transducers and dimension measurement measurement systems · measurement systems by: yidnekachew...

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Transducers and Measurement systems

By: Yidnekachew Messele

Dimension Measurement

Rules and Tapes Rules and tapes are the simplest way of measuring larger

dimensions. Steel rules are generally only available tomeasure dimensions up to 1 m. Beyond this, steel tapes(measuring to 30 m) or an ultrasonic rule (measuring to 10 m)is used.

The steel rule is the measurement accuracy is much dependentupon the skill of the human measurer and, at best, theinaccuracy is likely to be at least 0.5%.

The ultrasonic rule consists of an ultrasonic energy source,an ultrasonic energy detector, and battery-powered, electroniccircuitry housed within a handheld box.

3AAiT Instrumentation and measurement Chapter 5

Both source and detector often consist of the same type ofpiezoelectric crystal excited at a typical frequency of 40 kHz.

Energy travels from the source to a target object and is thenreflected back into the detector.

The time of flight of this energy is measured and this isconverted into a distance reading by the enclosed electronics.

Maximum measurement inaccuracy of 1% of the full-scalereading is claimed.


2

Calipers Calipers are generally used in situations, where measurement of

dimensions with a rule or tape is not accurate enough.

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Determination of the point, where the two scales coincide enables very accurate measurements to be made, with typical inaccuracy levels down to 0.01%.

AAiT Instrumentation and measurement Chapter 5

Micrometers Micrometers provide a means of measuring dimensions to high

accuracy. Measurement is made between two anvils, one fixed and one that

is moved along by the rotation of an accurately machined screwthread. One complete rotation of the screw typically moves theanvil by a distance of 0.5 mm.

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The most commonmeasurement ranges areeither 0-25 mm or 25-50mm, with inaccuracylevels down to 0.003%.

AAiT Instrumentation and measurement Chapter 5

Position and Displacement Sensors

Displacement sensors are basically used for the measurement ofmovement of an object.

Position sensors are employed to determine the position of anobject in relation to some reference point.

One method of determining a position, is to use either"distance", which could be the distance between two points suchas the distance travelled or moved away from some fixed point, orby "rotation" (angular movement). For example, the rotation of a robots wheel to determine its distance

travelled along the ground. Either way, Position Sensors can detect themovement of an object in a straight line using Linear Sensors or by itsangular movement using Rotational Sensors.


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The most commonly used of all the "Position Sensors", is thepotentiometer because it is an inexpensive and easy to useposition sensor.

The potentiometer can be of linear or angular type. It works onthe principle of conversion of mechanical displacement into anelectrical signal.

The sensor has a resistive element and a sliding contact (wiper).The slider moves along this conductive body, acting as a movableelectric contact.

The object of whose displacement is to be measured is connectedto the slider by using a rotating shaft (for angular displacement) a moving rod (for linear displacement)

9AAiT Instrumentation and measurement Chapter 5 10AAiT Instrumentation and measurement Chapter 5

Application These are typically used on machine-tool controls, elevators,

liquid-level assemblies, forklift trucks, automobile throttlecontrols.

In manufacturing, these are used in control of injectionmolding machines, woodworking machinery, printing,spraying, robotics, etc. These are also used in computer-controlled monitoring of sports equipment.


Measuring Linear Displacement Very small displacements:

Strain Gauges Capacitive Sensors Inductive Sensors (LVDT)

Medium displacements Slide Wire / Film Wire wound potentiometer

Large Displacements (above range of most ‘pure’ linear transducers) Convert linear to angular motion and measure the angular motion with

an angular displacement transducer Measure velocity and integrate signal to obtain displacement


4

Linear Displacement - Resistive Methods

Resistance is defined by thefollowing equation

Therefore if the length,thickness or resistivity of anobject changes with respect todisplacement we can use theresistance as a way to measure it

AlR


Linear Displacement - Resistive Methods (Slide Wire/Film)

This is the simplest way of measuring displacement between a moving and a stationary object

A piece of wire or film is connected to a stationary object A slide, which makes contact with the wire, is attached to the moving object This acts as a very basic potentiometer A potentiometer is an electromechanical device containing an electrically

conductive wiper that slides against a fixed resistive element according to the position or angle of an external shaft.

AAiT Instrumentation and measurement Chapter 5 14


Slide Wire Range

± 1 – 300mm

Advantages Simple Good Resolution Low Cost

Disadvantages Wire does not have high resistance, film is better (±200 to 500Ω/cm) Wear Frictional Loading Inertial Loading


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Linear Displacement - Resistive Methods (Wire

Wound Potentiometer)

Wire Wound potentiometersuse the same principle asslide wire sensors exceptthat they use a coil ofinsulated resistance

The slider runs on onesurface of the coil that is notinsulated


Potentiometer

Vex

V=0 to VexRp

Rx

maxVV

maxRR

exRR

pmax

x

xx

xx

VV

Rx

xR

ex

p

x

p

x

xmax x

X can also be the degrees of turns.

Linear potentiometer is a device in which the resistance varies as a function of the position of a slider.

V


Potentiometers Resolution

± 1mm – 4m Advantages

Simple Robust

Disadvantages Resolution dependant on wire diameter Continuous use over portion of the wire will cause wear Frictional Loading Inertial Loading


6

Linear Displacement - Resistive Methods (Strain Gauges)

Strain gauge: it is an electrical conductor whose resistancechanges as it is strained.

Attach the strain gauge to the object When the object is in tensionor compressed it will result in a change in the resistance of thestrain gauge.

This is used to measure the change in length of the object


Strain Gauges

Advantages: Relatively easy to understand and attach Cheap

Disadvantages Need temperature compensation


Linear Displacement - Capacitive Methods

Capacitance is defined as

Therefore we could use the change in Plate Area Permittivity of the dielectric Distance between the plates

as a way to measure displacement

dAC r 0


If we have two electrodes and one moves relative to the other ina linear direction we will get an effective change in the area ofthe plates

This results in a change in the capacitance which can be relatedto displacement.


(Variable Area)


d

wxAC r 0

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Linear Displacement - Capacitive Methods(Distance Between the Plates)

If we have two electrodes, one fixed and the other movable wecan arrange it that the distance between the plates changes fora change in displacement


xAC r 0


(Distance Between the Plates)

This type of capacitive arrangementconsists of two fixed outer plates andone central moveable plate .

The central plate can move towardseither of the plates which essentiallychanges the capacitance between themoveable plate and the fixed plates.

If the moveable plate is in the centerof the capacitor, voltages V1 and V2will be equal.



(Permittivity)

The dielectric moves relative to the plates and this causes achange in the relative permittivity of the dielectric


Linear Displacement - Inductive Methods

Inductive methods use very similar principles to resistive andcapacitive methods

The inductance of a coil is given by the following equation

Where N is the number of turns in the coil, µ is the effectivepermeability of the medium in and around the coil, A is thecross sectional area and l is the length of the coil in m.

As with the other examples if we change any one of theseparameters we get a change in the inductance

][ Henrys l

ANL 2


8


(Linear Variable Differential Transformers LVDTs)

LVDTs are accuratetransducers which are oftenused in industrial andscientific applications tomeasure very smalldisplacements



(Linear Variable Differential Transformers LVDTs)

An LVDT consists of a centralprimary coil wound over the wholelength of the transducer and twoouter secondary coils

A magnetic core is able to movefreely through the coil

The primary windings areenergized with a constant amplitudeAC signal (1 – 10kHz)


This produces an alternating magnetic field which induces asignal into the secondary windings

The strength of the signal is dependent on the position of thecore in the coils.

When the core is placed in the center of the coil the outputwill be zero.

Moving the coil in either direction causes the signal to increase The output signal is proportional to the displacement


Linear Variable-Differential Transformer (LVDT)

-x

Vo=V1-V2

Vi

V1 V2

V1 > V2 Vi

Vo

LVDTs are devices to measure displacementby modifying spatial distribution of analternating magnetic field.

Oscillating excitation voltage-50 Hz to 25 kHz


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X=0

Vo=V1-V2

Vi

V1 V2

V2 = V1Vi

Vo



+x

Vo=V1-V2

Vi

V1 V2

V2 > V1Vi

Vo

So, the direction of displacement can be determined from the relative phase of the signal.


LVDTs Range: ±2.5nm - ±10cm Advantages: Good resolution

Disadvantages: Needs shielding to prevent interference from magnetic

sources


Pressure Measurements

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Pressure definition Pressure is the action of one force against another over, a

surface. The pressure P of a force F distributed over an area A isdefined as:

P = F/A

System Length Force Mass Time Pressure

MKS Meter Newton Kg Sec N/M2 = Pascal

CGS CM Dyne Gram Sec D/CM2

English Inch Pound Slug Sec PSI


How Much is a Pascal (Pa) 1 atmosphere (14.7 psi, 750mmHg) is approximately

100 kPa = 1 bar 1 kPa is about 7 mmHg 1% of a gas at our altitude is about 7 mmHg

How is pressure generated? Collision of molecule with wall Momentum of mass with x velocity Sum collisions over area to get force


Static, Dynamic, and Impact pressures

Static pressure is the pressure of fluids or gases that are stationary or not inmotion.

Dynamic pressure is the pressure exerted by a fluid or gas when it impacts ona surface or an object due to its motion or flow. In Fig., the dynamic pressure is(B − A).

Impact pressure (total pressure) is the sum of the static and dynamic pressureson a surface or object. Point B in Fig. depicts the impact pressure.


Definition Of Pressure Absolute pressureThe pressure is referenced to

zero absolute. Absolutepressure can only have apositive value.

Gauge pressureThe pressure is referenced to

atmospheric pressure and byconvention is measured inthe positive direction.

Vacuum pressureThe pressure is referenced to

atmospheric pressure and byconvention is measured inthe negative direction.


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Pressure MeasurementA number of measurement units are used for pressure. They are as follows:1. Bar (1.013 atm) = 100 kPa2. Pascals (N/m2) or kilopascal (1000Pa)3. Pounds per square foot (psf) or pounds per square inch (psi)4. Atmospheres (atm)5. Torr = 1 mm mercury6. Pascals (N/m2) or kilopascal (1000Pa)

Pressure Units As previously noted, pressure is force per unit area and historically a great

variety of units have been used, depending on their suitability for theapplication.

For example, blood pressure is usually measured in mmHg becausemercury manometers were used originally.

Atmospheric pressure is usually expressed in mmHg for the same reason. Other units used for atmospheric pressure are bar and atm.

1 psi= 51.714 mmHg= 2.0359 in.Hg

= 27.680 in.H2O= 6.8946 kPa

1 bar= 14.504 psi1 atm. = 14.696 psi


Wet Meters (Manometers)



Manometer basics Characterized by its inherent accuracy and simplicity of

operation. It’s the U-tube manometer, which is a U-shaped glass tube

partially filled with liquid. This manometer has no moving parts and requires no calibration. With both legs of a U-tube manometer open to the atmosphere

or subjected to the same pressure, the liquid maintains thesame level in each leg, establishing a zero reference.

With a greater pressure applied to the left side of a U-tubemanometer, the liquid lowers in the left leg and rises in the rightleg.

The liquid moves until the unit weight of the liquid, as indicatedby h, exactly balances the pressure.


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Pressure in open tankA container filled with a liquid has a pressure (due to the weight of the liquid) at a point in the liquid of:

P = F/AP = W/AP = ρgV/AP = ρghA/AP = ρghP = pressure

F = forceA = AreaW = weight of the liquid V = volume above the Areag = gravitationρ = mass densityh = distance from the surface

A

h


When the liquid in the tube is mercury, for example, theindicated pressure h is usually expressed in inches (ormillimeters) of mercury. To convert to pounds per squareinch (or kilograms per square centimeter), P2 = ρh

Where P2 = pressure, (kg/cm2), ρ = density, (kg/cm3), h = height, (cm) Gauge pressure is a measurement relative to atmospheric

pressure and it varies with the barometric reading. A gauge pressure measurement is positive

when the unknown pressure exceedsatmospheric pressure (A), and is negativewhen the unknown pressure is less thanatmospheric pressure (B).


Variations on the U-Tube Manometer The pressure reading is always the

difference between fluid heights,regardless of the tube sizes.

With both manometer legs open tothe atmosphere, the fluid levels arethe same (A).

With an equal positive pressureapplied to one leg of eachmanometer, the fluid levels differ,but the distance between the fluidheights is the same (B).


Reservoir (Well) Manometer In a well-type manometer, the cross-sectional area of one

leg (the well) is much larger than the other leg. Whenpressure is applied to the well, the fluid lowers only slightlycompared to the fluid rise in the other leg.

In this design one leg is replaced by a large diameter well sothat the pressure differential is indicated only by the heightof the column in the single leg.

The pressure difference can be read directly on a singlescale. For static balance,

Where A1 = area of smaller-diameter leg A2 = area of well

2 1 1 2(1 / )P P A A h If the ratio of A1/A2 is small compared withunity, then the error in neglecting this termbecomes negligible, and the static balancerelation becomes2 1P P h


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Pressure Sensing Pressure is sensed by mechanical

elements such as plates, shells, and tubesthat are designed and constructed todeflect when pressure is applied.

This is the basic mechanism convertingpressure to physical movement.

Next, this movement must be transducedto obtain an electrical or other output.

Finally, signal conditioning may beneeded, depending on the type of sensorand the application. Figure illustrates thethree functional blocks.

Pressure

Signal Conditioner

Sensing Element

Transduction element

displacement

electric

V or I output


The main types of sensing elements are Bourdon tubes, diaphragms, capsules, and bellows .

All except diaphragms provide afairly large displacement that isuseful in mechanical gauges andfor electrical sensors that requirea significant movement.

The basic pressure sensing element can be configured as a C-shaped Bourdontube (A); a helical Bourdon tube (B); flat diaphragm (C); a convoluteddiaphragm (D); a capsule (E); or a set of bellows (F).



Primary Pressure Elements Capsule, Bellows & Spring Opposed Diaphragm


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Bellows


In general a bellows can detect a slightly lower pressure than a diaphragm

The range is from 0-5 mmHg to 0-2000 psi Accuracy in the range of 1% span


Bourdon Tube In “C” type Bourdon tube, a section of tubing that is closed at

one end is partially flattened and coiled. When a pressure is applied to the open end, the tube uncoils. This movement provides a displacement that is proportional to

the applied pressure. The tube is mechanically linked to a pointer on a pressure

dial to give a calibrated reading.


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Bourdon Tubes


(a) C-type tube.(b) Spiral tube. (c) Helical tube

Bourdon Tubes


Diaphragm Gauges To amplify the motion that a diaphragm capsule produces, several

capsules are connected end to end. Diaphragm type pressure gauges used to measure gauge, absolute, or

differential pressure. They are normally used to measure low pressures of 1 inch of Hg, but

they can also be manufactured to measure higher pressures in the rangeof 0 to 330 psig.


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Diaphragm


A diaphragm usually is designed so that the deflection-versus-pressure characteristics are as linear as possible over aspecified pressure range, and with a minimum of hysteresisand minimum shift in the zero point.

(a) flat diaphragm; (b) corrugated diaphragm


CapsuleA capsule is formedby joining theperipheries of twodiaphragms throughsoldering or welding.

Used in some absolutepressure gages.


Use of capsule element in pressure gage


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Potentiometric type sensor A mechanical device such as a diaphragm is used

to move the wiper arm of a potentiometer as the input pressure changes.

A direct current voltage (DC) V is applied to thetop of the potentiometer, and the voltage that isdropped from the wiper arm to the bottom of the potis sent to an electronic unit.

It normally cover a range of 5 psi to 10,000 psi. Can be operated over a wide range of temperatures. Subject to wear because of the mechanical contact

between the slider and the resistance element. Therefore, the instrument life is fairly short, and

they tend to become noisier as the pot wears out.


Bellows Resistance Transducer Bellows or a bourdon tube with a

variable resistor. Bellow expand or contract causes the

attached slider to move along theslidewire.

This increase or decrees theresistance.

Thus indicating an increase ordecrease in pressure.


Inductance-Type Transducers The inductance-type transducer consists of three parts: a coil, a

movable magnetic core, and a pressure sensing element. An AC voltage is applied to the coil, and, as the core moves,

the inductance of the coil changes.


LVDT Another type of inductance transducer, utilizes two coils

wound on a single tube and is commonly referred to as aDifferential Transformer or sometimes as a Linear VariableDifferential Transformer (LVDT).


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Piezoelectric Piezoelectric elements are bi-directional transducers capable

of converting stress into an electric potential and vice versa. One important factor to remember is that this is a dynamic

effect, providing an output only when the input is changing. This means that these sensors can be used only for varying

pressures. The piezoelectric element has a high-impedance output and care must

be taken to avoid loading the output by the interface electronics. Somepiezoelectric pressure sensors include an internal amplifier to provide aneasy electrical interface.


•Piezoelectric sensors convert stress intoan electric potential and vice versa.•Sensors based on this technology areused to measure varying pressures.


Strain Gauge Pressure Sensors Strain gauge sensors originally used a metal diaphragm with strain gauges

bonded to it. the signal due to deformation of the material is small, on the order of 0.1% of

the base resistance Semiconductor strain gauges are widely used, both bonded and integrated

into a silicon diaphragm, because the response to applied stress is an order ofmagnitude larger than for a metallic strain gauge.

When the crystal lattice structure of silicon is deformed by applied stress,the resistance changes. This is called the piezoresistive effect. Following aresome of the types of strain gauges used in pressure sensors.

Deposited strain gauge. Metallic strain gauges can be formed on a diaphragmby means of thin film deposition. This construction minimizes the effects ofrepeatability and hysteresis that bonded strain gauges exhibit. These sensorsexhibit the relatively low output of metallic strain gauges.


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Range of Elastic-Element Pressure Gages


Dead-weight pressure gauge A cylindrical piston 1 is placed inside a stainless-steel cylinder 2. The measuring pressure is supplied through the vent 8 to the fluid 4. The gravitational force developed by calibrated weights 3 can balance this force

and the piston itself.. The balance should be achieved for a certain position of the piston against a

pointer 9 of the stainless-steel cylinder. A manual piston pump 5 is used to achieve approximate force balance (to

increase pressure in the system), whereas a wheel-type piston pump 6 serves foraccurate balancing.

A Bourdon-type pressure gauge 7 is used for visual reading of pressure.


1

2

3

4

5

6

7

8

2

9

Calibration of Pressure Sensing Devises


Mass, Force and Torque

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Mass (Weight) Measurement The mass of a body is always quantified in terms of a

measurement of the weight of the body, this being the downwardforce exerted by the body when it is subject to gravity.

The first method of measuring the downward force exerted by amass subject to gravity involves the use of a load cell. The loadcell measured the downward force F, and then the mass M iscalculated from the equation:

Since the values of g vary by small amounts at different pointsaround the earth’s surface, the value of M can only be calculatedexactly if the value of g is known exactly.


Several different forms of load cells are available. Most loadcells are now electronic, although pneumatic and hydraulictypes also exist.

Within an electronic load cell, the gravitational force on thebody being measured is applied to an elastic element. Thisdeflects according to the magnitude of the body mass. Massmeasurement is thereby translated into a displacementmeasurement task.

The elastic elements used are specially shaped and designed.



The design aims to obtain a linear output relationship betweenthe applied force and the measured deflection and to make theinstrument insensitive to forces that are not applied directlyalong the sensing axis.

Elastic force transducers based on differential transformers(linear variable differential transformers (LVDTs)) to measuredefections are used to measure masses up to 25 tonne.

Piezoelectric device used to measure masses in the range of 0-1000 tonne.

Piezoelectric crystals replace the specially designed elasticmember normally used in this class of instrument, allowing thedevice to be physically small.


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Pneumatic and hydraulic load cells translate mass measurementinto a pressure measurement task, though they are now lesscommon than the electronic load cell.


Mass-Balance (Weighing) Instruments


Force Sensors Force is a quantity capable of changing the size, shape, or

motion of an object. There are four basic forces in nature:

gravitational, magnetic, strong nuclear, and weak nuclear forces.

The weakest of the four is the gravitational force. It is also theeasiest to observe, because it acts on all matter and it is alwaysattractive, while having an infinite range. Its attractiondecreases with distance, but is always measurable.


Force Sensors The fundamental operating principles of force, acceleration, and

torque instrumentation are closely allied to the piezoelectric andstrain gage devices used to measure static and dynamic pressures.

Piezoelectric sensor produces a voltage when it is "squeezed" by aforce that is proportional to the force applied.


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Difference between these devices and static force detectiondevices such as strain gages is that the electrical signalgenerated by the crystal decays rapidly after the application offorce.

The high impedance electrical signal generated by thepiezoelectric crystal is converted to a low impedance signalsuitable for such an instrument as a digital storageoscilloscope.

Depending on the application requirements, dynamic force canbe measured as either compression, tensile, or torque force.

Applications may include the measurement of spring or slidingfriction forces, chain tensions, clutch release forces.


Strain gauge. The electrical resistance of a length of wire varies in direct

proportion to the change in any strain applied to it. That’s theprinciple upon which the strain gauge works.

The most accurate way to measure this change in resistance isby using the wheatstone bridge.

The majority of strain gauges are foil types, available in a widechoice of shapes and sizes to suit a variety of applications.

They consist of a pattern of resistive foil which is mounted ona backing material.


They operate on the principle that as the foil is subjectedto stress, the resistance of the foil changes in a definedway.


Strain gauge Configuration The strain gauge is connected

into a wheatstone Bridge circuitwith a combination of fouractive gauges(full bridge),twoguages (half bridge) or,lesscommonly, a single gauge(quarter bridge).


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Guage factor A fundamental parameter of the strain guage is its

sensitivity to strain, expressed quantitatively as theguage factor (GF).

Guage factor is defined as the ratio of fractionalchange in electrical resistance to the fractionalchange in length (strain).


Strain guage contd.. The complete wheatstone brigde is excited with a stabilized

DC supply. As stress is applied to the bonded strain guage, a resistive

change takes place and unbalances the wheatstone bridgewhich results in signal output with respect to stress value.

As the signal value is small the signal conditioningelectronics provides amplification to increase the signal.


Shear Force Measurement The strain gauges are bonded on the flat upper and lower sections of the load cell at

points of maximum strain. This load cell type is used for low capacities and performs with good linearity. Its disadvantage is that it must be loaded correctly to obtain

consistent results

VO

VS

VO= k PVS

Where:

A is the cross-sectional area

E is the modulus of elasticity

v is Poissin’s ratio of the material

Sg is a gauge factor

Therefore Force P is measured interms of Voltage out put as VO


24

Torque Sensors Torque is measured by either sensing the actual shaft deflection caused by a twisting

force, or by detecting the effects of this deflection. The surface of a shaft under torque will experience compression and tension, as shown in

Figure. To measure torque, strain gage elements usually are mounted in pairs on the shaft, one

gauge measuring the increase in length (in the direction in which the surface is undertension), the other measuring the decrease in length in the other direction.


Torque Sensor Torque is a measure of the forces that causes an object to

rotate. Reaction torque sensors measure static and dynamic torque

with a stationary or non-rotating transducer. Rotary torque sensors use rotary transducers to measure

torque.


Technology Magnetoelastic : A magnetoelastic torque sensor detects changes

in permeability by measuring changes in its own magnetic field. Piezoelectric : A piezoelectric material is compressed and

generates a charge, which is measured by a charge amplifier. Strain guage : To measure torque,strain guage elements usually

are mounted in pairs on the shaft,one guage measuring theincrease in length the other measuring the decrease in the otherdirection.


Figures showing Torque sensors


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Applications of force/torque sensors In robotic tactile and manufacturing applications In control systems when motion feedback is employed. In process testing, monitoring and diagnostics applications. In measurement of power transmitted through a rotating

device. In controlling complex non-linear mechanical systems.


Flow Measurement

Since 1989 there were at least 23 distinct type of technologiesavailable the measurement of flow in closed conduit.

Flow meters selection are part of the basic art of the instrumentengineer, and while only handful of these technologiescontribute to the majority of installations.

And wide product knowledge is essential to find the most costeffective solution to any flow measurement application.


Types of Flows Reynolds Number

The performance of flow meters is also influenced by a dimensionlessunit called the Reynolds Number. It is defined as the ratio of theliquid's inertial forces to its drag forces.

The Reynolds number is used for determined whether a flow is laminaror turbulent. Laminar flow within pipes will occur when the Reynolds number is below the

critical Reynolds number of 2300 and turbulent flow when it is above 2300. The value of 2300 has been determined experimentally and a certain range around

this value is considered the transition region between laminar and turbulent flow.


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Differential Pressure (Obstruction-Type) Meters

Venturi Meter

• In the venturi meter velocity is increased and the pressuredecreased in the upstream cone.

• The pressure drop from points F to I can be used to measure therate of flow through the meter.

• Venturi meters are most commonly used for liquids, especiallywater.



27


Venturi MeterSince friction cannot be eliminated in the venturi meter a permanent loss inpressure occurs. Because of the small angle of divergence in the recoverycone, the permanent pressure loss is relatively small (about 10% of theventuri differential pa–pb).


28

Orifice Meter

The orifice meter consists of an accurately machined and drilled plateconcentrically mounted between two flanges. The position of the pressuretaps is somewhat arbitrary.


Orifice MeterThe orifice meter has several practical advantages when compared to venturi meters.• Lower cost• Smaller physical size• Flexibility to change throat to pipe diameter ratio to

measure a larger range of flow ratesDisadvantage:• Large power consumption in the form of irrecoverable

pressure loss


–


29



There is a large pressure drop much of which is not recoverable. This can be a severe limitation when considering use of an orifice meter.


30

AAiT Instrumentation and measurement Chapter 5 117 AAiT Instrumentation and measurement Chapter 5 118


31

ComparisonVenturi Orifice

High Capital Cost Low Capital Cost

Low Operating Cost(good p recovery)

High Operating Cost(poor p recovery)

Not Flexible(β fixed)

More Flexibility(interchangeable)

Large Physical Size Compact


Rotameters• Rotameters fall into the category of flow measurement

devices called variable area meters.• These devices have nearly constant pressure and

depend on changing cross sectional area to indicateflow rate.

• Rotameters are extremely simple, robust devices thatcan measure flow rates of both liquids and gasses.

• Fluid flows up through the tapered tube and suspendsa ‘float’ in the column of fluid. The position of thefloat indicates the flow rate on a marked scale.


RotametersThree types of forces must be accounted for when analyzing rotameter performance:

• Flow• Gravity• Buoyancy

Flow

Buoyancy

Gravity

For our analysis neglect drag effect


RotameterMass BalanceAssume Gradual Taper

SQVV

SVSV

21

21

Flow Between Float and Tube

313 S

SVSS

QVf

S3 is annular flow area at plane 3124AAiT Instrumentation and measurement Chapter 5

32

RotameterMomentum BalanceNote:• p3 = p2• Must account for force due to float

fff gVVzSgSppVVQ 2113

bf

SgV

SS

SQzgp

3

2

1


RotameterMechanical Energy Balance

fhpzgVVW

21

232

1ˆ

0

2

23VKh Rf Assume: (Base velocity head on smallest flow

area)

2

3

21

2

3

21

212

1SSVK

SSVVzgp

R


Rotameter

2

3

2

3

2

11211

SSK

SQ

SgV

SS

SQ

Rbf

Combining Momentum and Mechanical Energy Balance

After Some Manipulation

f

f

f

fR

f

SgV

SSKSS

SQ2

1 23


Rotameter

f

f

fR S

gVCSQ

23

Assuming Sf ≈ S a discharge coefficient can be defined 211 RR KC

CR must be determined experimentally. As Q increases the float rides higher, the assumption that Sf = S is poorer, and the previous expression is more nearly correct.


33

Turbine Meter

Measure by determining RPM of turbine (3) via sensor (6). Turbine meters accurate but fragile.

129AAiT Instrumentation and measurement Chapter 5 AAiT Instrumentation and measurement Chapter 5 130

Level Measurement

Dipsticks Dipsticks offer a simple means of measuring the level of liquids

approximately. The ordinary dipstick is the cheapest deviceavailable.

This consists of a metal bar on which a scale is etched


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Sight-glass level indicator


Pressure-Measuring Devices (Hydrostatic Systems) Pressure-measuring devices measure liquid level to a better

accuracy and use the principle that the hydrostatic pressure dueto a liquid is directly proportional to its depth and hence to thelevel of its surface.


Where liquid-containing vessels are totally sealed, the liquidlevel can be calculated by measuring the differential pressurebetween the top and bottom of the tank


Capacitive Devices Capacitive devices are widely used for measuring the level of

both liquids and solids in powdered or granular form.


35

Ultrasonic Level Gauge Ultrasonic level measurement is one of a number of noncontact

techniques available. It is primarily used to measure the level ofmaterials that are either in a highly viscous liquid form or in asolid (powder or granular) form.

The principle of the ultrasonic level gauge is that energy from anultrasonic source above the material is reflected back from thematerial surface into an ultrasonic energy detector.

Measurement of the time of flight allows the level of the materialsurface to be inferred.

AAiT Instrumentation and measurement Chapter 5 137 AAiT Instrumentation and measurement Chapter 5 138

Nucleonic (or Radiometric) Sensors Nucleonic, sometimes called radiometric, sensors are relatively

expensive. They use a radiation source and detector systemlocated outside a tank in the manner shown in Figure


Temperature Measurement

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Temperature Measurement Temperature measurement is a

crucial part of many industrialprocesses.

Examples of industries where it isimportant are mineral processing,plastics, petrochemical, food etc.

There are a large number of differentmethods to measure temperature .

These use different physicalproperties. We will discuss some ofthese and look at some commontemperature measurement sensors


What is Temperature? Temperature is a measure of the

average kinetic energy of particles ina medium

The international unit for temperatureis Kelvin (K) or degrees Celsius (ºC)where

K = °C + 273.15

The measurement of low to medium temperatures (-273 ºC - ~500 ºC) is defined as thermometry while the measurement of higher temperatures it is known as pyrometry.


Thermal Measurement Method: Linear Expansion of a Solid

Rod thermometers and bimetallicthermometers are based on thisprinciple.

These indicate temperature dueto the different thermalexpansion of two differentmetals.

A solid bar will change in lengthwhen it experiences a change intemperature

Where is the linear temperaturecoefficient, L is the length of the barand dT is the change in temperature

If the original length of the bar is L0at T0 , the new length L1 at T1 can becalculated as follows:

LdTdL

TL TTLLL

10

01001


Thermal Measurement Method: Thermal Expansion of Liquids Used in liquid glass thermometers for a

direct indication of temperature The principle is similar to that in solids

except that we consider a volumetrictemperature coefficient

It is considered constant over a limitedrange

TVV 101


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Thermal Measurement Method: Vapour Pressure of Liquids

Vapor pressure is dependant on temperature

The equation for an ideal gas is

Where p is pressure, v is aspecific volume, T is thetemperature and R is the molargas constant

Therefore temperature can bemeasured in two ways:

Measuring the volume change at a constant pressure

Measuring the pressure difference at a constant volume

RTpv


Temperature Measurement with Electrical Sensors Convert temperature to an electrical

signal. They often require some form of

power source. Great advantage is that the signals

from these sensors aretransmittable over long distanceswhich makes remote measurementfeasible.


Conductivity in Metals Good conductivity in metals is due to the freely mobile

electrons in the atomic lattice. The number of free electrons and their kinetic energy are

functions of temperature. As the temperature increases, theamplitude and frequency of vibration increases.

The free electrons’ movement is now hindered through themedium and therefore the resistance of the materialincreases.

If an increase in temperature causes an increase in resistanceof a material it is said to have a Positive TemperatureCoefficient (PTC).


The relationship between the temperature of metals and its electricalresistance is not liner but can be described by the followingequation:

Where R1 is the resistance at temperature T1, R0 is the resistance ofthe material at a reference temperature T0 .

a, b and c are the temperature coefficients of resistivity and aredependant on the metal. They are only constant over a specificrange

However, for certain materials it is possible to neglect the higherterms for specific temperature ranges without introducing too largean error

This reduces the equation to a linear relationship

....)(

32

01

01

1 TcTbTaRtR

TTT


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Any metal used for temperature measurement should meetthese requirements: Good long term stability in terms of resistance High temperature coefficient of resistivity Resistant to corrosion and chemical impurities Not effected by other physical quantities such as pressure Good reproducibility of change in resistance as a function of

temperature Platinum (Pt) and Nickel (Ni) satisfy most of the above

requirements


Electrical Temperature Measurement: Resistance Temperature Detectors (RTDs)

Use the fact that certain materialsresistance changes in a predictableway with a change in temperature.

They are mainly made from metallicconductors and mostly of platinum.

They are becoming the temperaturesensor of choice in industry fortemperature measurements below 600ºC.


The most common types of RTD are:

Wire-wound in a ceramic insulator Wires which are encapsulated in glass

Resistance often measured using a Wheatstone Bridgearrangement

)1()( 0 TRTR

151AAiT

Advantages: High accuracy and can therefore be used in precision

applications Has low drift with time Wide operating temperature range

Disadvantages: Are not often used above 660ºC as it is difficult to keep the

platinum pure


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Thomas Johan Seebeck A German-Estonian Physicist who

discovered that a voltage wasproduced across a metal bar when atemperature difference existed in thebar in 1821.

From this he formulated the SeebeckPrinciple which is used in sometemperature measurement devices.


The Seebeck Effect If two different metals are joined together to

form a continuous loop and their junctionsare at different temperatures, an e.m.f. willbe generated which cause a current to flow.

If a millivoltmeter is inserted into the loop,its output reading will give us an indicationof the temperature difference between thetwo junctions of the loop.

This concept forms the basis of athermocouple.


Electrical Temperature Measurement: Thermocouples Are the most commonly used

electronic temperature measurementdevices.

Consists of two dissimilar metals whichare joined together at both ends.

One of the conductors is broken in themiddle. A potential difference isgenerated across the break if thejunctions are held at differenttemperatures

Therefore if one end of a thermocoupleis held at a known reference, thetemperature of the other end can becalculated.


Thermocouples Types

ANSICODE ALLOY COMBINATION

MAXMIUMTEMPERATURE RANGE

mVOUTPUT

BEJKNRST

Platinum/RhodiumChromel/ConstantanIron/ConstantanChromel/AlumelNicrosil/NisilPlatinum/Rhodium PlatinumPlatinum/Rhodium PlatinumCopper/Constantan

0°C to +1700°C–200°C to +900°C

0°C to +750°C–200°C to +1250°C–270°C to +1300°C

0°C to +1450°C0°C to +1450°C

–200°C to +350°C

0 to +12.426–8.824 to +68.783

0 to +42.283–5.973 to +50.633–4.345 to +47.502

0 to +16.7410 to +14.973

–5.602 to +17.816

• Various metal combinations can be used for different temperature andvoltage ranges, the following are examples of common combinations:

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40

Thermocouples Advantages:

Wide operating temperature range can be used at high temperatures

Fairly cheap Interchangeable Have standard connectors

Disadvantages: Lack of precision


Electrical Temperature Measurement: Semiconductor Sensors

Silicon Measuring Resistors (PTC) Small non-linearity -70ºC - 160ºC gives a resistance change of 14W to 4kW

Semiconductor Diodes If supplied with a constant current, the conducting voltage is

a function of absolute temperature Almost linear between -50ºC - 150ºC


Thermistors A semiconductor used as a temperature sensor. Mixture of metal oxides pressed into a bead, wafer or other

shape. The resistance decreases as temperature increases, negative

temperature coefficient (NTC) thermistor.


Thermistors Most are seen in medical

equipment markets. Thermistors are also used are

for engine coolant, oil, andair temperature measurementin the transportation industry.


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Thermistors

High sensitivity to smalltemperature changes

Temperaturemeasurements becomemore stable with use

Copper or nickelextension wires can beused

Limited temperature range

Fragile Some initial accuracy

“drift” Decalibration if used

beyond the sensor’s temperature ratings

Lack of standards for replacement

Advantages Disadvantages


Electrical Temperature Measurement: Radiation Thermometers Also known as pyrometers. They are non-contact sensors. Used in the measurement range

-100 ºC - 3500 ºC They are used to measure the

temperature of a surface if it isvisible.

Often used for objects withrapid temperature changes,moving objects and smallobjects.


Infrared Thermometry Infrared thermometers measure the amount of

radiation emitted by an object. Peak magnitude is often in the infrared region. Surface emissivity must be known. This can add a lot

of error. Reflection from other objects can introduce error as

well. Surface whose temp you’re measuring must fill the

field of view of your camera.


Benefits of Infrared Thermometry Can be used for Moving objects Non-contact applications where

sensors would affect results orbe difficult to insert orconditions are hazardous

Large distances Very high temperatures


transducers and dimension measurement measurement systems · measurement systems by: yidnekachew...

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