Week 1

INSTRUMENTATION & MEASUREMENTS(EE-302)

WEEK 1

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The Course…..

Instrumentation and measurements constitute the fundamental skills of an engineer.

In the field you can’t work without the proper knowledge of instrumentation and consequently measurements.

Instrumentation has a extremely wide scope involving analog as well as digital sides.

In this course we are going to learn some very exciting, practical and application oriented measurements techniques as well as get the knowledge of a number of basic as well as advanced instruments and processes.

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Lets formally define ‘measurements’

The measurement of a given parameter or quantity is the act or result of a quantitative comparison between a pre-defined standard and an unknown quantity to be measured!

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Meaningful result would require…

The comparison standard is accurately defined.

The procedure and the instrument used for obtaining the comparison must be provable!

A measuring instrument is defined as a device for determining the value or magnitude of a quantity or variable.

An electronic measuring instrument is the one in which electronic/electrical principles for its measurement function are used.

The physical, chemical, electrical quantity, property, process, variable or a condition to be measured is referred as measurand.

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Advantages of Electronic measurement

Involvement of Transducers that could measure almost any physical quantity.

Ease of digital conversion.

Amplification, Filtering, Multiplexing and Sampling possible.

Long distance transmission of signals easier.

Multiple measurements could be conducted simultaneously.

Extremely short time duration as well as weak signals could be measured.

Higher sensitivity, low power consumption, high reliability.

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Functional Elements of an instrument

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Primary Sensing Element is a transducer.

Transducer is a device that could convert one form of energy into the other, for our case of electronic transducer, it can convert any physical quantity into an analog electrical signal.

A variable conversion element makes the output of the transducer suitable for measurement and recording (think of a A/D converter)

A variable manipulation element is a signal conditioning element or a data conditioning element.

Data Transmission Elements are used when the system’s elements are physically separated.

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Data Presentation Element is used to present the data in the format so that it could be used either to control, observe or manipulate the system.

Data and its handling could be explained by a simple example of the ammeter block as follows:

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A simple analog Ammeter

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Performance Characteristics

Two basic categories:

Static Characteristics

Dynamic Characteristics

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Static & Dynamic Characteristics

The set of criteria defined for instruments which are used to measure the quantities which are slowly varying with time or mostly constant i.e do not vary with time is called static characteristics.

When the quantity under measurement changes rapidly with time, it is necessary to study the dynamic relations between the input and the output generally expressed with differential equations, thus the set of criteria defined based on such differential equations is called dynamic characteristics.

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Calibration

Calibration is the process of making an adjustment or marking a scale so that the readings of an instrument agree with the accepted and the certified standard.

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Accuracy

Precision

Resolution

Error

Sensitivity

Threshold

Reproducibility

Zero Drift

Stability

Linearity

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Accuracy is the degree of closeness with which the instrument reading approaches the true value of the quantity to be measured.

Precision is the measure of consistency or repeatability of measurements.

High degree of precision does not necessarily guarantees the accuracy. Calibration makes accurate measurements possible.

Precision is composed of Conformity and Number of Significant figures.

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Precision mathematically…

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Explaining Conformity and Number of Significant Figures

A resistor having a true value of 2358692Ω might be represented by the instrument as 2.4 MΩ. This has been estimated due to the non availability of the proper scale. This value is estimated by the reader from the available scale. The error created due to the limitation of the scale reading is the precision error.

An accurate instrument should be precise but a precise instrument may not be accurate.

Likewise a resistance 110Ω specified by an instrument may be closer to 109Ω or 111Ω, thus there are three significant figures. But when it is specified as 110.0Ω the number of significant figures changes from 3 to 4.

The greater the number of significant figures, the greater the precision of the measurement.

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Error & Sensitivity

The algebraic difference between the indicated value and the true value of the quantity to be measured is called an error.

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Absolute Error

Absolute error is a measure of how far 'off'a measurement is from a true value or anindication of the uncertainty in ameasurement

Absolute Error = Actual Value - Measured Value

Relative Error

Relative error expresses how largethe absolute error is compared withthe total size of the object

Relative Error = Absolute Error / Actual Value

Sensitivity

The sensitivity denotes the smallest change in themeasured variable to which the instrument responds.

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Sensitivity

If the calibration of the curve is not linear, the sensitivityVaries with the input.

Deflection factor is expressed as the reciprocal of sensitivity.

The sensitivity of the instrument should be as high as possible, and to achieve this the range of an instrument should not greatly exceed the value to be measured.

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Sensitivity

Some basic definitions

Resolution: It is the smallest increment of quantity being measured which can be detected with certainty, thus the resolution means the smallest measureable input change.

Threshold: The smallest measurable input.

Linearity: Indicates the straight line nature of the calibration curve.

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Zero Drift: The deviation in the instrument’s output with time, from its zero value, when the variable to be measured is constant.

Reproducibility: The degree of closeness with which a given value may be repeatedly measured.

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Stability: The ability of the instrument to retain its performance throughout its specified operating / storage life.

Tolerance: The maximum allowable error in the measurement . This is related to accuracy.

Range of Span: Minimum and Maximum values of a quantity for which an instrument is designed.

Bias: The constant error which exists over the entire range of measurement of an instrument.

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Hysteresis: Change in theresponse when the input ischanged from negative topositive or positive tonegative values is calledhysteresis. (The magneticdomains example of anelectromagnet) .

Dead Space: The range ofinput after which the outputresponds to input and getschanged.

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When the instrument is subjected to rapidly varying input, the output also varies instant to instant.

The behaviour of the system under such conditions is called the dynamic response of the system.

The dynamic response of the system is checked after applying certain basic known and pre-determined variations of the input to the sensing element and such a response is expressed mathematically by differential equations.

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Dynamic Inputs

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Dynamic Characteristics of instruments

Speed of response

Fidelity

Lag

Dynamic Error

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Speed of Response: Rapidity with which the system responds to the changes in the being measured quantity.

Fidelity: How faithfully the system reproduces the changes in the input.

Lag: Retardation or delay in the response of a system.

Dynamic Error: Assuming zero static error the difference in the true and measured values of a system’s variable.

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Dynamic Error Explained…

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Instrument Classification

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Active/Passive Instruments

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Null/Deflection Type Instruments

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Monitoring/Control Instruments

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Analog/Digital Instruments

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Absolute and Secondary Instruments

Absolute Instruments: That give the measured value in terms of the physical constants of the instrument.

Secondary Instruments: That give the measured value directly.

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Methods of Measurement

Direct Method: The quantity to be measured is used to produce certain effect that directly gives the indication on the meter. Further classified into Deflection Methods and Comparison methods.

Indirect Method: The quantity to be measured is not directly measured but other parameters related to the quantity are measured.

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Calibration Methodologies

Direct Comparison

Indirect Comparison

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Units

The SI International System of Units (abbreviated SI from French: Le SystèmeInternational d'Unités) is divided into :

Fundamental Unites

Supplementary Units

Derived Units

The units which are independently chosen and not dependent upon any other unit are called base or fundamental units.

In the these fundamental units are Meter (m), Kilogram (kg), Second (s), Ampere (A), Kelvin (K), Mole (mol), Candela (cd)

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http://en.wikipedia.org/wiki/French_language

The two Supplementary Units added into the SI system of units are:

Radian for the plane angles

Steradian for the Solid Angle

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Units

Derived Units

Units other than fundamental and supplementary are derived from the fundamental and supplementary units and are primarily classified into:

Mechanical Units like mass, velocity, acceleration, force, weight, torque, work, energy, power etc.

Electric and Magnetic Units such as power, energy, ohms, farads, henries, magnetic flux in webers, tesla etc.

Thermal Units such as latent heat specific heat capacity, sensible heat, calorific value etc.

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Problems

A set of solved problems related to Units shall be provided soon.

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Week 1

Engineering

Transcript of Week 1