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7/30/2019 Cep Assignment (Autosaved)

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QUESTIONS AND ANSWERS

1. STATIC AND DYNAMIC CHARACTERSTICS OF MEASUREMENT SYSTEM?WHEN IT IS USED MEASURE RAPIDLY VARYING QUANTITES WITH

REFERNCE TO :

(i) ACCURACY

(ii) PRECISION

(iii) RESOLUTION

(iv) THRESHOLD

(v) SENSITIVITY

A)Static Characteristics

Static characteristics refer to the characteristics of the system when the

input is either held constant or varying very slowly. The static

characteristics of measuring instruments describe the performance of

the instruments related to the steady-state input/output variables only.

The various static characteristics are destined for quantitative

description of the instruments perfections and they are well presented in

the manufacturer's manuals and data sheets.

Dynamic Characteristics

Dynamic characteristics is the relation between system input and output

when the measured quantity is varying rapidly. It is necessary to find

dynamic response characteristics input varies from instant to instant, so

dynamic I/p. Dynamic input may be Periodic, Transient, random.

The static and dynamic characterstics with reference to

accuracy, precision, resolution, threshold and sensitivity are:(i) ACCURACY

Accuracy of a measuring system is defined as the closeness of the

instrument output to the true value of the measured quantity. It is also

specified as the percentage deviation or inaccuracy of the measurement

from the true value. For example, if a chemical balance reads 1 g with an

error of 10 -2 g, the accuracy of the measurement would be specified as

1%.

The accuracy of the instruments can be specified in either of the


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following forms:

1. Percentage of true value =(measured value - true value /true

value)*100

2. Percentage of full - scale deflection =( measured value - true

value/maximum scale value)*100

(ii) PRECISION

Precision is defined as the ability of the instrument to reproduce acertain set of readings within a given accuracy. For example, if aparticular transducer is subjected to an accurately known input and ifthe repeated read outs of the instrument lie within say 1 %, then theprecision or alternatively the precision error of the instrument would bestated as 1%. Thus, a highly precise instrument is one that gives thesame output information, for a give input information when the readingis repeated a large number of times.

(iii) RESOLUTIONIt is defined as the smallest increment in the measured value that can bedetected with certainty by the instrument. In other words, it is the degreeof fineness with which a measurement can be made. The least count ofany instrument is taken as the resolution of the instrument. For example,a ruler with a least count of 1 mm may be used to measure to the nearest

0.5 mm by interpolation. Therefore, its resolution is considered as 0.5mm. A high resolution instrument is one that can detect smallestpossible variation in the input.

(iv) THRESHOLDIt is a particular case of resolution. It is defined as the minimum value ofinput below which no output can be detected. It is instructive to notethat resolution refers to the smallest measurable input above the zero

value. Both threshold and resolution can either be specified as absolute

quantities in terms of input units or as percentage of full scale deflection.Both threshold and resolution are not zero because of various factors likefriction between moving parts, play or looseness in joints (more correctlytermed as backlash), inertia of the moving parts, length of the scale,spacing of graduations, size of the pointer, parallax effect, etc.

(v) SENSITIVITYSensitivity (also termed as scale factor or gain) of the instrument isdetermined from the results of static calibration. This staticcharacteristic is defined as the ratio of the magnitude of response(output signal) to the magnitude of the quantity being measured (input


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signal). Sensitivity is represented by the slope of the input-output curveif the ordinates are represented in actual units.

2. PRESSURE MEASURING DEVICES USE ELASTIC MEMBERS FOR SENSING

PRESSURE AT THE PRIMARY STAGE? DESCRIBE DIFFERENT TYPES OF

ELASTIC MEMBERS WITH REFRENCE T0

(i) BOURDON TUBES

(ii) DIAPHRAGMS

(iii) BELLOWS

A) Most pressure measuring devices use elastic members for sensingpressure at the primary stage. These elastic members are of many typesand convert the pressure into mechanical displacement which islater converted into an electrical form using a secondary transducer.

(i) Bourdon TubesThese are designed in various forms like:

(i) C type(ii) Spiral

(iii) twisted tube(iv) helical

The Bourdon tubes are made out of an elliptically sectioned flattenedtube bent in such a way as to produce the above mentioned shapes.One end of the tube is sealed or closed and physically held. The otherend is open for the fluid to enter. When the fluid whose pressure is to bemeasured enters the tube, the tube tends to straighten out on account ofthe pressure. This causes the movement of the free end and thedisplacement of this end is amplified through mechanical linkages. The

amplified displacement of the free end is used to move a pointer over ascale calibrated in units of pressure. Bourdon tubes normally measure


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gauge pressure. The materials used for Bourdon tubes are brass,phosphor bronze, beryllium copper, and steel.

(ii) DIAPHRAGMSThe movement of a diaphragm is a convenient way of sensing lowPressures. A diaphragm is a circular disc of thin, springy metal firmlyfixed at its rim. The unknown pressure is applied to one side of thediaphragm and since the rim of the diaphragm is rigidly fixed there is adeflection of the diaphragm. The displacement of the centre of thediaphragm is directly proportional to the pressure and therefore can beused as a measure of pressure.The diaphragms are of two types:

(i) Flat(ii) Corrugated

(iii) BELLOWSThe bellows element consists of a cylindrical metal box with corrugated

walls of thin springy material like brass, phosphor bronze, or stainlesssteel. The thickness of walls is typically 0.1 mm. Bellows are used inapplications where the pressures involved are low. The pressure insidethe bellows tends to extend its length. This tendency is opposed by thespringiness of the metals, which tends to restore the bellows to its


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original size. Pressure on the outside of the bellows tends to reduce itslength and this tendency also, is opposed by the springiness of metal.

3. IN THE MEASUREMENTS SYSTEMS DESCRIBE VARIOUS TYPES OF

ERRORS IN PERFORMANCE PARAMETER DEPENDING ON THE TYPE OF

INSTRUMENTS AND NATURE OF APPLICATION OF INSTRUMENTS?

A) ERRORS IN PERFORMANCE PARAMETERS

The various static performance parameters of the instruments are

obtained by performing certain specified tests depending on the

type of instrument, the nature of the application, etc. Some salient static

performance parameters are periodically checked by means of a staticcalibration. This is accomplished by imposing constant values of 'known'

inputs and observing the resulting outputs.

RANDOM ERRORS SYSTEMATIC ERRORS

Types of Errors

Error is defined as the difference between the measured and the true

value (as per standard). The different types of errors can be broadly

classified as follows.


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Systematic or Cumulative Errors

Such errors are those that tend to have the same magnitude and sign for

a given set of conditions. Because the algebraic sign is the same, they

tend to accumulate and hence are known as cumulative errors. Since

such errors alter the instrument reading by a fixed magnitude and withsame sign from one reading to another, therefore, the error is also

commonly termed as instrument bias. These types of errors are caused

due to the following:

Instrument errors:

Certain errors are inherent in the instrument systems. These may be

caused due to poor design / construction of the instrument. Errors

in the divisions of graduated scales, inequality of the balance arms,

irregular springs tension, etc., cause such errors. Instrument errorscan be avoided by (i) selecting a suitable instrument for a given

application, (ii) applying suitable correction after determining the

amount of instrument error, and (iii) calibrating the instrument against

a suitable standard.

Environmental errors:

These types of errors are caused due to variation of conditions external

to the measuring device, including the conditions in the area

surrounding the instrument. Commonly occurring changes in

environmental conditions that may affect the instrument characteristics

are the effects of changes in temperature, barometric pressure, humidity,

wind forces, magnetic or electrostatic fields, etc.


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Loading errors:

Such errors are caused by the act of measurement on the physical system

being tested. Common examples of this type are: (i) introduction of

additional resistance in the circuit by the measuring millimetre whichmay alter the circuit current by significant amounted.

Accidental or Random Errors:

These errors are caused due to random variations in the parameter or

the system of measurement. Such errors vary in magnitude and may be

either positive or negative on the basis of chance alone. Since these

errors are in either direction, they tend to compensate one another.

Therefore, these errors are also called chance or compensating type of

errors. The following are some of the main contributing factors to

random error.


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4) EXAMPLES OF INSTRUMENTS TO ENNUMERATE THE DEVELOPMENT OF

SUCH DEVICES IN THE MEASUREMENTS OF VARIABLE AND QUANTIES?

A)The history of development of instruments encompasses three phases of

instruments, viz.

(i) mechanical instruments

These instruments are very reliable for static and stable conditions.

Major disadvantage is unable to respond rapidly to measurements of

dynamic and transient conditions. This is due to the fact that these

instruments have moving parts that are rigid, heavy and bulky and

consequently have a large mass. Another disadvantage of mechanical

instruments is that most of them are a potential source of noise and

cause noise pollution.

Ruler and scales

Callipers

Venire calliper


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Micrometre

Feeler gauge

(ii) Electrical instruments

Electrical methods of indicating the output of detectors are more rapid

than mechanical methods. Electrical system normally depends upon a

mechanical meter movement as indicating device. This mechanical

movement has some inertia and therefore these instruments have a

limited time (and hence, frequency) response.

Voltmeter

Ammeter

Ohm meter


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Wattmeter

Power-factor meter

(iii) electronic instrumentsSince in electronic devices, the only movement involved is that of

electrons, the response time is extremely small on account of very smallinertia of electrons. This is particularly important in the area of Bio-instrumentation since Bio-electric potentials are very weak i.e., lowerthan 1 mV.

Digital Millimetres

Capacitors

Function Generators

Transistors


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Diodes

5) ANSWER THE FOLLOWING QUESTIONS:

(i) GIVE THE CLASSIFICATION OF MEASUREMENTS AND EXPLAIN

EACH CLASS WITH EXAMPLE (DIRECT OR INDIRECT

MEASUREMENT)?

A) Measurement

Measurements provide us with a means of describing variousphenomena in quantitative terms. It has been quoted "whatever exists,exists in some amount". The determination of the amount ismeasurement.

The methods of measurement may be broadly classified into twoCategories:Direct MethodsThese methods, the unknown quantity (also called the measurand) isdirectly compared against a standard. The result is expressed as anumerical number and a unit. Direct methods are quite common for themeasurement of physical quantities like length, mass and time.Examples are:

(i) Dip Stick(ii) Resistance tapes(iii) Sight glass(iv) Floats(v) Ultrsonic

In-Direct MethodsIn engineering applications Measurement Systems are used. Thesemeasurement systems use indirect methods for measurement purposes.

A measurement system consists of a transducing element which convertsthe quantity to be measured into an analogous signal. The analogous


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signal is then processed by some intermediate means and is then fed tothe end devices which present the results of the measurement.Examples are:

(i) Hydrostatic head methods

(ii) Load cells(iii) Capacitance(iv) Conductivity

(ii) WHAT DO YOU MEAN BY IPTS (INTERNATIONAL PRACTICALTEMPERATURE SCALE)?

A) A temperature scale adopted by international agreement in 1968, and

revised in 1990, based on thermodynamic temperature and using

experimental values to define 16 fixed points. The lowest is the triplepoint of an equilibrium mixture of orthohydrogen and parahydrogen (-

259.34C) and the highest the freezing point of copper (1084.62C).

(iii) DIFFERENT TYPES OF TEMPERATURE SCALES?

A) Fahrenheit, Celsius, Kelvin and Rankin are the four most commonly

used temperature scales. The scales use degrees with ratios defined by

the boiling and freezing points of water and a value called absolute zero.

Fahrenheit ScaleThe Fahrenheit scale, named after physicist Daniel Gabriel Fahrenheit,

was used in most English-speaking countries until the 1970s, when mostof those countries switched to the Celsius scale. When writing atemperature on the Fahrenheit scale, the number value is generallyfollowed by a degree sign and the letter "F."This scale features a water

boiling point of 212 F and a water freezing point of 32 F. Absolute zerohas a value of minus 459.67 F. The only point on the Fahrenheit andCelsius temperature scales at which Fahrenheit and Celsius equal each

other is at minus 40 F and, therefore, minus 40 Converting atemperature from Fahrenheit to Celsius requires subtracting 32 from theFahrenheit degree number; then that number needs to be divided by 9/5or 1.8.

Celsius ScaleThe Celsius, or Centigrade, scale received its name from astronomer

Andrew Celsius. This scale was the standard in science even before itspost-1970s prominence. It is based on a water freezing point of 0 C and a

water boiling point of 100 C. The 100-degree difference between those

values explains the alternate name of Centigrade. The Celsius value forabsolute zero is minus 273.15. To convert from Celsius to Fahrenheit


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requires multiplying the Celsius degree value by 9/5 or 1.8 and thenadding 32.

Kelvin Scale

The Kelvin scale was named for the physicist William Thomson, BaronKelvin. The scale has degrees equivalent in size to the Celsius scale, butthe Kelvin scale has an absolute zero of 0 compared to Celsius' minus273.15. The standard degree unit of thermodynamic temperature, Kelvintemperatures generally are written without a degree symbol between thenumbers and the Water boils at 373.15 K and freezes at 273.15.Conversion from Celsius to Kelvin requires adding 273.15 to the Celsiusreading. To convert from Kelvin to Celsius merely requires subtracting273.15 from the Kelvin reading.

(iv) HOW TO MEASURE TEMPERATURE (NON ELECTRICAL, ELECTRICAL,

RADIATION)?

A)Temperature is measured by observing the effect that temperaturevariation causes on the measuring device. Temperature measurementmethods can be broadly classified as follows:

(i) NON-ELECTRICAL METHODSThe non-electrical methods of temperature measurement can be basedon anyone of the following principles:1. change in the physical state,2. change in the chemical properties, and3. change in the physical propertiesExample are:

BIMETALLIC THERMOMETER

LIQUID-IN-GLASS THERMOMETER

PRESSURE THERMOMETERS


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MERCURY-IN-GLASS THERMOMETER

(ii) ELECTRICAL METHODS

Electrical methods are in general preferred for the measurement oftemperature as they furnish a signal which can be easily detected,amplified or used for control purposes. There are two main electricalmethods used for measuring temperature. They are:1. Thermo-resistive type i.e., variable resistance transducers and2. Thermo-electric type i.e., emf generating transducersExamples are:

ELECTRICAL RESISTANCE THERMOMETERS

METALLIC RESISTANCE THERMOMETERS

THERMO-ELECTRIC SENSORS

(iii) RADIATION METHODSRadiation Thermometers (Pyrometers, if you will) are non-contacttemperature sensors that measure temperature from the amount ofthermal electromagnetic radiation received from a spot on the object ofmeasurement.

Examples are:

INFRARED THERMOMETERS

PYROMETERS

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