Instrumentation and Measurement

Lecture 02

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

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

LIMITING ERROR

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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

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Hysteresis effect

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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

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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

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Passive Instrument requires no external source of power

Pressure Gauge

Active and Passive Instruments

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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

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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

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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

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

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

Ampere’s Law

DC

Electromegnatism…

Recall…..

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

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

Greater the intensity of current, stronger the magnetic field.

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

Example: Galvanometer.

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

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

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

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.

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

PMMC

U-shaped permanent magnet.

Movable coil.

A pointer.

A scale.

Coil is suspended between the poles of the magnet.

Construction and Working

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

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..

Restoring torque is given by

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

At steady state or equilibrium position,

Which in turn,

◦ And

◦ Where G is the galvanometer Constant and

A =

I = ….?

B = …?

T = …?

= radians N = ….

Units

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

Assignment 1

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

Shunt is a name given to low resistance/resistor.

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

◦ Small…why?

Ammeter

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

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

Then voltage drop at galvanometer is

Since shunt resistance is parallel to galvanometer so

Current through shunt is

By 1 and 2

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

than then…

Putting the above value in 3

Note:In several books = and =

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

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

42

R1 R2 R3 R4

+

_

+

_Rm

Multi range Ammeter

S

Consider

Recall equation (5)…

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

Calculations

  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…

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

Current should be negligible through the voltmeters.

So the resistance of the voltmeter should be high.

The series resistor is called a "multiplier" resistor

To determine the value of ‘multiplier’ resistor

Consider the above figure…

Calculations

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….???

R = 10 Ω = = 50 mA

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

Numerical

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

Also… Sensitivity or

Sensitivity

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

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…