Introduction to instrumentation system

Electrical
Measurement-II
Prof. Yogesh K. Kirange M.E. (Electrical Machines & Drives)
Assistant Professor
Department of Electrical Engineering
R.C.Patel Institute of Technology, Shirpur

Unit-I
Introduction to Instrumentation
• Definition, purpose, measurement – definitions, types of instruments
• Classification of instruments
• Generalized measurement system
• Standards and calibrations.
• Instrument Response: Instrument Response to step, ramp, sinusoidal
i/p up to second order system.
• Errors – types – gross, systematic, random, limiting & Numericals
• Sources of errors, techniques to minimize them.
T.E. Electrical , Electrical Measurement-II Presented By : Y. K. Kirange
Unit-I Introduction to Instrumentation Department of Electrical Engineering , RCPIT, Shirpur
1

Measurements-
“The measurement of a given quantity is essentially an act or the result of
comparison between the quantity (whose magnitude is unknown) and a
predefined standard”. Since two quantities are compared the result is
expressed in numerical values.
In order that the results of the measurement are meaningful, there are
two basic requirements :
(i) The standard used for comparison purposes must be accurately defined
and should be commonly accepted, ·
(ii) The apparatus used and the method adopted must be provable.
T.E. Electrical , Electrical Measurement-II Presented By : Y. K. Kirange 2

Significance of Measurements-
(i) The advancement of Science and Technology is dependent upon
a parallel progress in measurement techniques.
(ii) It can be safely said that the quickest way to assess a nation's
progress in Science and Technology is to examine the type of
measurements that are being made and the way in which the data is
acquired by measurements and is processed.
(iii) There are two major functions of all branches of engineering :
Design of equipment and processes and Proper operation,
maintenance of equipment and processes.

Classification of Instruments-
Instrument- “ A device for determining the value or magnitude of a
quantity or variable.” OR
“The device used for comparing the unknown quantity with the unit of
measurement or standard quantity is called a Measuring Instrument.”
OR
“A machine or system which is designed to maintain functional
relationship between prescribed properties of physical variables & could
include means of communication to human observer.”

Electrical instruments may be divided into two categories, that are;
1. Absolute instruments,
2. Secondary instruments.
Absolute instruments gives the quantity to be measured in term of
instrument constant & its deflection. (e.g. Tangent Galvanometer and
Rayleigh's current balance.
Secondary instruments the deflection gives the magnitude of electrical
quantity to be measured directly. These instruments are required to be
calibrated by comparing with another standard instrument before
putting into use. (e.g. a voltmeter, a glass thermometer and a
pressure gauge)

Electrical measuring instruments may also be classified according to the
kind of quantity, kind of current, principle of operation of moving
system.
CLASSIFICATION OF SECONDARY INSTRUMENTS
• Secondary instruments can be classified into three types;
i. Indicating instruments;
ii. Recording instruments;
iii. Integrating instruments.

CLASSIFICATION OF SECONDARY INSTRUMENTS
Indicating Instruments:
It indicate the magnitude of an electrical quantity at the time
when it is being measured. The indications are given by a pointer
moving over a graduated dial.

Recording Instruments:
The instruments which keep a continuous record of the variations of
the magnitude of an electrical quantity to be observed over a
defined period of time.

Integrating Instruments:
The instruments which measure the total amount of either quantity of
electricity or electrical energy supplied over a period of time. For
example energy meters.

ESSENTIALS OF INDICATING INSTRUMENTS
A defined above, indicating instruments are those which indicate
the value of quantity that is being measured at the time at which it
is measured. Such instruments consist essentially of a pointer which
moves over a calibrated scale & which is attached to a moving
system pivoted in bearing. The moving system is subjected to the
following three torques:
1. A deflecting ( or operating) torque;
2. A controlling ( or restoring) torque;
3. A damping torque.

 Instruments can classified into many categories, one
classification is given as under.
1. Active/Passive instruments
2. Null or deflection type
3. Monitoring or control type
4. Analogue or digital
5. Absolute or secondary

1. Active/Passive Instruments
Passive Instruments: (Easy design, cheap)
In which the output produced depends entirely on quantity being
measured.
Example: Analogue ammeter, Pressure gauge
Active Instruments: (Difficult to design, costly)
In which the quantity being measured activates the magnitude of some
external power input, which in turn produces the measurement.
Example: Liquid Level Indicator, LUX meter using LDR.

2. Null/Deflection
Null type instruments:
In which a zero or null indication leads to the determination of magnitude
of the quantity being measured.
Example: DC potentiometer
Deflection type instrument:
In which the quantity being measured
produces some effect due to which pointer
deflects.
Example: PMMC instrument.

3. Monitoring /Control
Monitoring type instruments:
In which some indication or condition of parameter value under study
is obtained.
Example: All deflection type or digital instruments
Control type instruments:
These are used in automatic control systems in the feedback path, to
send a feedback signal from the output of a process to its input.
Example: Automatic air- conditioning system, AVR.

4. Analogue/digital
Analogue type instruments:
In which output varies in continuous fashion as quantity being measured,
having infinite values in a given range.
Example: Deflecting Instruments are good examples of
analogue instruments.
Digital Instruments:
In which output varies in discrete step and thus give finite values in a
given range.
Example: Digital Multi-meter

 Any instrument or measuring can be represented by a block
diagram, that indicates necessary elements and its functions.
 The entire operation of a measuring system can be understand
from the following block diagram.
Generalized measurement system

1. Primary Sensing Element
2. Variable Conversion Element
3. Variable Manipulation Element
4. Data Transmission Element
5. Data Storage Element (Not necessary)
6. Data presentation element

Primary Sensing Element
 measurand is first detected by primary sensor.
 conversion of measurand into an analogous electrical signal.
 This is done by a transducer (a device which converts energy from
one form to another)
Variable Conversion Element
 Some times output is not suited to the system.
 For the instrument to perform, the desired function, it may be necessary
to convert this output to some other suitable form while preserving the
information content of the original signal.
 Therefore we will use an A/D converter.

Variable Manipulation Element
 The function of this element is to manipulate the signal presented to
it preserving the original nature of the signal.
 Manipulation here means a change in numerical value of the signal.
Data Transmission Element
When the elements of an instrument are actually physically separated, it
becomes necessary to transmit data from one to another. The element that
performs this function is called a Data Transmission Element. For example
space-crafts are physically separated from the earth where the control
stations guiding their movements are located. Therefore control signals arc
sent from these stations to space-crafts by a complicated telemetry systems
using radio signals.

 Just take an example of analogue meter used to measure current
(Ammeter), all necessary elements are shown in block diagram.

Standards-
A standard is a physical representation of a unit of measurement. The term
'standard' is applied to a piece of equipment having a known measure of
physical quantity· They are used for the purpose of obtaining the values of the
physical properties of other equipment by comparison methods.
Standards of Measurement is classified by their Junction and application in
the following categories :
(i) International standards
(ii) Primary standards
(iii) Secondary standards
(iv) Working standards.

International Standards-
 The international standards are defined on the basis of international
agreement.
 They represent the units of measurements which are closest to the possible
accuracy attainable with present day technological and scientific methods.
 International standards are checked and evaluated regularly against absolute
measurements in terms of the fundamental units.
 The International Standards are maintained at the International Bureau of
Weights and Measures and are not available to the ordinary user of
measuring instruments for the purposes of calibration or comparison.

Primary Standards-
 Primary standards are absolute standards of such high accuracy that they
can be used as the ultimate reference standards.
 These standards are maintained by national standards laboratories in different
parts of the world.
 It represent the fundamental units and some of the derived electrical and
mechanical units, are independently calibrated by absolute measurements
at each of the national laboratories.
 Primary standards are not available for use outside the national laboratories
One of the main functions of the primary standards is the verifications and
calibration of secondary standards.

The following points must be taken into serious consideration when a primary
standard is built:
1. The materials should have a long time stability.
2. The temperature coefficient of the materials should be as small as possible.
3. The deterioration of the materials caused by moisture and other
environmental conditions should be eliminated as far as possible.
4. The machining of parts should be accurate.
5. The measurement of physical dimensions, on which the accuracy of the
standard depends predominantly, should be done with most sophisticated
techniques available.
6. The rigidity of the construction should be insured.

Secondary Standards-
 The secondary standards are the basic reference standards used in industrial
measurement laboratories.
 The responsibility of maintenance and calibration of these standards lies with
the particular industry involved.
 These standards are checked Iocally against reference standards available
in the area.
 Secondary standards are normally sent periodically to the national standards
laboratories for calibration and comparison against primary standards.
 The secondary standards are sent back to the industry by the national
laboratories with a certification as regards their measured values in terms of
primary standards. ·

Working Standards-
 The working standards are the major tools of a measurement laboratory.
 These standards are used to check and calibrate general laboratory
instruments for their accuracy and performance.
 For example, a manufacturer of precision resistances, may use a Standard
Resistance (which may be a working standard) in the quality control
department for checking the values of resistors that are being
manufactured.
 This way, he verifies that his measurement set up performs within the limits
of accuracy that are specified.

For example of Standards for Mass:-
Primary Standard of Mass:-
We have stated before that the material representation of unit of mass is the
Prototype kilogramme preserved at the International Bureau of Weights and
Measures at Severes near Paris. The primary unit of mass is a Prototype
kilogramme kept at National Physical Laboratories of every country. This has an
accuracy of 1 part in 108 and is occasionally verified against the standard kept
at the International Bureau.
Secondary standards of Mass:-
The secondary standards of mass are kept by industrial laboratories. The
standards have an accuracy of 1 ppm and are checked against the primary
standard.
Working Standards of Mass:- The working standards of mass are available i a
wide range of vaIues so that they suit any kind of application. The accuracy
of working standards are of the order of 5 ppm. The standards are verified
against the secondary standard.

Calibration is “checking the accuracy of a measurement instrument by
comparing it to reference standards.” The result of equipment calibration is
higher accuracy. Electrical & electronic calibration is one of three main types of
calibration methods used today. Other types of modern calibration methods
include mechanical and thermal calibration. Electrical/Electronic calibration
deals with the calibration of electric and electronic instruments.
What is calibration ?

The formal definition of calibration by the International Bureau of Weights and
Measures is the following: "Operation that, under specified conditions, in a first
step, establishes a relation between the quantity values with measurement
uncertainties provided by measurement standards and corresponding
indications with associated measurement uncertainties (of the calibrated
instrument or secondary standard) and, in a second step, uses this information to
establish a relation for obtaining a measurement result from an indication.“
This definition states that the calibration process is purely a comparison, but
introduces the concept of measurement uncertainty in relating the accuracies
of the device under test and the standard.
…contd

 Calibration in measurement technology and metrology is the comparison
of measurement values delivered by a device under test with those of
a calibration standard of known accuracy.
 Such a standard could be another measurement device of known accuracy,
a device generating the quantity to be measured such as a voltage, or a
physical artefact, such as a metre ruler.
The outcome of the comparison:-
1. It can result in no significant error being noted on the device under test
2. A significant error being noted but no adjustment made
3. An adjustment made to correct the error to an acceptable level
Strictly speaking, the term calibration means just the act of comparison, and
does not include any subsequent adjustment.
…contd

1. Electrical/Electronic calibration involves either stimulating an electrical signal
or measuring the electrical signal of the instrument being calibrated with respect
to that of a master (standard) instrument.
2. Known reference standards are used for the calibration to ensure traceability.
3. These international standards include Volts, Watts and Amperes, amongst
others.
How electrical/ electronic calibration is carried out?

1. A new instrument
2. After an instrument has been repaired or modified
3. When a specified time period has elapsed
4. When a specified usage (operating hours) has elapsed
5. Before and/or after a critical measurement
Calibration may be required the following reason;

6. After an event, for example
i. After an instrument has been exposed to a shock, vibration, or physical
damage, which might potentially have compromised the integrity of its
calibration
ii. Sudden changes in weather
7. Whenever observations appear questionable or instrument indications do not
match the output of surrogate instruments
8. As specified by a requirement, e.g., customer specification, instrument
manufacturer recommendation.
Calibration may be required the following reason;

In order to understand the concept of errors in measurement, we should know
the two terms that defines the error and these two terms are written below:
True Value
It is not possible to determine the true value of a quantity by experiment means.
True value may be defined as the average value of an infinite number of
measured values when average deviation due to various contributing factor will
approach to zero.
Errors in Measurement

Measured Value
It may be defined as the approximated value of true value.
It can be found out by taking means of several measured readings during an
experiment, by applying suitable approximations on physical conditions.
Now we are in a position to define static error.
Static error is defined as the difference of the measured value and the true
value of the quantity.
Mathematically we can write an expression of error as, dA = Am - At
where, dA is the static error; Am is measured value and At is true value.
“NOTE:- The absolute value of error can not be determined as due to the fact
that the true value of quantity can not be determined accurately.
Errors in Measurement

Limiting Errors or Guarantee Errors
1. The concept of guarantee errors can be cleared if we study this kind of error
by considering one example.
2. Suppose there is a manufacturer who manufactures an ammeter, now he
should promise or declare that the error in the ammeter that he is selling is not
greater than the limit he sets.
3.This limit of error is known as limiting errors
Relative Error or Fractional Error
It is defined as the ratio of the error and the specified magnitude of the quantity.
Mathematically we write as,
Where, dA is the error and A is the magnitude.
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐸𝑟𝑟𝑜𝑟 =
𝑑𝐴
𝐴

Now here we are interested in computing resultant limiting error under the
following cases:
(a)By taking the sum of two quantities:
Let us consider two measured quantities a1 and a2. The sum of these two
quantities can be represented by A. Thus we can write A = a1 + a2. Now the
relative incremental value of this function can be calculated as
𝑑𝐴
𝐴
=
𝑑(𝑎1 + 𝑎2)
𝐴
Separating the each term as shown below and by multiplying and dividing
a1 with the first term and a2 with the second term we have
𝑑𝐴
𝐴
=
𝑎1 𝑑𝑎1
𝐴𝑎1
∗
𝑎2 𝑑𝑎2
𝐴𝑎2

From the above equation, we can see that the resultant limiting error is equal
to the sum of products formed by multiplying the individual relative limiting
errors by the ratio of each term to the function.
Same procedure can be applied to calculate the resultant limiting error due
to summation of more than two quantities. In order to calculate the resultant
limiting error due to difference of the two quantities just change the addition
sign with subtraction and rest procedure is same.

(b) By taking the product of two quantities:
Let us consider two quantities a1 and a2. In this case the product of the two
quantities are expressed as A = a1.a2. Now taking log both sides and
differentiating with respect to A we have resultant limiting errors as
𝑑𝐴
𝐴
=
𝑑𝑎1
𝑎1
+
𝑑𝑎2
𝑎2
From this equation we can see that the resultant error is summation of
relative errors in measurement of terms.

Types of Errors:
Gross Errors
 This category of errors includes all the human mistakes while reading,
recording and the readings. Mistakes in calculating the errors also come
under this category.
e.g while taking the reading from the meter of the instrument he may read
21 as 31. All these types of error are come under this category.
 Gross errors can be avoided by using two suitable measures and they are
written below: 1. A proper care should be taken in reading, recording the
data. Also calculation of error should be done accurately.
2. By increasing the number of experimenters we can reduce the gross
errors. If each experimenter takes different reading at different points, then
by taking average of more readings we can reduce the gross errors.

i. Instrumental Errors
 These errors are inherent in instruments due to wrong construction, calibration
of the measuring instruments.
 These types of error may arise due to friction or may be due to hysteresis.
These types of errors also include the loading effect and misuse of the
instruments.
 Misuse of the instruments results in the failure to the zero adjustment of the
instruments.
In order to minimize the gross errors in measurement various correction factors
must be applied and in extreme condition instrument must be re-calibrated
carefully.
Systematic Errors

ii. Environmental Errors
This type of error arises due to external conditions to the instrument.
External condition includes temperature, pressure, humidity or it may include
external magnetic field.
In order to minimize the environmental errors:
 Try to maintain the temperature and humidity of the laboratory constant
by making some arrangements.
 Ensure that there should not be any external magnetic or electrostatic
field around the instrument.

Observational Errors
As the name suggests, these types of errors are due to wrong observations.
The wrong observations may be due to PARALLAX.
In order to minimize the PARALLAX error highly accurate meters are required,
provided with mirrored scales.
Random Errors
After calculating all systematic errors, it is found that there are still some errors
in measurement are left. These errors are known as random errors. Some of
the reasons of the appearance of these errors are known but still some
reasons are unknown. Hence we cannot fully eliminate these kinds of error.

Sources of Errors:-
The errors arise from the following sources also
1.Noise: It is defined as any signal that does not convey useful information .
 Noise signals/errors can be reduced to a minimum level through filtering,
careful selection of components, shielding and isolation of the entire
measuring system .
2.Response Time: It is defined as the time taken by instrument to show 63.2 %
change in a reading to step input . This factor contributes to the uncertainty
of the measurement
3.Design Limitations: In the design of instrument, certain inevitable factor
such as friction and resolving power lead to uncertainty of measurements.

Continued...
4. Energy exchanged by interaction:- As earlier pointed out, whenever the
energy required for the operating the measuring systems is extracted from
the measurand, the value of the latter is altered to a greater or lesser extent.
This alternation is dependent upon the capacity of the system.
5. Transmission:- During the transmission of information from the primary
sensing element to the indicator, the signal may be attenuated due to any
of the following reasons:
 It may suffer loss through leakage
 it may be absorbed or otherwise consumed in the communication
channel
 it may be distorted by resonance , attenuation or delay phenomenon
whose action are selective on various signal components

Continued...
6.Detremination of measuring system:-
The examples of some type of the deterioration which occur measuring
system and result in a source of error in measurement are :
 Change in resistance of a circuit element through strain relief
 Alteration of thermocouple characteristics through oxidizing or reducing
atmosphere
7. Ambient influences on measuring systems
8. Errors of observation and interpretation

Introduction to instrumentation system

Introduction to instrumentation system

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