Displacement & Pressure Marks

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Archibald Nelson
5 years ago
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construction and working of low pressure and

1 Displacement & Pressure Marks Course Outcome: The students will be able to explain working of displacement transducers, construction and working of low pressure and high pressure measuring instruments.

2 Introduction Displacement: Classification of Instruments: Linear Capacitive Transducer, Resistive Transducer (Potentiometer), Inductive Transducer (LVDT) Angular Capacitive Transducer, Resistive Transducer (Rotary Potentiometer), Inductive Transducer (RVDT), Optical Encoder, 2

3 Capacitive Transducer Working Principle: Ability of body to store electrical charge. Capacitance of a parallel plate capacitor C.A d r. d o.a Advantage: small force to operate, good frequency response, extremely sensitive, low power requirement, locations having magnetic field, low cost. Disadvantage: Non linear behaviour, High output impedance, Temperature sensitive q V..r d : Permittivity of the medium A: Overlapping area of plates d: Distance between plates 2 3

4 Capacitive Transducer Construction: Plates / Cylinder, Di electric material (Air, Mica, Oil etc) Working: Capacitance can be changed by 3 ways..a r. o.a q C d d V..r d 2 4

5 Capacitive Transducer Construction: Plates / Cylinder, Di electric material (Air, Mica, Oil etc) Working: Capacitance can be changed by 3 ways..a r. o.a q C d d V..r d 2 5

6 Capacitive Transducer Application: For measurement of both linear & angular displacement. (0.1 X 10-6 mm) For measurement of force & pressure. Used directly as pressure transducer where dielectric constant changes with pressure. Used directly for mesurement of humidity as dielectric constant changes with humidity. Used in conjuction with mechanical modifiers for measurement of volume, density, liquid level etc..a C d r. d o.a q V..r d 2 6

7 Capacitive Transducer 7

8 Potentiometer It consists of a resistive element provided with a sliding contact. Wiper could be transational or rotational. Potentiometer is a passive instrument. Power source both ac or dc. R ρ.l A 8

9 Potentiometer Working Principle: Positioning of slider changes resistance of a POT or a bridge ckt. Xt Xi Output Voltage, Eo Resistanceat o/p terminal.(v)i/p Resistanceat i/p terminal Output Voltage, Eo θi. θt.ei Output Voltage, Eo Xi. Xt.Ei 9

10 Potentiometer Wire Wound POT: Material: Pt, Cr, Ni, Ni-Cu. Resolution: mm 0.05 mm Carry large current at high tempearture Non Wire POT: Continuous POT Material: Cermet, Hot Moulded Carbon, Carbon Film, Thin Metal Film Resolution: Unlimited More sensitive to temperature Higher wipre contact Carry moderate currents. 10

11 Potentiometer Advantages: Inexpensive Simple Operation Useful for large displacement High Efficiency High Output (No requirement of amplification) Χ Disadvantages: Large force to move wiper Wiper (Wear, Noise, Misalignment) Terms: Resolution: Ratio of potential difference between two adjacent coil to input PD R= 2.l.d + δ Noise: Fluctuation in output voltage due to wiper bounce, dirt & wear etc. 11

12 Applications of Potentiometer 12

13 Applications of Potentiometer Audio Control Motion Control Television 13

14 Applications of Potentiometer 14

15 Linear Variable Differential Transformer Linear Displacement / motion. Variable Inductance / Voltage. Differential Difference between two secondary outputs. Transformer functions as a transformer. Principle Differetial volatage between two secondary windings can be varied by changing the position of iron core by external force. 15

16 Linear Variable Differential Transformer 16

17 Linear Variable Differential Transformer Based on magnetic characteristics of an electrical circuit. 1.Change in Mutual Inductance: M k. L 1.L 2 L1 & L2: Self inductance of two coils k: coefficient of coupling If coils are connected in series then Inductance will vary between L1+L2-2M to L1+L2+2M 17

Linear Variable Differential Transformer Constuction:

cylindrical former, S1 & S2 have equal no of turns &

18 Linear Variable Differential Transformer Constuction: 1 Primary Winding, 2 Secondary Windings wound on cylindrical former, S1 & S2 have equal no of turns & identically placed on either side of primary. An a/c power source connected to P1. A movable soft iron core is placed inside the former. Core is having high permiability. 18

19 Linear Variable Differential Transformer Working: Power a/c (P1)Magnetic field a/c Current a/c (S1 & S2) 19

20 Linear Variable Differential Transformer Advantages: High Range: 1.25 mm to 250 mm Friction & Electrical Isolation: Frictionless, Infinite Resolution Immunity From External Effects: High Input & High Sensitivity: Ruggedness: Low Hystersis: Low Power Consumption: Disdvantages: Relatively large displacement needed. Sensitive to stray magnetic field Performance can be affected by vibration. Receiving Instrument should run on a/c Performance can be affected by temperature 20

21 Linear Variable Differential Transformer Application: To measure displacement from a few mm to a few cm. As a secondary transducer to measure force, displacement, pressure. 21

22 Rotary Variable Differential Transformer Principle: Differetial volatage between two secondary windings can be varied by rotating the iron core by external force. Constuction: 1 Primary Winding, 2 Secondary Windings wound on cylindrical former, S1 & S2 have equal no of turns & identically placed on either side of primary. An a/c power source connected to P1. A movable round soft iron cam shaped core Core is having high permiability. Working: Power a/c (P1)Magnetic field a/c Current a/c (S1 & S2) 22

23 Pressure Pressure : Force per unit area. For an ideal gas, Pressure P= (n.m.v rms2 )/3 Static Pressure: Liquid is at equillibrium. Dynamic Pressure: Due to pressure gradient. Velocity Pressure: Total Pressure Static Pressure 23

24 Pressure Absolute Pressure: Gauge Pressure: Vaccum Pressure: Atmospheric Pressure: Atmospheric Pressure: 760 mm of Hg pascal m of H2O 24

25 Pressure Measuring Transducers Gravitational Type: Liquid Columns ( manometer), Piston & Weight ( dead weight type tester) Direct Acting Elastic Type: unsymmetrical loaded tube, symmetrical loaded tube, elastic diaphragm, bellow Passive Type: Thermocouple Gauge, Pirani Gauge, Ionosation Gauge, Bridheman Gauge etc 25

26 Pressure Measurment Devices Low Pressure Measurement: McLeod Gauge, Thermal Conductivity Gauge, Ionization Gauge, Thermocouple Vacuum Gauge, Pirani Gauge High Pressure Measurement: Diaphragm, Bellows, Bourdon tube, Electrical Resistance Type, Photoelectric Pressure Transducers, Piezoelectric Type, Variable Capacitor Type 26

27 McLeod Gauge Working Principle: Boyle s Law P1V1 = P2V2 Construction: Reference Column Reference Capillary Bulb & Measuring Capillary Mercury Reservoir & Piston 27

28 McLeod Gauge Working: P2 = P1+h P1V1 = P2V2 P1V1= (P1+h).a.h P1 = ah 2 / (V1 ah) As ah is very small P1 = ah 2 / V1 28

McLeod Gauge Advantages: It is independent of gas composition. Serves as reference standard for other low pressure gauges. A linear relationship exists between pressure & height.

29 McLeod Gauge Advantages: It is independent of gas composition. Serves as reference standard for other low pressure gauges. A linear relationship exists between pressure & height. Disdvantages: It cannot give a continuous output The gas whose pressure is to be measured should obey the Boyle s law. Moisture traps are required to avoid any considerable vapor into the gauge. Application Low pressure measurement & calibration of low pressure gauges. 29

30 Thermal Conductivity Gauge Working Principle: Modes of heat Transfer: Conduction, Convection & Radiation 30

31 Thermal Conductivity Gauge Working Principle: Higher pressure higher density higher conductivity Reduced filament temperature less resistance of filament Thermal Conductivity of gas decresess when pressure drops below one torr. 1 Torr ( Evangelista Torricelli) = 1 mm of Hg 31

32 Thermocouple Vaccum Gauge Working Principle: Construction: Working Resistive Element Thermocouple 32

33 Thermocouple Vaccum Gauge Advantages: Rugged & inexpensive. Gives continuos reading. Output can be in digital form. Disadvantages: Need frequent calibration. Possibility of burnout when exposed to atmosphere. 33

34 Pirani Gauge Working Principle: Construction: Pirani Gauge Chamber Platinum Filament Compensating Chamber Wheat Stone Bridge Working 34

35 Pirani Gauge Advantages: Rugged & inexpensive. Gives continuos reading. Disadvantages: Need calibration for different gases. Electric Power is essential. Accurate / linear / Remote Indicate. 35

36 Ionisation Gauge [ Bayard Alpert / Hot Cathode ] Working Principle: Ionisation Construction: A triode vaccum tube Cathode ( Coated Iridum) Outside Anode Grid (Nickel) Midway Spiral Ion Collector ( Tungsten) Center 36

37 Ionisation Gauge [ Bayard Alpert / Hot Cathode ] Working: Heated Cathode Emits Electron Electrons Anode (Grid) Electrons Collide With Gas Molacules Gas Molacules Get Ionised Ion Collector (Plate) is Negative Ip ano. Of Ions ano. Of Gas Molecules adensity apressure 1 Ip P. K I g Ip: Plate / Ionisation Ig: Grid / Emission Current K: Sensitivity of Gauge 37

Ionisation Gauge [ Bayard Alpert / Hot Cathode ] Sensitivity: Gauge Design Electrode Geometry : Cathode to Grid spacing, Collector wire location & diameter, Grid diameter Used gas : He < D2 < H2

38 Ionisation Gauge [ Bayard Alpert / Hot Cathode ] Sensitivity: Gauge Design Electrode Geometry : Cathode to Grid spacing, Collector wire location & diameter, Grid diameter Used gas : He < D2 < H2 < N2. Air < O2 < CO < H2O < NO < Ar < CO2 < Kr <Xe Temperature : Inversely with the square root of the absolute temp. of the gas. Magnetic Field Pressure Range : mm of Hg to 1mm of Hg 38

Ionisation Gauge Advantages: Wide range. Constant sensitivity for a given gas. Accurate / linear / remote indicate. Continuos measurement. Disadvantages: High initial cost & complex electric ckt.

39 Ionisation Gauge Advantages: Wide range. Constant sensitivity for a given gas. Accurate / linear / remote indicate. Continuos measurement. Disadvantages: High initial cost & complex electric ckt. Need calibration for different gases. Careful control of filament current. Decomposition of gases can occur at high temperature. Excessive pressure causes rapid deterioration of filament. Filament burns quickly if exposed to atmosphere. 39

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41 Bourdon Pressure Gauge Most Popular, Eugene Bourdon, Elastic Pressure Element Types: C, Spiral, Helical 41

42 Bourdon Pressure Gauge Material: Brass, Alloy steel, SS, Bronze, Phosphor Bronze, Beryllium Copper, Monel, K Monel & Ni-span C Choice Of Material: Range of masurement, Process medium, Temperature of medium, Corrosive resistance of medium Phosphor Bronze ( Low Pressure) Stainless Steel ( High Pressure & Corrosion) 42

43 C Type Bourdon Tube Δa 0.05.a.P E Construction A seamless tube having elliptical cross section Tube enclosed in a socket which cotains pressure inlet Free end of tube is known as tip Socket, tube & tip are welded, brazed or soldered Tip attached to link Link is attached to gear sector Gear sector meshes with pinion Pinion carries pointer Working Due to pressure tube tends to become straighter Tip moves upwards, Causes movement of link Which in turn rotates sector & pinion gear Pinion gear moves pointer. r t 0.2. x y x t 3 43

44 Spiral Type Bourdon Tube Construction: Δa 0.05.a.P E Δa varies inversly with wall thickness t & directly with length of arc a Value of a depends on angle subtended ( How tube is bent) Angle more than 360 can be made by spiral or helix formation. Increased displacement of free end eliminates need of magnification Absence of gears means no backlash, less friction Spiral tubes are made by winding several turns of tubes. r t 0.2. x y x t Working Due to pressure tube tends to uncoil Produce long movement of tip Accuracy of Spiral is more than C type. 3 44

45 Helix Type Bourdon Tube Construction: A central shaft installed within the helical element Pointer is driven by shaft by connecting links System transmits only the circular motion of the tip of pointer Working Due to pressure tube tends to uncoil Produce long movement of tip The no of coils employed depends on degree of pressure Displacement of helical bourdon is more than spiral type. 45

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49 Bellows Construction: A bellow is a series of circular parts These joined / formed parts expand / contract axially Material: thin, flexible, ductile, fatigue strength Material: Brass, Bronze, Beryllium Copper, SS, Monel, Alloys of Ni & Cu 49

50 Bellows Construction: Normally bellows expands too much To increase accuracy & life of bellow spring is used Movement of bellow is opposed by spring force Such system is called spring loaded bellows. 50

51 Bellows Arrangement to measure absolute, gauge, & differential pressure 51

pressures Low drift & hysteresis Χ Disdvantage: Need temperature compensating devices

52 Bellows Advantage: Simple & rugged construction Moderate price Usefulness for low,medium,high pressure measurement Applicability for gauge, absolute, differential pressures Low drift & hysteresis Χ Disdvantage: Need temperature compensating devices to avoid errors Not suitable for dynamic measurement due greater mass & longer relative movement 52

Diphragm Construction: A diaprahm is a flexible disc ( flat / corrugated ) Deflection of diapragm is proportional to applied pressure The movement is small thus spring is not required Movement

53 Diphragm Construction: A diaprahm is a flexible disc ( flat / corrugated ) Deflection of diapragm is proportional to applied pressure The movement is small thus spring is not required Movement depends on diapragm thickness & diameter Material: Brass, Bronze, Beryllium Copper, SS, Monel, Ni Span C, Inconel Non metallic ( Buna N rubber, Teflon, Nylon) Pressure Range: 00 to 200 KN / m 2 Accuracy: (+-) 0.5 % to (+-) 1.25 % 53

54 Diphragm Construction: Two diaprahms are bonded together by soldering to form a capsule One or more capsules are connected together axially 54

55 Diphragm 55

56 Electrical Resistance Type Pressure Gauge Bridgman Gauge: Change of resistance with change of pressure If a wire is subjected to hydrostatic pressure, Resistance of wire changes according to relation R= R1.(1+b. p); R1= resistance at 1 atm, b= pressure coefficient of resistance, p= gauge pressure In most common metal wires, the resistance decreases with increase in pressure, while for antimony, bismuth, lithium, and manganin, it increases. 56

57 Electrical Resistance Type Pressure Gauge Bridgman Gauge: Material of wire: Gold-chrome, Manganin ( 84Cu, 12Mn, 4Ni) Pressure up to 100,000 atm can be measured. Advantages: Good dynamic response, High sensitivity, Rugged in construction 57

58 Photoelectrical Type Pressure Gauge Optical Pressure Sensor: Incident energy on a sensitivity surface causes an emission of electrons. Consists of a light source, phototube & a vane / seperater. Vane is attached to force summing element (diapragm, capsule, bellow) Pressure force summing member vane light Advantages: Good dynamic response, High accuracy Disadvantages: More time for stabilization, large displacement of fsm is needed 58

Piezoelectrical Type Pressure Gauge Piezoelectric Pressure Sensor: Piezoelectric transducer generates emf when it deforms under pressure.

59 Piezoelectrical Type Pressure Gauge Piezoelectric Pressure Sensor: Piezoelectric transducer generates emf when it deforms under pressure. Pressure is converted into displacement Quartz, Rochelle Salt, Lithium Sulphate For more sensitivity synthetic material is used Advantages: Good dynamic response, High accuracy, Active transducer, Compact Disadvantages: Sensitive to temperature, Unsuitable for static measurement 59

60 Capacitive Transducer Working Principle: Ability of body to store electrical charge. Capacitance of a parallel plate capacitor C.A d r. d o.a Advantage: small force to operate, good frequency response, extremely sensitive, low power requirement, locations having magnetic field, low cost. Disadvantage: Non linear behaviour, High output impedance, Temperature sensitive q V..r d : Permittivity of the medium A: Overlapping area of plates d: Distance between plates 2 60

61 Capacitive Transducer Construction: Plates / Cylinder, Di electric material (Air, Mica, Oil etc) Working: Capacitance can be changed by 3 ways..a r. o.a q C d d V..r d 2 61

62 Capacitive Transducer Application: For measurement of both linear & angular displacement. (0.1 X 10-6 mm) For measurement of force & pressure. Used directly as pressure transducer where dielectric constant changes with pressure. Used directly for mesurement of humidity as dielectric constant changes with humidity. Used in conjuction with mechanical modifiers for measurement of volume, density, liquid level etc..a C d r. d o.a q V..r d 2 62

63 Capacitive Transducer 63

64 Capacitive Transducer Application 64

65 Capacitive Transducer Application 65

66 Failing To Prepare Means Preparing To Fail!!! MAC MAEER's 66

EMaSM. Principles Of Sensors & transducers

EMaSM. Principles Of Sensors & transducers EMaSM Principles Of Sensors & transducers Introduction: At the heart of measurement of common physical parameters such as force and pressure are sensors and transducers. These devices respond to the parameters