MEASUREMENT AND MEASUREMENT AND INSTRUMENTATIONINSTRUMENTATION

MeasurementsMeasurements

Measurement of a given quantity is essentially an act or result of comparison between the quantity (whose magnitude is unknown) and predetermined or predefined standards.

Two quantities are compared the result is expressed in numerical values.

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Basic requirements for a Basic requirements for a meaningful measurementmeaningful measurement

The standard used for comparison purposes must be accurately defined and should be commonly accepted.

The apparatus used and the method adopted must be provable (verifiable).

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Significance Of Significance Of Measurement Measurement

Importance of Measurement is simply and eloquently expressed in the following statement of famous physicist Lord Kelvin: ”I often say that when you can measure what you are speaking about and can express it in numbers, you know something about it; when you cannot express in it numbers your knowledge is of meager and unsatisfactory kind”

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Two major functions of all Two major functions of all branch of engineering branch of engineering Design of equipment and

processesProper Operation and

maintenance of equipment and processes.

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Methods of Measurement Methods of Measurement

Direct Methods

Indirect Methods

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DIRECT METHODS: In these methods, the unknown quantity (called the measurand ) is directly compared against a standard.

INDIRECT METHOD: Measurements by direct methods are not always possible, feasible and practicable. In engineering applications measurement systems are used which require need of indirect method for measurement purposes.

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Instruments and Measurement Instruments and Measurement Systems.Systems.

Measurement involve the use of instruments as a physical means of determining quantities or variables.

Because of modular nature of the elements within it, it is common to refer the measuring instrument as a MEASUREMENT SYSTEM.

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Evolution of Instruments.Evolution of Instruments.a)Mechanical b)Electrical c) Electronic Instruments. • MECHANICAL: These instruments are very reliable for static and stable conditions. But their disadvantage is that they are unable to respond rapidly to measurements of dynamic and transient conditions.

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ContdContdELECTRICAL: It is faster than mechanical,

indicating the output are rapid than mechanical methods. But it depends on the mechanical movement of the meters. The response is 0.5 to 24 seconds.

ELECTRONIC: It is more reliable than other system. It uses semiconductor devices and weak signal can also be detected.

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Elements of Generalized Elements of Generalized Measurement SystemMeasurement System

• Primary sensing element.• Variable conversion element.• Data presentation element.• PRIMARY SENSING ELEMENT: The quantity under measurement makes its first contact with the primary sensing element of a measurement system.• VARIABLE CONVERSION ELEMENT: It converts the output of the primary sensing element into suitable form to preserve the information content of the original signal.

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Contd..Contd..DATA PRESENTATION ELEMENT: The

information about the quantity under measurement has to be conveyed to the personnel handling the instrument or the system for monitoring, control or analysis purpose.

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Functional Elements of an Functional Elements of an Instrumentation System Instrumentation System

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PRIMARY SENSING ELEMENT

VARIABLE CONVER-SION ELEMENT

VARIABLE MANIPULATI-ON ELEMENT

DATA TRANSMISSIO-N ELEMENT

DATA CONDITIONING ELEMENT

INTERMEDIATE STAGE DETECTOR TRANSDUCER STAGE

TERMINATING STAGE

QUANTITY TO BE MEASURED

DATA PRESENTATION ELEMENT

Activity – 1 (Answers)Activity – 1 (Answers)Physical Quantities

Standard Unit Symbol

Length Meter mMass Kilogram kgTime Second sTemperature Kelvin ACurrent Ampere KLuminous Intensity

Candela cd

Matter Mole molPlane Angle Radian radSolid Angle Steradian srMagnetic flux Weber WbMagnetic flux density

Tesla T

Static Characteristics Of Static Characteristics Of Measurement systemMeasurement system

Application involved measurement of quantity that are either constant or varies slowly with time is known as static.

Accuracy Precision Tolerance Range or Span Linearity Sensitivity of measurement Threshold Resolution Drift Hysteresis effect Dead space

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ACCURACY: The accuracy of an instrument is a measure of how close the output reading of the instrument is to the correct value.

PRECISION: It describes an instruments degree of freedom from random errors.

TOLERANCE: Defines the maximum error that is to be expected in some value.

Range or Span: The range or span of an instrument defines the minimum and maximum values of a quantity that the instrument is designed to measure.

Linearity: It is normally desirable that the output reading of an instrument is linearly proportional to the quantity being measured.

Sensitivity of Measurement: It is a measure of the change in instrument output that occurs when the quantity being measured changes by a given amount.

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Threshold: If the input to an instrument is gradually increased from zero the input will have to reach a certain minimum level before the change in the instrument output reading is of large enough magnitude to be detectable. This minimum level of input is known as thereshod.

Resolution: When an instrument is showing a particular output reading there is a lower limit on the magnitude of the change in the input measured quantity that produces an observable change in instruments input.

Drift: Standard ambience conditions are usually defined in the instrument specification. As variation occurs in ambience conditions changes certain static instrument characteristics changes and drift is a measure of the magnitude of this change.

Dead space: It is defined as the range of different input values over which there is no change in output value.

Hysteresis:If the input measured quantity to the instrument is steadily increased from a negative value, the output reading varies as in curve (a). It the input steadily decreased, the output varies as shown in curve (b). This non coincidence between these curves is known as hysteresis.

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Characteristics of Measuring Characteristics of Measuring InstrumentsInstrumentsDynamic Characteristics

◦These characteristics are related with the rapidly changing quantities therefore in order to understand these types of characteristics we are required to study the dynamic relations between the input and the output.

The dynamic characteristics The dynamic characteristics of instrument is defined and of instrument is defined and evaluated by evaluated by Speed of responseMeasurement lagFidelityDynamic error

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Dynamic characteristicsDynamic characteristicsSpeed of Response:-

It is the rapidity with which the system responds to the changes in the quantity to be measured.It gives the information about how fast the system reacts to changes in the input

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Dynamic characteristicsDynamic characteristicsFidelity:-It indicates how much faithfully the

system reproduces the changes in the input.

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Dynamic characteristicsDynamic characteristicsMeasurement of lagIt is the retardation or delay in the

reponse of an instrument to changes in the measured quantity.

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Dynamic characteristicsDynamic characteristicsDynamic error:-It is the difference between the

true value of the variable to be measured changing with time and the value indicated by the measurement system assuming zero static error

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Activity 2 (Who Am I ?) Activity 2 (Who Am I ?) NAME PURPOSE

Measures current

Measures the capacitance of a component

Measures the amount of energy dissipated

Measures the frequency of the current

Measures the inductance, capacitance and resistance of a component

Measures power at microwave frequencies

General purpose instrument measures voltage, current and resistance (and sometimes other quantities as well)

Measures the resistance of a component

Displays waveform of a signal

Generates signals for testing purposes

Measures the power

Measures the potential difference between two points in a circuit.

Activity 2 (Answers)Activity 2 (Answers)NAME PURPOSE

Ammeter Measures current

Capacitance meter Measures the capacitance of a component

Electricity meter Measures the amount of energy dissipated

Frequency counter Measures the frequency of the current

LCR meter Measures the inductance, capacitance and resistance of a component

Microwave power meter Measures power at microwave frequencies

Multimeter General purpose instrument measures voltage, current and resistance (and sometimes other quantities as well)

Ohmmeter Measures the resistance of a component

Oscilloscope Displays waveform of a signal

Signal generator Generates signals for testing purposes

Wattmeter Measures the power

Voltmeter Measures the potential difference between two points in a circuit.

Parameters involved in Parameters involved in Designing Measuring Designing Measuring Instruments Instruments Indicating Instruments

◦Has dial and pointer to measure the quantity

Eg: Analog VoltmeterRecording Instruments

◦Has nozzle nob, pen or plotter Eg : ECG plotter

Integrating Instruments◦Has coils and wires

Eg: Energy Meters

Out Class ActivityOut Class Activity• Students are requested to compare

ancient (vs) modern instruments to measure few parameters.

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