Measurment displacement &...

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18 Marks

Displacement & Pressure -------------------------------------

Course Outcome:

The students will be able to explain working of

displacement transducers, construction and working of

low pressure and high pressure measuring instruments.

Introduction

2 MIT Polytechnic, Pune 17528

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,

Capacitive Transducer


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



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





d

.r.

V

q

d

.A.

d

.AC

2

or



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



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.

A

ρ.LR

Potentiometer


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)


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


Applications of Potentiometer Audio Control

Motion Control

Television


Applications of Potentiometer


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.


Linear Variable Differential Transformer



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



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.



Working:

Power a/c (P1)Magnetic field a/c Current a/c (S1 & S2)



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


Application: To measure displacement from a few mm to a few cm. As a secondary transducer to measure force, displacement, pressure.



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


Pressure


Absolute Pressure: Gauge Pressure: Vaccum Pressure: Atmospheric Pressure:

Atmospheric Pressure: 760 mm of Hg 101325 pascal 10.33 m of H2O

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


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


McLeod Gauge

Working Principle: Boyle’s Law

P1V1 = P2V2

Construction:

Reference Column

Reference Capillary

Bulb & Measuring Capillary

Mercury Reservoir & Piston


McLeod Gauge

Working: P2 = P1+h P1V1 = P2V2 P1V1= (P1+h).a.h P1 = ah2 / (V1 – ah) As ah is very small P1 = ah2 / V1


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.


Thermal Conductivity Gauge

Working Principle:

Modes of heat Transfer: Conduction, Convection & Radiation


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


Thermocouple Vaccum Gauge

Working Principle:

Construction:

Resistive Element

Thermocouple

Working


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.


Pirani Gauge

Working Principle:

Construction:

Pirani Gauge Chamber

Platinum Filament

Compensating Chamber

Wheat Stone Bridge

Working


Pirani Gauge

Advantages:

Rugged & inexpensive.

Gives continuos reading.

Accurate / linear / Remote Indicate.

Disadvantages:

Need calibration for different gases.

Electric Power is essential.


Ionisation Gauge [ Bayard Al p er t / Hot Cathode ]

Working Principle: Ionisation

Construction:

A triode vaccum tube

Cathode ( Coated Iridum) Outside

Anode Grid (Nickel) Midway Spiral

Ion Collector ( Tungsten) Center



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

Ip: Plate / Ionisation Ig: Grid / Emission Current

K: Sensitivity of Gauge


g

p

I

I.

K

1P


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 : 0.000001mm of Hg to 1mm of Hg


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.



Bourdon Pressure Gauge Most Popular, Eugene Bourdon, Elastic Pressure Element

Types: C , Spiral, Helical


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)


C Type Bourdon Tube


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


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


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 .




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

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.

Bellows


Arrangement to measure absolute, gauge, & differential pressure

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

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 / m2

Accuracy: (+-) 0.5 % to (+-) 1.25 %

Diphragm


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

Diphragm


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.

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

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

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



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





d

.r.

V

q

d

.A.

d

.AC

2

or



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 Application


Capacitive Transducer Application


MAC 17528 MAEER's MIT Polytechnic, Pune 66

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