INTRODUCTION MEASUREMENT STANDARDS AND UNITS

Chapter Objectives Contents

To define some measurement terms To describe basic measurement units and relate to derivative units To characterize instruments To differentiate between instrument and indicators

Definition and measurement

Errors in measurement process

Classification of instruments

Instrument Elements

Application Area

DEFINITION & MEASUREMENT

Definition

Instrumentation is a technology of measurement which serves not only science but all branches of engineering, medicine, and almost every human endeavor.Electronic Instrumentation – the application of measurement technology in Electronic-related field.Instrument A device or mechanism used to determine the present value of the quantity under measurement.

Measurement The process of determining the amount, degree, or capacity by comparison (direct or indirect) with the accepted standards of the system units being used.Accuracy The degree of exactness (closeness) of a measurement compared to the expected (desired) value.Resolution The smallest change in a measured variable to which an instrument will respond.

Definition

Precision A measure of the consistency or repeatability of measurements, i.e. successive reading do not differ. (Precision is the consistency of the instrument output for a given value of input).Expected value The design value, i.e. the most probable value that calculations indicate one should expect to measure.

Error The deviation of the true value from the desired value.Sensitivity The ratio of the change in output (response) of the instrument to a change of input or measured variable.

Measurement MeasurandThe process of comparing an unknown quantity with an accepted standard quantity.The process of determining the amount, degree, or capacity by comparison (direct or indirect) with the accepted standards of the system units being used.

Displacement Strain Vibration Pressure Flow Temperature Force Torque

Measurand

Displacement: Vector representing a change in position of a body or a point with respect to a reference.Strain: Relative deformation of elastic, plastic, and fluid materials under applied forces.Vibration: Oscillatory motion which can be described in term of amplitude (size), frequency (rate of oscillation) and phase (timing of the oscillation relative to fixed time).Pressure: Ratio of force commonly acting on a surface to the area of the surface.

Flow: Stream of molten or liquidified material that can be measured in term of speed and quantity

Temperature: Measure of relative warmth or coolness of an object compared to absolute value.

Force: Defined as a quantity that changes the motion, size, or shape of a body.

Torque: Defined as the tendency of a force to rotate the body to which it is applied.

Unit Base UnitInternational System of Units (abbreviated SI from the French le Système international d'unités)It is the world's most widely used system of measurement, both in everyday commerce and in science.The SI was developed in 1960 from the old metre-kilogram-second system.

Length – meter (m) Mass – kilogram (kg) Time – second (s) Electric current –

ampere (A) Temperature – kelvin

(K) Luminous intensity –

candela (cd) Amount of substance –

mole (mol)

Derivative Unit

•Electric charge – coulomb (C)•Electric potential difference – volt (V)•Electric resistance – ohm (Ω)•Electric capacitance – farad (F)•Electric inductance – henry (H)

Energy – joule (J) Force – newton (N) Magnetic flux –

weber (Wb) Power – watt (W)

ERROR IN MEASUREMENT

Direct Analysis FormulaError is the degree to which a measurement nears the expected value. It can be expressed as:

Absolute error Percentage of error

Accuracy can be calculated based on error.

nn XYe e = absolute errorYn = expected valueXn = measured value

Formula

%E = percentage of errore = absolute errorYn = expected valueXn = measured value

a = percentage of accuracyA = relative accuracye = absolute error

A = relative accuracy e = absolute error Yn = expected value Xn = measured value

Yn = expected valueXn = measured value

100100% xY

XYx

Y

eE

n

nn

n

n

nn

n Y

XY

Y

eA

11

10011001100 xYn

XnYnx

Y

eAxa

n

Formula (Cont..)P= Precision

Where = value of the nth measurement. = average set of measurement

n

nn

x

xxP 1

nx

nx

Example

1. The expected value of the voltage across a resistor is 80 V. However, the measurement gives a value of 79 V. Calculate :

(i) absolute error (ii) percentage of error (iii) relative accuracy (iv) percentage of

accuracy.

nn XYe

100100% xY

XYx

Y

eE

n

nn

n

n

nn

n Y

XY

Y

eA

11

10011001100 xYn

XnYnx

Y

eAxa

n

Statistical Analysis

Can be used to determine the uncertainty of the test results.The analysis require a large number of measurement (data) to be taken.

Arithmetic Mean xn is nth data taken and n is

the total of data or measurement.

Deviation from mean dn is the deviation of the

nth data with the arithmetic mean.

Average deviations Indicate the precision of

the instrument used, lower value of average deviation specify a highly precise instruments.

Standard deviation Small value of standard

deviation means that the measurement is improved.

n

xx

n

nn

1

Example 2

For the following given data, calculate (i) Arithmetic mean;(ii) Deviation of each

value; (iii)Algebraic sum of

the deviations; (iv) Average deviation; (v) Standard deviation.Given x1 = 49.7; x2 = 50.1; x3 = 50.2; x4 = 49.6; x5 = 49.7

xxd nn

ntotal dddd ....21

||....|2||1| nav dddD

n

xx

n

nn

1

Source of Error

Errors in measurement can be broadly defined in three categories:

Gross errors Systematic errors Random errors

Gross Errors Because of the human

mistakes. Improper or incorrect

installation or use of measurement instrument.

Failure to eliminate parallax during reading or recording the measurement.

Cannot be remedied mathematically.

Systematic Errors Random ErrorsBecause of the instrument.Three types of systematic errors:

Instrumental errorsEnvironmental

errorsObservational

errorsProduce constant uniform deviation.

Occur when different results in magnitude or sign obtained on repeated measurement of one or the same quantity.

The effect can be minimized by taking the measurement many times.

This error can be handled mathematically.

CLASSIFICATION OF INSTRUMENTS

Absolute Secondary•Provide magnitude of the quantity under measurement in terms of physical constant of the instrument.

•Provide magnitude of the quantity under measurement only from the observation of the output from instrument.•Most instrument used in practice are secondary.

Operation type

DeflectionOnly one source of

input required.Output reading is

based on the deflection from the initial condition of the instrument.

The measured value of the quantity depends on the calibration of the instrument.

NullRequire two input –

measurand and balance input.

Must have feedback operation that compare the measurand with standard value.

More accurate and sensitive compared to deflection type instrument.

Signal Type

AnalogProduce the signal that vary in continuous way.Infinite range of value in any given range.

DigitalProduce the signal that vary in discrete steps.Finite different values in a given range.

INSTRUMENT ELEMENTS

Important element is sensor which can convert the physical variable into signal variable.

Signal variable can be displayed, recorded or integrated into secondary instrument system.

Signal variable may also be used as an input signal of a control system.

Model

Block Diagram

Block Diagram (Simplified)

Subsystems

Transducers Power Supply Signal Conditioning Circuits Filter / Amplifier Data Processors Process Controller Command Generator Recorder

Elements of Electronic Instrumentation Transducers

Device that converts a change in physical quantity into a change of electrical signal magnitude.

Power Supply Provide energy to drive the transducers.

Signal Conditioning Circuits Electronic circuits that manipulate,

convert the output from transducers into more usable electrical signal.

Elements of Electronic Instrumentation (cont.) Amplifiers

Amplify low voltage signal from transducers or signal conditional circuit.

Recorders Used to display the measurement for

easy reading and interpretation. Data Processors

Can be a microprocessor or microcontroller.

Elements of Electronic Instrumentation (cont.) Process Controllers

Used to monitor and adjust any quantity of the specified level or value.

Command Generator Provide control voltage that represents

the difference of the parameter in a given process.

APPLICATION AREA

APPLICATION AREA

Engineering AnalysisProcess Control

Monitoring Automation

APPLICATION AREA

Engineering Analysis To validate new design of structure,

component or system by theoretical and experimental approach

Process ControlMonitoring process: provide real-time

data that allow operator to respond.Automatic process: provide real-time

feedback data to the control system.