------------------------------------- 18 Marks Displacement & P ressure ------------------------------------- Course Outcome: The students will be able to explain working of displacement transducers, construction and working of low pressure and high pressure measuring instruments.
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Measurment displacement & pressuremitpolytechnic.ac.in/downlaods/09_knowledge-bank/04_mechanical/SEM-5/... · 18 Marks Displacement & Pressure ----- Course Outcome: The students will
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Working Principle: Ability of body to store electrical charge.
Capacitance of a parallel plate capacitor
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
d
.r.
V
q
d
.A.
d
.AC
2
or
€: Permittivity of the medium A: Overlapping area of plates d: Distance between plates
Capacitive Transducer
4 MIT Polytechnic, Pune 17528
Construction: Plates / Cylinder, Di electric material (Air, Mica, Oil etc)
Working: Capacitance can be changed by 3 ways.
d
.r.
V
q
d
.A.
d
.AC
2
or
Capacitive Transducer
5 MIT Polytechnic, Pune 17528
Construction: Plates / Cylinder, Di electric material (Air, Mica, Oil etc)
Working: Capacitance can be changed by 3 ways.
d
.r.
V
q
d
.A.
d
.AC
2
or
Capacitive Transducer
6 MIT Polytechnic, Pune 17528
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.
d
.r.
V
q
d
.A.
d
.AC
2
or
Capacitive Transducer
7 MIT Polytechnic, Pune 17528
Potentiometer
8 MIT Polytechnic, Pune 17528
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.
A
ρ.LR
Potentiometer
9 MIT Polytechnic, Pune 17528
Working Principle: Positioning of slider changes resistance of a POT or a bridge ckt.
Xt
Xi
.EiXt
Xi.Eo Voltage,Output
(V)i/p . terminali/pat Resistance
terminalo/pat ResistanceEo Voltage,Output
.Eiθt
θi.Eo Voltage,Output
Potentiometer
Wire Wound POT: Material: Pt, Cr, Ni, Ni-Cu.
Resolution: 0.025 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.
17528 10 MIT Polytechnic, Pune
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)
17528 11 MIT Polytechnic, Pune
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.
Applications of Potentiometer
17528 12 MIT Polytechnic, Pune
Applications of Potentiometer Audio Control
Motion Control
Television
17528 13 MIT Polytechnic, Pune
Applications of Potentiometer
17528 14 MIT Polytechnic, Pune
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 MIT Polytechnic, Pune 17528
Linear Variable Differential Transformer
16 MIT Polytechnic, Pune 17528
17528 17 MIT Polytechnic, Pune
Based on magnetic characteristics of an electrical circuit.
1.Change in Mutual Inductance:
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
21.LLk.M
Linear Variable Differential Transformer
17528 18 MIT Polytechnic, Pune
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.
Linear Variable Differential Transformer
17528 19 MIT Polytechnic, Pune
Working:
Power a/c (P1)Magnetic field a/c Current a/c (S1 & S2)
Linear Variable Differential Transformer
17528 20 MIT Polytechnic, Pune
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:
Linear Variable Differential Transformer
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
17528 21 MIT Polytechnic, Pune
Application: To measure displacement from a few mm to a few cm. As a secondary transducer to measure force, displacement, pressure.
Linear Variable Differential Transformer
17528 22 MIT Polytechnic, Pune
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)
Rotary Variable Differential Transformer
Pressure Pressure : Force per unit area.
For an ideal gas , Pressure P= (n.m.vrms2)/3
Static Pressure: Liquid is at equillibrium.
Dynamic Pressure: Due to pressure gradient.
Velocity Pressure: Total Pressure – Static Pressure
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 : 0.000001mm of Hg to 1mm of Hg
38 MIT Polytechnic, Pune 17528
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 MIT Polytechnic, Pune 17528
40 MIT Polytechnic, Pune 17528
Bourdon Pressure Gauge Most Popular, Eugene Bourdon, Elastic Pressure Element
Types: C , Spiral, Helical
41 MIT Polytechnic, Pune 17528
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 MIT Polytechnic, Pune 17528
C Type Bourdon Tube
43 MIT Polytechnic, Pune 17528
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
30.330.2
t
x.
y
x.
t
r.
E
0.05.a.PΔa
Spiral Type Bourdon Tube
44 MIT Polytechnic, Pune 17528
Construction: 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
30.330.2
t
x.
y
x.
t
r.
E
0.05.a.PΔa
Working Due to pressure tube tends to uncoil Produce long movement of tip Accuracy of Spiral is more than C type .
Helix Type Bourdon Tube
45 MIT Polytechnic, Pune 17528
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 .
17528 46 MIT Polytechnic, Pune
17528 47 MIT Polytechnic, Pune
17528 48 MIT Polytechnic, Pune
Bellows
49 MIT Polytechnic, Pune 17528
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
Bellows
50 MIT Polytechnic, Pune 17528
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.
Bellows
51 MIT Polytechnic, Pune 17528
Arrangement to measure absolute, gauge, & differential pressure
Bellows
52 MIT Polytechnic, Pune 17528
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
Diphragm
53 MIT Polytechnic, Pune 17528
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 / m2
Accuracy: (+-) 0.5 % to (+-) 1.25 %
Diphragm
54 MIT Polytechnic, Pune 17528
Construction: Two diaprahms are bonded together by soldering to form a capsule One or more capsules are connected together axially
Diphragm
55 MIT Polytechnic, Pune 17528
Electrical Resistance Type Pressure Gauge
56 MIT Polytechnic, Pune 17528
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.
Electrical Resistance Type Pressure Gauge
57 MIT Polytechnic, Pune 17528
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
Photoelectrical Type Pressure Gauge
58 MIT Polytechnic, Pune 17528
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
Piezoelectrical Type Pressure Gauge
59 MIT Polytechnic, Pune 17528
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
Capacitive Transducer
60 MIT Polytechnic, Pune 17528
Working Principle: Ability of body to store electrical charge.
Capacitance of a parallel plate capacitor
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
d
.r.
V
q
d
.A.
d
.AC
2
or
€: Permittivity of the medium A: Overlapping area of plates d: Distance between plates
Capacitive Transducer
61 MIT Polytechnic, Pune 17528
Construction: Plates / Cylinder, Di electric material (Air, Mica, Oil etc)
Working: Capacitance can be changed by 3 ways.
d
.r.
V
q
d
.A.
d
.AC
2
or
Capacitive Transducer
62 MIT Polytechnic, Pune 17528
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.