Transcript Chapter 1 Introduction

MEASUREMENT STANDARDS AND UNITS
Chapter Objectives
 To define some
measurement terms
 To describe basic
measurement units and
relate to derivative units
 To characterize
instruments
 To differentiate between
instrument and indicators
Contents
Definition and measurement
Errors in measurement process
Classification of instruments
Instrument Elements
Application Area
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
Error The deviation of the true
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).
value from the desired value.
Expected value The design value,
i.e. the most probable value that
calculations indicate one should
expect to measure.
Sensitivity The ratio of the change
in output (response) of the
instrument to a change of input or
measured variable.
Measurement
Measurand
The process of comparing an
 Displacement
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.
 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 Unit
International 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 metrekilogram-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)
Direct Analysis
Error 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.
Formula
e  Yn  X n
e = absolute error
Yn = expected value
Xn = measured value
Formula
Yn  X n
e
% E  x100 
x100
Yn
Yn
%E = percentage of error
e = absolute error
Yn = expected value
Xn = measured value
Yn  X n
e
A  1
 1
Yn
Yn




A = relative accuracy
e = absolute error
Yn = expected value
Xn = measured value


 Yn  Xn
e


a  Ax100  1 
x
100


1



Y
Yn

n


 a = percentage of accuracy
 A = relative accuracy
 e = absolute error

 x100

 Yn = expected value
 Xn = measured value
Formula (Cont..)
P= Precision

xn  x n
P
1


xn





Where
x n = value of the nth
measurement.
xn = average set of
measurement
Example
1. The expected value of
e  Yn  X n
the voltage across a
resistor is 80 V.
Yn  X n
e
% E  x100 
x100
However, the
Yn
Yn
measurement gives a
value of 79 V. Calculate :
Yn  X n
e
(i) absolute error
A  1
 1
Yn
Yn
(ii) percentage of error
(iii) relative accuracy



e
 x100  1 
(iv) percentage of accuracy. a  Ax100  1 


Yn 

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.
x
n 1
 dn is the deviation of the nth
data with the arithmetic
mean.
 Average deviations
 Indicate the precision of the
n
x
 Deviation from mean
n
n
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.
Arithmetic Mean
 xn is nth data taken and n is
the total of data or
measurement.
Example 2
n
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
x
x
n 1
n
n
dn  xn  x
dtotal  d 1  d 2  ....dn
Dav | d 1 |  | d 2 | .... | dn |
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
Because of the instrument.
Three types of systematic
errors:
 Instrumental errors
 Environmental errors
 Observational errors
Produce constant uniform
deviation.
Random Errors
 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.
Absolute
Secondary
•Provide magnitude of the
•Provide magnitude of the
quantity under measurement
in terms of physical constant
of the instrument.
quantity under measurement
only from the observation of
the output from instrument.
•Most instrument used in
practice are secondary.
Operation type
Deflection
Null
 Only one source of input
 Require two 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.
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
Analog
Produce the signal that
vary in continuous way.
Infinite range of value in
any given range.
Digital
Produce the signal that
vary in discrete steps.
Finite different values in
a given range.
Model
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.

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
Engineering Analysis
Process Control
Monitoring
Automation
APPLICATION AREA
 Engineering Analysis
 To validate new design of structure, component or
system by theoretical and experimental approach
 Process Control
 Monitoring process: provide real-time data that
allow operator to respond.
 Automatic process: provide real-time feedback
data to the control system.