EE121 MEASUREMENTS

AND INSTRUMENTATION

Syllabus (5 Chapters)
1Introduction
Measurements

to

Instrumentation
and
(2 lectures)

2- DC and AC meters

(6 lectures)

3- Bridges

(2 lectures)

4- Instrument transformers

(2 lectures)

5- Oscilloscopes

(2 lectures)
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Assessment
Mid-term
Laboratory
Final Examination
TOTAL

30
30
90
150

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 Chapter

#1 Measurement
and Error

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Objectives
At the end of this chapter, you should be
able to:
discuss and calculate various types of
error in measurement.
Explain the meaning of some terms in
instrumentation field.

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Outlines
The outline of this chapter is as follows:

Principles of instrumentation and measurements

Error in measurement
Some terms +definitions in instrumentations.

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INTRODUCTION

Measurement is the process of comparing unknown

quantity with an accepted standard quantity. It involves
connecting a measuring instrument into the system under
consideration and observing the resulting response of the
instrument.

CLASSIC ELECTRICAL MEASUREMENTS

A typical digital multimmete

A typical analogue multimmeter
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CLASSIC ELECTRICAL MEASUREMENTS

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MEASUREMENTS IN INDUSTRIAL PROCESS

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MEASUREMENTS IN INDUSTRIAL PROCESS

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Typical Measurement System Architecture

Noise and Interference
Process
or
Test

Sensor
or
Transducer

Amp

OUR TOPIC IS HERE

Proces
s

Controller
and control
over the process or experiment

Signal
Conditioner

ADC
Converter

PC comp
and data
storage

Electric/Electronic Instrument
Basic elements of an electronics instrument
Transducer

Signal
Modifier

Indicating
Device

1) Transducer
- convert a non electrical signal into an electrical signal
- e.g: a pressure sensor detect pressure and convert it to
electricity for display at a remote gauge.
2) Signal modifier
- convert input signal into a suitable signal for the indicating
device
3) Indicating device
- indicates the value of quantity being measure

SIGNIFICANCE OF MEASUREMENTS
The advancement of science and technology is dependent upon a
parallel progress in measurement technique
There are two major function of all branches of engineering

(i) Design of equipment and processes

(i) Proper operation and maintenance of equipment and process

Measuring Instruments
Classification According to
1) Measuring Method
Direct
Measuring
Instruments

AC

DC

Ammeters, Voltmeters, Wattmeters, Flux

Meters, Oscilloscope, etc

Comparison
Measuring
Instruments

AC/DC

AC

DC

Bridges, Potentiometers

CLASSIFICATION MEASUREMENT INSTRUEMNT

ACCORDING TO 2) TYPE

Mechanical instrument ( this instrument is very

reliable for stable and steady state value)
Electrical instrument ( this instrument is more
accurate than mechanical one, and can detect the
variation with time)
Electronic instrument ( more accurate than the
previous two types and can be used to record and
store the data)

CLASSIFICATION MEASUREMENT INSTRUEMNT

ACCORDING TO 3) READING

Absolute

instrument

( which gives the

reading in terms of physical constants of the
instrument like GALVANOMETER )

Secondary

instrument (these instruments

are so constructed that the quantity being

measured can only be measured by observing the
output indicated by the instrument, and these
instrument are calibrated by comparison with an
absolute instrument) ( it is used in analogue and
digital devices)

A tangent galvanometer is an early measuring

instrument used for the measurement of electric
current. It works by using a compass needle to
compare a magnetic field generated by the
unknown current to the magnetic field of the
Earth.

CLASSIC ELECTRICAL MEASUREMENTS

A typical digital multimmete

A typical analogue multimmeter
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CLASSIFICATION ACCORDING TO 4) FUNCTION

Monitoring of process and operation

Control of process and operation
Experimental engineering analysis

ELEMENTS OF GENERALIZED MEASURING SYSTEM

1) Primary sensing
element

Element

3) Data Presentation
Element

2) Variable
Conversion Element

CHARACTERISTIC OF INSTRUMENTS AND

MEASUREMENT SYSTEM

Static characteristic
Dynamic characteristic (depends on the dynamic
function of the measured value and the system
elements response )

Static
Characteristic

Accuracy

Sensitivity

Reproducibility

Drift

Static Error

Dead Zone

Definitions
Instrumentation is a technology of
measurement which serves sciences,
engineering, medicine and etc.
Measurement is the process of determining
the amount, degree or capacity by
comparison with the accepted standards of
the system units being used.
Instrument is a device for determining the
value or magnitude of a quantity or variable.
Electronic instrument is based on electrical or
electronic principles for its measurement
functions.

BASIC DEFINITIONS
Accuracy: It is the closeness of the measured value with
the true value.
Precision: It is the degree of agreement within a group of
readings
Sensitivity, S: The ratio of the output signal or response
of the instrument to a change in the input variable.
Resolution , R : Smallest change in a measured value to
which the instrument will respond (can identify it).
True Value: The Expected value with no error
Error, e: Deviation of the measured value from the true
one
e = measured value true value
1.1

BASIC DEFINITIONS (CONT.)

Absolute error, ea :
ea = e = xm xt
where
ea = absolute error.
xt = expected value (true value).
xm = measured value.

1.2

Percentage relative error :

ea
absolute error
e%
100 %
100 %
true value
xt

1.3

ACCURACY AND PRECISION

Accuracy: refers to the degree of closeness or conformity
to the true value of the quantity under measurement.
Precision: refers to the degree of agreement within a
group of measurement or instruments.
Good accuracy: means good precision.
Good precision: does not have to mean good accuracy.

The accuracy, precision and error depend on the

instrument and the person. But the sensitivity and
resolution depend on the instrument only.

SIGNIFICANT FIGURES

An indication of the precision of the measurement is

obtained from the number of significant figure in whish
the result is expressed. For example if a resistor is
specified as having a resistance of 68 then the number
of significant figure is two. But if a resistor is specified as
having a resistance of 68.0 then the number of
significant figure is three.

Case 1 : 67 , 68 ,
Case 2 : 67.9 , 68.0 ,

69
68.1

Error = 1
Error = 0.1

TYPES OF ERRORS
Any measurement is affected by many variables therefore
the results is deviated from the expected value for example
connecting the measuring instruments into the circuit
under consideration always disturbs (changes) the circuit
causing the measurement to differ from the expected value.
Some factors that affect measurements are related to the
instruments themselves, other factors are related to the
person using the instrument, i.e. errors may come from
different sources.

TYPES OF ERRORS (CONT.)

1)Gross errors
2) Systematic errors
3)Random errors

GROSS ERRORS
Covers human mistakes in reading or bad usage of
the instruments or incorrect calculating the
measurement results.
Also covers the incorrect adjustment for the
instruments. (Zero adjustment).

Error Classification: Gross Error

An example of gross error

Error Classification: Gross Error

An example of gross error

Error Classification: Gross Error

An example of gross error

Error Classification: Gross Error

An example of gross error

TYPES OF ERRORS (CONT.)

1)Gross errors
2) Systematic errors
3)Random errors

SYSTEMATIC ERRORS
1. Instrumental errors:
o
These errors arise due to three main reasons
o
(i) due to inherent shortcomings in the instrument
(ii) due to misuse of the instrument
o
o
(iii) due to loading effects of instruments
o
Which are inherent in measuring instruments because of
their mechanical structure such as movement friction in
the bearing, irregular spring tension, reduction in
tension or overloading.
o
These errors may be avoided by selecting a suitable
instrument, applying correction factors after
determining the amount of instrumental error and
calibrating the instrument against standard.

SYSTEMATIC ERRORS (CONT.)

2. Environmental errors
These errors are due to the external conditions such
as temperature, magnetic field, and humidity that
can effect in the reading. These errors may be avoided
by shielding the measuring instrument.

3- Observational errors
There are many sources of observational errors.
As an example, the pointer of a voltmeter
rests slightly above the surface of the scale.
Thus an arror on account (PARALLAX)

TYPES OF ERRORS (CONT.)

1)Gross errors
2) Systematic errors
3)Random errors

RANDOM ERRORS

These errors are due to unknown reasons and occur even

when all systematic errors have been accounted for. The
only way to avoid or decrease these errors is by
increasing the number of reading and using statistical
analysis to obtain best approximation.

Random Errors
Random errors are not inherent to the measuring
process.
Frequently they are introduced by external factors that
cause a scattering of the measured data.
When the scattering is distributed equally about the
true value, the error can be mitigated somewhat by
making multiple measurements and averaging the data.

Vibration in mechanical devices produces random errors.

In electronic devices, noise produces random errors.

Limiting Error ( Guarantee Error)

The manufacturer has to specify the deviations from the nominal value of a
particular quantity.
The limits of these deviations from the specified value are defined as Limiting
Errors or Guarantee Errors

Combination of quantities with limiting error

Statistical Treatment of Data

The experimental data is obtained in two forms of tests

Multi-sample Test
Repeated measurement of a
given quantity are done
using different test
conditions such as different
instruments, different ways
of measurements, and
different observers

Single-sample
Test
Repeated under identical
conditions excepting for time

REPEATABILITY/ REPRODUCIBILITY
Both terms describe the spread of output
readings for the same input.

This spread is referred to as repeatability

if the measurement conditions are
constant and as reproducibility if the
measurement conditions vary.
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REPEATABILITY/ REPRODUCIBILITY
Repeatability describes the closeness of output readings
when the same input is applied repetitively over a short
period of time, with the same measurement conditions,
same instrument and observer, same location and same
conditions of use maintained throughout.

Reproducibility describes the closeness of output readings

for the same input when there are changes in the method of
measurement, observer, measuring instrument, location,
conditions of use and time of measurement.
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Statistical analysis
When a number of multisampling observations are
taken experimentally there is a scatter of the data
about some central value. One method presenting test
results in the form of a Histogram.
Length

Number
of
readings

99.7

99.8

99.9

12

100.0

19

100.1

10

100.2

100.3

A histogram represents the

number of observed reading
( frequency of occurrence) a
histogram
is
called
frequency distribution curve

Deviation from the mean value =

Precision from the mean =
P= 1 X X X
i

The average deviation is an indication of the

precision of the instruments used in making the
measurements,
Highly precise instruments yield a low average deviation between readings

Standard Deviation (S.D)

is a measure of how spread out numbers are

1
2
(
d
i)

n i 1
For n > 20

Standard Deviation (S.D)

1
2
i)
(
d

( n 1) i 1
For n < 20

Variance (V)
is the mean square deviation
1
2
V
(n)

(d )

For n>20

i 1

n
1
2
2
V s
(di )

(n 1) i 1

For n<20