (direct), Movement of microphone diaphragm to quantify liq- uid movement through heart (Indirect). A Displacement Sensor measures and detects changes (displacement) in a physical quantity. The Sensor can measure the height, width, and thickness of an object by determining the amount of displacement of that object. A Measurement Sensor is a device that measures the dimensions of an object by converting changes in amount of light into electrical signals when the object interrupts a wide laser beam. A Measurement Sensor measures the position and dimensions of an object. Figure: Displacement Sensor REVA University Rotary Encoders A Rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital signal. It is used for sensing in myriad applications on motors paired with drives and automated machinery for everything from consumer electronics, elevators, and conveyor speed monitoring to position control on au- tomated industrial machines and robotics. They track the turning of motor shafts to generate digital position and motion information. There are two main types: Absolute and Incremental (relative). The output of absolute en- coders indicates the current position of the shaft, making them angle transducers. The output of incremental encoders provides information about the motion of the shaft, which is typically further processed elsewhere into information such as speed, distance and position. Figure: Rotary Encoder Sensor Photoelectric sensor A photoelectric sensor emits a light beam (visible or infrared) from its light-emitting element. A reflective-type photoelectric sensor is used to detect the light beam reflected from the target. A thrubeam type sensor is used to measure the change in light quantity caused by the target crossing the optical axis. Figure: Photoelectric sensor Figure: Through Beam REVA University The transmitter and receiver are separated. When the target is between the transmitter and receiver, the light is interrupted. Both the light emitting and light receiving elements are contained in same housing. The light from the emitting element hits the reflector and returns to the light receiving element. When a target is present, the light is interrupted. Strain Gauge Sensor 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, Figure: Retro reflective model etc. The majority of strain gauges are foil types, available in a wide choice of shapes and sizes to suit a variety of applications. They consist of a pattern of resistive foil which is mounted on a backing material. They operate on the principle that as the foil is subjected to stress, the resistance of the foil changes in a defined way. Pressure sensor Since a long time, pressure sensors have been widely used in fields like Automobile, Manufacturing, Aviation, Bio medical measurements, Air conditioning, Hydraulic measurements etc. A few prominent areas where the use of pressure sensors is inevitable are: 1. Touch Screen Devices: The computer devices and smart phones that have touch screen displays come with pressure sensors. Whenever slight pressure is applied on the touch screen through a finger or the stylus, the sensor determines where it has been applied and accordingly generates an electric signal that informs the processor. Usually, these sensors are located at the corners of the screen. So when the pressure is applied, usually two or more such sensors act to give precise location information of the location. 2. Automotive Industry: In automotive industry, pressure sensors form an integral part of the engine and its safety. In the engine, these sensors monitor the oil and coolant pressure and regulate the power that the engine should deliver to achieve suitable speeds whenever accelerator is pressed or the brakes are applied to the car. For the purpose of safety, pressure sensors constitute an important part of AntiLock Braking system (ABS). This system adapts to the road terrain and makes sure that in case of braking at high speeds, the tires don’t lock and the vehicle doesn’t REVA University skid. Pressure sensors in the ABS detail the processor with the conditions of the road as well as the speed with which the vehicle is moving. Air bag systems also use pressure sensors so that the bags get activated to ensure the safety of the passengers whenever high amount of pressure is experienced by the vehicle. 3. Bio Medical Instrumentation: In instruments like digital blood pressure monitors and ventilators, pressure sensors are needed to optimize them according to patient’s health and his requirements. 4. Industrial Uses: Pressure sensors are used to monitor gases and their partial pressures in industrial units so that the large chemical reactions take place in precisely controlled environmental conditions. In oil industry, sensors detail with the depth that the oil rig has reached while exploring. Aviation: In the airplanes, these sensors are needed to maintain a balance between the atmospheric pressure and the control systems of the airplanes. This not only protects the circuitry and various internal components of the airplane but also gives exact data to the system about the external environment. Also, particular levels of air pressure need to be maintained in the cockpit and the passengers lobby to provide nominal ground like breathing conditions. 5. 6. Marine Industry: For ships and submarines, pressure sensors are needed to estimate the depth at which they are operating and for detailing the marine conditions so that the electronic systems can remain safe. Oxygen requirements of under water projects are also regulated by the pressure sensors. The sensing element reacts to the force or pressure of the process, creating an output signal that can be interpreted by a readout device or a data-collection device. The sensing element, therefore, is the heart of the transducer or load cell. Figure: Pressure Sensor REVA University REVA University