Introduction - Lect. 2-10-10

8/6/2019 Introduction - Lect. 2-10-10

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2 1 0 6

Instructor: Dr. P.Muthukumar

Associate Professor, Mechanical En ineerin


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You can ex ect to learn the followin

Basic rinci les of o eration of various

instruments

Design of ExperimentsDesign of Instruments

Ex erimental data anal sis

Error analysis, etc.


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Whatever exists, does in some measurable quantity

Everything that exists has some quantity and quality

Measurements can be said to be the process of finding the

quantity

Measurements provides the quantitative information on the

actual state of physical variables.


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WHY DO WE MEASURE

To estimate the amount of something

Measurement is the basis for R &D, Innovation, etc.

Routine monitoring of some industrial process

It is also the basis for control

o es a s an en orce s an ar s

Sharing resources

Performance evaluation

Numerical Validation


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What is standard

A standard rovides the reference for a measured uantit

A standard should be internationally known and accepted

Physical representation of the unit of measurement. Generallyc osen w re erence o an ar rary ma er a s an ar or o

a natural phenomenon which includes physical and atomic

constants

Definition of mass, length and timeRefer text

History of time, mass an engt books


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Length (m) Force (N = kg.m/s2)

Mass k Fre uenc Hz = 1/s

Time (s) Energy (J = N.m)

Temperature (K) Power (W = J/s)

Electric Current (A) Pressure , stress (Pa = N/m2)

Amount of substance (mol) Electrical Resistance ( = V/A)Luminous intensity (candela, ca) Electrical Capacitance (S = A/V)

Supplementary Units

Plane angle : radian (rad)

o ang e : s era an sr


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Meter

International prototype: 1889 at the first General conference

on Weights and Measures, France (1889 1960)

stance etween two nes on a stan ar ar compose o an

alloy of ninety percent platinum and ten percent iridium,

measured at the meltin oint of ice. (uncertaint : 0.1-0.2 m)Updated definition

interval of 1/299,792, 458 of a second. (uncertainty : 0.1 nm)

This definition fixed based on the s eed of li ht in a vacuum at

299,792,458 meter per second

Refer wikipedia for Definitions of the metre since 1795


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Time

The solarday was divided into 24 hours, each of which

contained 60 minutes of 60 seconds each, so the second was1

86400 .1967 to 1997: Time elapsed during 9,192,631,770 periods of the

two hyperfine levels of the ground state of the caesium 133

atom. (This definition refers to a caesium atom at rest at aempera ure o .- up a e

Refer wikipedia for various definitions of the time


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Mass

:Mass of Platinum Iridium bar kept at specific

condition Int Bureau of Wei hts and Measures, located inSevres)

Prototype of mass is also kept at NIST

empera ure

Kelvin (K): SI unit of thermodynamic temperature, is a

rac on o . o e ermo ynam ctemperature of the triple point of water.


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Methods of comparison

Direct comparison:

Indirect method :

Measurement is not direct, but inferential, based on the known

factors.

Eg. Measurement of electron charge by JJ Thompson,

easurement pressure, etc.


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General Measurement system


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In case of commercial balance,

r mary sens ng e emen : pr ng

Variable conversion, manipulation

Data transmission : Spring and

Presentation : Scale and pointer


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Vapour Pressure Thermometer


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Essential requirements of an ideal instrument

Low cost

Eas to use

High sensitivity (small measurement volume that provideshigh resolution)

High accuracy

Wide range of operating range ?

easure e quan y rec y

Should give both digital and analog outputs.

Should have interfacing facility and temporary storage


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Various inputs to an instrument

An instrument is subject to the following inputs;

Desired inputs

Interfering inputs

Modifying / compensating inputs

Manometer

with

interference

inputs

Example of eliminating

interference in uts

Mechanical vibrations minimized by isolating the device

Electrical interferences minimized b insulation

Different methods

Design the instrument such that undesirable inputs are compensated

By making suitable correction


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Transducers

To absorb some energy (input) from the system and convert it into

other form

Do not require any external energy

Require external energy for

operation

e.g. ermocoup es, ermometer . . ,

flow meters, etc.


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Classification of signals

Static signals: Measurement do not change with time e.g. Length and mass

measurement

Dynamic signals: Measurement change with time e.g. Speed of a vehicle, vibration,

etc..

Periodic signals: Measurement in a periodic e.g. Oscillation of a simple pendulum.


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Representation of various signals

Digital Analog

Output can take only Infinite values

Digital to analog comparison

a finite number ofvalues.

Inference with

computer is easy.

A/D conversion is

required

Accuracy ispreserved.

Accuracy may

decrease duringtransmission


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Classification of Instruments based on the

principles of measurements

deflection zero by applying an

effect opposite to that produced by

the measured quantity. e.g.

opposing physical effect which can

be observed. eg. Spring balance,

thermometer,etc.

Platform balance

Accuracy depends on the accuracy

of the opposing effect that is given

Accuracy depends on calibration

Takes more time and may berequired some skill

Small and intermittent scale

For large scale application

application

Only Static measurement Both static and dynamicmeasurements


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Calibration

rocess o eterm n ng t e re at on etween a measure va ue an a

standard.

Act of applying a known value of input(standard input) to a measurement

system for the purpose of observing the system output.

Calibration procedure establishes the correct output scale for the

Example of calibration


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Pressure

calibration

Make : Keller, Swiss

Model : 21 SC

-

mA Reg.

value

Measu.

4 0 -0.395

8 50 50.821

12 100 102.037

.

20 200 204.469


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Instruments Characteristics

Static Characteristic : Input and output are static

Dynamic Characteristic: Input and output are function of time

s o s a c c arac er s cs a ng o ns rumen per ormance

1. Accuracy 8. Threshold

. .

3. Static Sensitivity 10. Zero drift

. .

5. Range or Span 12. Dead space

. .

7. Hysteresis


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Accuracy

Refers to the ability of the instrument to indicate the true value

Accuracy (FULL SCALE)

Precision% ( ) 100

easured value true value

of accuracy FS Maximum scale value

=

The difference between the instruments outputs during repeatedmeasurements of the same quantity.

Free from random errors


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Static sensitivity

, -linear.

Is the measure of change in indicated output with a given change in static

input.

Should be sensitive enough to indicate small change in inputs.

,

value.


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Zero Error or Bias

Is the value of the output when the input is zero.

Zero Error is undesirable. Output should be corrected by repeating the

experiments.


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Range or Span

The difference between the maximum and minimum limits of an

instrument.

The maximum allowable in ut is also called Full Scale FS

Range and accuracy are inter related. In order to have higher accuracyand precision, range should be kept as low as possible.

To avoid extrapolation beyond the range of known calibration during

measurement since the behavior of the instrument is unpredictable in

these regions.


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Linearity

Linearity quantifies how close the calibrated data is to be a straight line.

Another way of expressing linearity is to fit the calibration data through

.

The output should be linear with input over its range


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Hysteresis

Refers to difference in the values found between upscale and downscale of

the measurements in a sequential test.

De endence of out ut to the direction of in ut.

Due to the looseness, friction and material characteristics.

e.g. Thermocouples, Pressure transducers


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Threshold

In ut below which no out ut can be detected Smallestmeasurable input)

Resolution It is the minimum input required to detect a change in

the output. Ie. the value of minimum sensible input.

Defined as the input increment that gives some small butdefinite numerical change in the output.

Both threshold and resolution values should be as small as possible


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Pressure Transducer of Rang 0-100 bar (100-2)

90

100

y = 10.226x - 3.3505

R2 = 0.9996

40

50

60

70

80

d

pressure(bar)

0

1020

30

Applie Threshold

Error between0-10 bar

Output voltage (V)

35

40

20

25

30

ure,

bar

5

10

15Pre

s

0

0 0.5 1 1.5 2 2.5 3 3.5 4

Out put, Volts


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Drift

ue to var ous reasons, t e ca rat on curve maychange with time. This phenomenon is called as drift.

, , .

Zero drift is the change of Sensitivity drift is the change of

zero error w metime

e.g. Pressure gage, due to change of spring or pressure sensing elements elasticity.


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Dead Space or Dead Zone

Dead space is the range of input values over which there is no

change in output value.

May be observed in a large span instruments


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Repeatability

obtained in different schedule.

The ability of a instrument to show the same value ofrepeated outputs of same input given in different time.

If the instrument yields the same outputs to the same input values, than it

is said to be having better repeatability.

Re roducibilit

If someone measures a input in a laboratory, and the same

input is measured by another person in another laboratory

and gets the same output, then the measurement is said to bereproducible.

Manufacturer claims of an instrument reproducibility must be based on

multiple repeatability tests performed in different labs on a single unit.

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Introduction - Lect. 2-10-10