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Instrumentation and Measurement Lecture 02
54

Lecture 02. The accuracy of measuring instrument is described with certain percentage (%). E.g manufacturer may specify the instrument to be accurate.

Dec 22, 2015

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Page 1: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Instrumentation and Measurement

Lecture 02

Page 2: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

The accuracy of measuring instrument is described with certain percentage (%) .

E.g manufacturer may specify the instrument to be accurate at 2 % .

LIMITING ERROR

Page 3: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

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If input measured quantity to the instrument is steadily increased the output reading varies in the manner shown in curve A

If the input is then steadily decreased the output varies in the manner shown in curve B

Non coincidence between the loading and unloading curves is known as hysteresis

Hysteresis is exhibited by instruments that contain springs and electrical windings formed around an iron core

Hysteresis effect

Page 4: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

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Hysteresis effect

Page 5: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

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Instrument System are divided into different types based on accuracy, performance and application

Active and Passive Instruments Null-Type and Deflection-Type Instruments Analogue and Digital Instruments

Instrument Types and Performance Characteristics

Page 6: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

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Active Instrument requires external source of power.

Either a battery or an external voltage source.

Examples of active components include transistors, LED lights, and opamps or other IC chips

Active and Passive Instruments

Page 7: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

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Passive Instrument requires no external source of power

Pressure Gauge

Active and Passive Instruments

Page 8: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

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Pressure Gauge is a Deflection Type Instrument

Value of quantity measured displayed in terms of movement of pointer

Calibration of spring is difficult therefore Deflection type instrument is less accurate

More convenient to use

Null & Deflection Type Instruments

Page 9: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

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Dead Weight Pressure Gauge is a Null Type Instrument, works on the principle that P= F/A

Weights are put on top of piston until the downward force balances the fluid pressure

Weights are added until the piston reaches a datum level known as the null point

Accuracy depends upon calibration of weights

Calibration of weights is easier therefore null type instruments are more accurate

Inconvenient to use, for calibration purposes only

Null & Deflection Type Instruments

Page 10: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

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Analogue Instruments give a continuously varying output as the quantity being measured changes◦ Deflection Type pressure Gauge

Digital Instruments give output that varies in discrete quantities

Analogue and Digital Instruments

Page 11: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Standard quantity of any physical quantity Global standard, the International System of

Units(SI). History of uniform systems/standards dates

back to Bronze age. Fundamental units Derived units Go through table 2-3 of your book … Conversion of units

Units

Page 12: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Standards of measurement◦ International◦ Primary◦ Secondary◦ Working

Electrical standards◦ Ampere◦ Resistance◦ Voltage◦ Capacitance◦ Inductance

Do not exceed four pages… Be concise and brief..

Assignment 1

Page 13: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Ampere’s Law

DC

Electromegnatism…

Recall…..

Page 14: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Devices use to measure, indicate or both are called meters…

Why Meters….??◦ Whilst Troubleshooting, testing, or repairing

equipment you need various meters …◦ To check for proper circuit voltages, currents,

resistances, and to determine if the wiring is defective. You may be able to connect these test instruments to a circuit and take readings.

Meters

Page 15: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

A stationary, permanent-magnet, moving-coil meter is the basic meter movement used in most measuring instruments used for servicing electrical equipment.

When current flows through the coil, a resulting magnetic field reacts with the magnetic field of the permanent magnet and causes the movable coil to rotate. 

BASIC METER MOVEMENT

Page 16: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Greater the intensity of current, stronger the magnetic field.

Stronger the magnetic field , greater the rotation of the coil.

Example: Galvanometer.

Page 17: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

A galvanometer is an instrument for detecting and measuring electric current.

The term "galvanometer", in common use by 1836, was derived from the surname of Italian electricity researcher Luigi Galvani

Galvanometers were the first instruments used to detect and measure electric currents.

Galvanometer

Page 18: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

All galvanometers are based upon the discovery by Hans C. Oersted that a magnetic needle is deflected by the presence of an electric current in a nearby conductor.

The extent to which the needle turns is dependent upon the strength of the current.

The early moving-magnet form of galvanometer had the disadvantage that it was affected by any external magnetic field.

Disadvantage: Mechanical fault in spring

Page 19: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.
Page 20: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

In the first galvanometers, a freely turning magnetic needle was hung in a coil of wire; in later versions the magnet was fixed and the coil made movable.

Modern galvanometers are of this movable-coil type and are called d'Arsonval galvanometers (after Arsène d'Arsonval, a French physicist)

D’Arsonval Galvonometers

Page 21: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Further improvement included replacement of fine wire suspension with a pivot, and provided restoring torque and electrical connections through spiral spring.

 If a pointer is attached to the moving coil so that it passes over a suitably calibrated scale, the galvanometer can be used to measure quantitatively the current passing through it. Such calibrated galvanometers are used in many electrical measuring devices.

Page 22: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.
Page 23: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Permanent magnet moving coil and errors due to PMMC…????◦ Its part of your assignment 1… brief and simple…

PMMC

Page 24: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

U-shaped permanent magnet.

Movable coil.

A pointer.

A scale.

Coil is suspended between the poles of the magnet.

Construction and Working

Page 25: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Coil twist in response to the interaction of the applied current through the coil and the magnetic field of the permanent magnet. 

When the driving force of the coil current is removed, the restoring force provided by the spring returns the coil to its zero position

Page 26: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.
Page 27: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.
Page 28: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Let B be the strength of the magnetic field, n be the number of turns in the coil, A be the area of the coil and I be the current flowing through the coil.

      The Torque will be                            

Some Maths..

Page 29: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Restoring torque is given by

Where C is the restoring constant and is the final deflection.

At steady state or equilibrium position,

Page 30: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Which in turn,

◦ And

◦ Where G is the galvanometer Constant and

Page 31: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

A =

I = ….?

B = …?

T = …?

= radians N = ….

Units

Page 32: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

N =100 A = 0.0012 I = 0.02 A B = 0.05 T T = ???

Assignment 1

Page 33: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Used to measure electric current through the circuit.

Modified form of galvanometer. How?

◦ Galvanometer is used for small currents..◦ What if large value of current??? We add an additional resistor…known as Shunt

ResistorWhat does the word shunt means..??

DC Ammeter

Page 34: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Shunt is a name given to low resistance/resistor.

Placed parallel to Galvanometer… why??? Alternate path.. Shunt will be

◦ Small…why?

Ammeter

Page 35: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Ammeter sensitivity is the amount of current necessary to cause full scale deflection (maximum reading) of the ammeter.

The smaller the amount of current, the more "sensitive" the ammeter…..why???

Ampere or milliampere…??

Ammeter Sensitivity

Page 36: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

How to calculate shunt….? Let be the maximum current that can be

passes through the galvanometer. galvanometer resistance = Shunt Resistance = Total current = Current through Shunt resistance = Current through galvanometer =

Calculations

Page 37: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Then voltage drop at galvanometer is

Since shunt resistance is parallel to galvanometer so

Current through shunt is

Page 38: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

By 1 and 2

Current is larger than Let n be the factor by which is greater

than then…

Page 39: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Putting the above value in 3

Note:In several books = and =

Page 40: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Shunt resistance = ?◦ When = 1 mA, = 100 Ω, = 10mA

◦ Ideally the shunt resistance should have value???◦ What will happen when you connect the ammeter

parallel to the circuit???

Question

Page 41: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Several values of shunt resistors and a rotary switch are used to select the desired range of current to measure.

An Aryton shunt uses a combination of resistors.

Multirange Ammeter

Page 42: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

42

R1 R2 R3 R4

+

_

+

_Rm

Multi range Ammeter

S

Page 43: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Consider

Recall equation (5)…

Using the above equation, we can derive for the unknown quantities and in general we have

Calculations

Page 44: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

  Ammeter inserted in series. Always set the range to the highest scale

and then reduce as needed. Observe polarity. Better quality analog meters include a

mirror along the scale. This eliminates parallax error.

Reminders…

Page 45: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Besides current, Voltage is also essential quantity to be measured …

Galvanometer is further altered … Voltmeter used to measure the potential

difference between any two points in a circuit…

Connected in parallel to the circuit.. No alteration/drawing the current flowing

through the circuit.

DC Voltmeter

Page 46: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Current should be negligible through the voltmeters.

So the resistance of the voltmeter should be high.

The series resistor is called a "multiplier" resistor

Page 47: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.
Page 48: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

To determine the value of ‘multiplier’ resistor

Consider the above figure…

Calculations

Page 49: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

If is the voltage across the voltmeter, is the current through voltmeter, is the resistance through Galvanometer and is the multiplier resistance then

Ideally, Multiplier resistance should be….???

Page 50: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

R = 10 Ω = = 50 mA

Show how it can be adopted to measure voltage upto 750 V ?

Numerical

Page 51: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

We can also find the value of multiplier resistance by determining the sensitivity of the meter.

Sensitivity is given by S and is determined by

Voltmeter Sensitivity

Page 52: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

Also… Sensitivity or

Sensitivity

Page 53: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

A DC voltmeter can be converted into a multi range voltmeter by connecting a number of resistors (multipliers) in series with the meter movement.

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MULTIRANGE VOLTMETER

Multirange voltmeter

R1 R2 R3 R4

+

_

V1V2

V3

V4

Rm

Im

Page 54: Lecture 02.  The accuracy of measuring instrument is described with certain percentage (%).  E.g manufacturer may specify the instrument to be accurate.

For extension of simple voltmeter… we utilize the product of voltmeter sensitivity and the given range followed by subtraction of internal/meter resistance from the product …

In other words…

Calculation…