Generalized measurement system

SUBJECT
MEASUREMENT SYSTEMS
UNIT NO 1
GENERALIZED MEASUREMENT SYSTEM
BY- PROF. P.B. BORAKHEDE
MGI-COET, SHEGAON

UNIT NO 1
Contents
 Measurement
 Need of Measurement
 Standard
 Instruments and Instrumentation
 Measurement Methods
 Generalized Measurement System
 Classification of Instruments
 Input output configuration
 Difference between mechanical and electrical devices.
2
Prof. P.B. Borakhede, MGI-COET, Shegaon

MEASUREMENT
 The word measurement is used to tell us the length.
Weight, temperature, color or change in one of these
physical entities of a material.
 Measurement is a result of opinion formed by one or
more observers about relative size or intensity of some
physical quantity.
 The opinion is formed by observer after comparing the
object with object of same kind taken as unit, called
standard.
 Measurement is comparison of physical quantity with
reference standard. 3

NEED OF MEASUREMENT
 To indicate the exact quantity of any entity measurement is
needed.
 For example:-
 We know that moon is at a very large distance from earth,
but exactly how much?????
 I need to buy some food like grains from shopkeeper, but
again exactly how much quantity???
 To ensure that parts to be measured confirms to established
standards.
 To determine true dimensions of parts.
 For evaluating the performance of the systems.
4

 Measurement Process
5

Example:-
1. Measuring a line with a ruler.
2. Weighing vegetables on pan balance.
6

 In case of process industries and industrial
manufacturing
1. To improve quality of product.
2. To improve efficiency of production.
3. To maintain proper operation.
7

STANDARD
Standard is a level of quality or attainment.
A standard is an exact quantity that people agree to
use for comparison.
For consistence and quantitative comparison of physical
parameters, certain standards of mass, length, time,
temperature and electrical quantities have been
established.
These standards are internationally accepted and well
preserved under controlled environmental conditions.
Examples- gram, kilogram, second, Kelvin ampere etc.
8

Organizations of standards
 ISO- International Organization for Standardization
 BIS- Bureau of Indian Standards
 ISI- Indian Standards Institute
 IBS- International bureau of standards.
9

Classification of standards
The accepted standards for the measurement of physical
quantities are classified into following categories:
1. Primary Standards:
 They constitute ultimate basis of reference and are used for
the purpose of verification and calibration of secondary
standards.
 They are material standard preserved under most careful
conditions.
 These are not used for directly for measurements.
 These are copies of international prototypes and are kept
throughout the world in national standard laboratories and
institutions for similar standing.
10

Following points are to be considered while setting up
primary standards:
 Low temperature coefficient and long time stability of
material.
 No deterioration in its characteristics with changes in
humidity, temperature and other environmental
conditions.
 Rigidity of construction and accuracy of machining.
 Physical dimensions which determine the accuracy of
standard are measured with most sophasticated
techniques.
11

2. Secondary Standards:
 These are reference standards designed and calibrated from the
primary standards.
 These standards are send periodically to national standards
laboratories for their calibration.
 The secondary standards are kept at industrial organizations and
measurement laboratories to check and calibrate general tools for
their accuracy and precision.
 For example suppose a weight in a pan balance is being used for
long time, there is deterioration in that weight after some interval of
time. This weight can be calibrated by comparing with secondary
standard weight which is placed in laboratory. 12

3. Working Standards
They are normal standards used by technicians and
workers who are actually carrying out the operational
measurement.
The working standards have accuracy lower than
secondary standards.
In a factory, the production measuring instruments are
checked against working standards.
13

INSTRUMENTS AND INSTRUMENTATION
Instruments
The human senses cannot provide exact quantitative
information about the knowledge of events occurring in
our environments.
The devices used to measure physical quantities are
called instruments.
Instrumentation
The technology of using instruments to measure and
control physical and chemical properties of materials is
called instrumentation.
14

 Uses:-
1. study the function of different components and
determine the cause of all functioning of the system
2. to test a product on materials for quality control
3. to discover effective components.
4. monitor a data in the interest of health and safety.
5. to develop new theories.
15

MEASUREMENT METHODS
 Direct and Indirect Measurements
 Primary, Secondary and Tertiary Measurements
Contact and Non Contact Type
16

1. Direct and Indirect Method
 Measurement is a process of comparison of the physical
quantity with a reference standard.
 Depending upon requirement and based upon standards
employed, there are two basic methods of measurement.
a) Direct Measurement Method:-
 The value of physical parameter ( measurand) is
determined by directly comparing it with reference
standards.
 Physical Quantities like mass, length, and time are
measured by direct comparison.
17

 Direct measurements are not to be preferred because
they involve human factors are less accurate and less
sensitive.
 Further, the direct methods may not always be possible,
feasible and practicable.
Examples:
Measurement of line by steel rule.
b) Indirect Methods
 The value of measurand is more generally determined
by indirect comparison with secondary standards
through calibration.
 The measurand is converted into an analogous signals
which is subsequently processed and fed to end device
that present the result of measurement. 18

 The indirect technique saves primary or
secondary standards from a frequent direct
handling.
Example:
Measurement of Pressure by Pressure Gauge.
Measurement of speed by tachometer.
19
Direct Measurement
Indirect Measurement

2. Primary, Secondary and Tertiary Methods
The complexity of an instrument system depends upon
the measurement being made and upon accuracy level
to which measurement is needed.
Based on complexity of measurement systems,
measurements are grouped into three categories:
1. Primary Measurement
2. Secondary Measurement
3. Tertiary Measurement
20

1. Primary Measurement:-
 In primary measurement, value of physical parameter
is determined by comparing it directly with reference
standard.
 The required information is obtained through sense
and touch.
 Examples:
a) Matching of two lengths when determining the length
of object with ruler
b) Matching of two colors when judging temperature of
red hot steel.
c) Measurement of time by counting number of strokes of
a clock.
d) Estimating temperature difference between contents of
containers by inserting fingers.
21

 Primary measurement provides subjective information
only.
 These measurements are not accurate and time
consuming.
 Primary measurement is also called direct method.
2. Secondary Measurement:-
The indirect measurement involving one translation are
called secondary measurements.
Secondary measurement is indirect measurement method.
Example:-
1. Measurement of temperature by thermometer.
22

2. Measurement of pressure by means of bellows.
3. Pressure measurement by manometers.
3. Tertiary Measurement
 The indirect measurement involving two or more
conversions are called tertiary measurements.
 Tertiary measurements are also called indirect
measurements. 23
Thermometer Bellow Pressure Gauge

Examples:
1. Measurement of static pressure by a bourdon tube
pressure gauge.
2. Measurement of speed by tachometer.
24
Bourdon Tube Pressure Gauge

3. Contact and Non Contact Measurements
Measurements can also be classified as:
1. Contact type Measurement
 In contact type measurement system the measurand is
in direct contact with measuring instruments.
 In this sensing element is in direct contact with
measurement.
Example: 1) Speed measurement by tachometer,
2) Temperature measurement by mercury in thermometer.
25

2. Noncontact Type Measurement
 In non contact type measurement method, sensor of
measuring instrument is not in direct physical contact with
measurand.
 Examples:
1. Measurement of temperature by Infrared thermometer.
2. Speed measurement by stroboscope.
26

GENERALIZED MEASUREMENT SYSTEM
 The principle of an instrument is the acquisition of
information by sensing, processing of information and its
final presentation to a human observer.
 For the purpose of analysis and synthesis, instruments
are considered as systems.
 Different components are called elements and they
perform certain definite and required steps in the act of
measurement.
 The whole operation can be described in terms of
functional elements.
27

 Measuring systems contain following functional
elements:
1. Primary Sensing Element
2. Variable Conversion Element
3. Manipulation Element
4. Data Transmission Element
5. Data Presentation Element.
28

29
Block Diagram of Generalized Measurement System

1. Primary Sensing Element:
 The quantity or the variable which is being measured
makes its first contact with the primary sensing element of
a measurement system.
 The measurement is thus first detected by primary sensor
or detector.
 The sensing element sense the condition , state(or) value
of the process variable by extracting a small part of energy
from the measurement and produces an output which is
proportional to the input.
 The measurement is then immediately converted into an
analogous electrical signal.
30

2. Variable Conversion Element:
 It is also call transducer element.
 It is an element that converts signal from one physical
form into another without changing information content
of the signal.
 The signal after transduction is more suitable for
purpose of measurement and control.
 The transduction may be from mechanical, electrical, or
optical to any other related form.
Examples:
 Bourdon tube and bellows which transforms pressure
into displacement.
 Rack and pinion which converts linear to rotary motion
and vice versa. 31

3. Data Transmission Elements:
 It is an element that transmits the signal from one
location to another without changing the information
content.
 Data may by transmitted over long distances (from one
location to another) or short distances (from a test
centre to a nearby computer).
 Data transmission elements may be shaft and gearing
assembly, transmitting signals via data cables, wires,
change in voltage current etc.
32

4. Variable Manipulation Elements:
 Variable manipulation means a change in numerical
value of the signal.
 The function of a variable manipulation element is to
manipulate the signal presented to this element while
preserving the original nature of the signal.
 For example, a voltage amplifier acts as a variable
manipulation element.
 The amplifier accepts a small voltage signal as input
and produces an output signal which is also voltage but
of greater magnitude.
 Another example is gearing arrangement. Gearing
arrangement increases or decreases rotating speed of
shaft without changing its nature.
33

5. Data presentation Elements:
 This element provides a display record or indication of
the output from the manipulation elements.
 To monitor the data, visual display devices are needed.
 These devices may be analogue or digital indicating
instruments .
34

Example:-
Functional elements of bourdon tube
measurement system in form of
generalized measurement system.
35

As an example of a measurement system, consider the
simple Bourdon tube pressure gauge.
In this case, the Bourdon tube acts as the primary
sensing element and a variable conversion element.
It senses the input quantity (pressure in this case).
This tube also converts pressure sensed into
displacement of air in the tube. Thus, the pressure is
converted into a small displacement.
The closed end of the Bourdon tube is connected
through mechanical linkage to a sector-pinion gearing
arrangement.
The gearing arrangement amplifies the small
displacement and makes the pointer to rotate through a
large angle. 36

The mechanical linkage thus acts as a data
transmission element while the gearing arrangement
acts as a data manipulation element.
The dial scale on the gauge body plays the function of
data presentation element and conveys the information
about the quantity being measured.
The information conveyed by this device is in analogue
form.
37

Pressure Actuated Thermometer
 This device is used as a thermometer for measurement
of temperature.
 We will discuss functional elements of this system.
38

 In this device the liquid filled bulb acts as a primary
sensor and variable conversion element.
 It senses the input ( temperature), receives input signal
in the form of thermal energy.
 After sensing input temperature by bulb there is thermal
expansion of fluid (mercury) in the bulb. 39

 The pressure tube is employed to transmit pressure to
the bourdon tube and thus functions as data
transmission element.
 The bourdon tube converts fluid pressure into
displacement of its tip so it acts as variable conversion
element.
 The displacement is manipulated by linkage and
gearing to give a pointer motion so linkage and gearing
acts as variable manipulation element.
 The scale and pointer serves as data presentation
element.
40

Digital Revolution Counter
This device is used for indicating rotation of shaft.
We will identify various functional elements of the
measurement system.
41

 Sensing arm is operated by rotating shaft, operates
micro switch and gives a sequence of electrical pulses.
 These pulses are transmitted over long distance to a
solenoid which converts electrical pulses to mechanical
reciprocation of plunger.
 Indications of the shaft revolutions are then given on the
mechanical counter.
 The measurement system can be considered to be
comprised of the following functional elements.
42

43
Sensing arm senses rotation of the shaft so it acts as
primary sensing device.
Sensing arm also mechanically oscillates so its also
acts as variable conversion element.
Micro switch converts mechanical oscillations into
electrical oscillations by micro switch. (Variable
conversion element).

 Transmission of electrical oscillations to solenoid by
electrical wires ( data transmission element).
 Conversion of electrical pulses into mechanical
reciprocation of the plunger by solenoid ( variable
conversion element).
 Mechanical counter manipulates the data ie
reciprocating motion of plunger into rotary motion of
digits. (manipulation element).
 It also gives the result to observer (presentation
element).
44

Classification of Instruments
Instruments are classified as follows:
1. Automatic and Manual Instruments
 Manual instruments required the service operator.
 Automatic instruments does not required service of an
operator.
For example temperature measurement by mercury in
glass thermometer is an automatic instrument as the
instrument indicates the temperature without requiring
any manual assistance.
However, the measurement of temperature by a
resistance thermometer incorporating; Wheatstone
brigde in its circuit is manual in operation as it needs an
operator.
45

2. Self Generating and power operated instruments
Self-generated instruments
 These instruments are also called as active instruments.
 In self generating instruments, output energy is supplied
entirely by input signal.
 The instrument does not require any out side power in
performing its function.
 Example:
In mercury in glass thermometer, thermal expansion of
mercury is wholly supplied by input it doesnot need any
external power source.
 Other examples are bourdon pressure gauge, pitot tube
for measuring velocity.
46

Power Operated Instruments
Some instruments needs external power source for
operation. These instruments are called power
operated instruments.
These instruments are also called passive instruments.
For example:
Infrared thermometers need photovoltaic cells to
measure temperature.
LVDT used in measurement of displacement, force,
pressure etc.
Resistance thermometers and thermisters.
47

3. Self contained and Remote Indicating
Instruments
The different elements of a self-contained instrument
are contained in one physical assembly.
In a remote indicating instrument, the primary
sensing element may be located at a sufficiently long
distance from the secondary indicating element.
In the modern instrumentation technology, there is a
trend to install remote indicating instruments where
the important indications can be displayed in the
central control rooms.
48

4. Deflection and Null Output Instruments
In null-type instruments, the physical effect caused by
the quantity being measured is nullified (deflection
maintained at zero) by generating an equivalent
opposing effect.
The equivalent null causing effect then provides a
measure of the unknown quantity.
In null type devices are slow in operation, have poor
dynamic response.
They do not interfere with state of quantity to be
measured.
Example:
Measurement of mass of object by pan balance.
49

A deflection type instrument is that in which the
physical effect generated by the measuring quantity
(measurand) is noted and correlated to the
measurand.
These instruments are simple in construction and
operation, have good dynamic response.
They interfare with state of measurand and as such
donot determine exact state/value/condition.
50

5. Analog and Digital Instruments
The signals of an analog unit vary in a continuous
fashion and can take on infinite number of values in a
given range.
Examples: Wrist watch, speedometer, fuel gauge,
ammeters, voltmeters etc.
Digital instruments converts a measured analog voltage
into digital quantity which is displayed numarically.
The digital instruments have advantage of high accuracy,
high speed as there is elimination of human operational
errors.
However these devices are unable to indicate quantity
which is part of step value of instruments.
51

Input-Output configuration of Measuring
Instruments
An instrument performs an operation on an input
quantity(measurand) to provide an output (measurement).
Input is denoted by “ i ” and output is denoted by “ o “.
According to performance of instrument it can be stated in
terms of an operational transfer function G.
52
i
G
o

The various inputs to a measurement system can be
classified into three categories:
1. Desired Input:
 A quantity for which the instrument is specifically
made to measure is called desired input.
 The desired input iD produces an output component
according to an input-output relation symbolized by GD
. GD represents mathematical operation necessary to
obtain output from input.
2. Interfering Input:
 A quantity to which the instrument is unintentionally
sensitive.
 The interfering input iI would produce an output
component according to input-output relation
symbolized by GI .
53

3. Modifying Input:
A quantity that modifies input-output relationship for both
desired and modified inputs is called modifying input.
The modifying input iM will cause change in GD and or
GI .
The output of measuring apparatus is instantaneous
algebraic sum of output component due to desired,
interfering and modifying inputs.
54

55
Output

Example:
Take an example of differential manometer.
 Manometer is used to measure pressure difference.
56

 Manometer consist of U tube filled with mercury and with
its ends connected to two points between which
pressure difference is to be measured.
 In figure 1 P1 and pressure P2 are desired inputs and
the difference is measured by scale h.
 Suppose manometer is placed on wheel which is
subjected to acceleration.
 Pressure differential gets created due to acceleration
and scale indicates reading even though pressures P1
and P2 at two ends are equal.
 Here the acceleration is an interfering input.
57

 Suppose the manometer is placed on wheels and it is
subjected to acceleration. If there is temperature
variation due to change of location of manometer, the
manometer scale will shows different reading of
pressure difference.
 Here temperature variation acts as modifying input.
 Here temperature variation affects on desired input as
well as interfering input.
58

Methods to minimize Interfering and Modifying Inputs
It is necessary to reduce effect of interfering and
modifying inputs.
Various methods to accomplish this task are as follows:
1. Signal Filtering
2. Compensating by opposing inputs
3. Output Corrections
59

1. Method of Signal Filtering
 Filters are introduced into the instruments in the path of
spurious inputs( interfering and modifying).
 The filters are designed so as they allows desired input
signals.
 The filters can be introduced into measurement system
at one of its three stages (input, intermediate, and output
stage), depends upon application.
2. Compensation by opposing inputs
 This method consists of intentionally introducing
interfering/ modifying inputs to cancel bad effects of
unavoidable inputs.
60

Example:
If a bourdon tube is measuring pressure of an object, but
because of ambient temperature there is error in
pressure reading.
To compensate this problem another bourdon tube is
placed near measuring tube, it will sensitive only
ambient temperature.
61

This tube will measure pressure due to temperature only.
That pressure reading will be minus from the reading of first
tube.
3. Output Correction
The method involves in calculating corrections which may
be added to or subtracted from the indicated output so as
to leave only that component which is associated with
desired input.
Example
In strain gauge knowledge of temperature coefficient of
resistance, temperature sensitivity of gauge and
operating temperature helps to obtain the correction to
the output.
62

Difference between Mechanical and
Electrical/ electronics instruments
Mechanical Instruments
Advantages
Salient Features of Mechanical Instruments are
 Long history of development and successful use.
 Simple in design and use
 Durable due to rugged construction
 Relatively low cost
 No need of external power supply
 Reliable and accurate for measurement
Disadvantages
 Poor frequency response to dynamic transient
measurement. 63

 Due to inertia of moving parts, instruments do not follow
rapid changes of input signals.
 Incompatibility when remote indication or control is
needed.
 Requirement of large forces to overcome mechanical
friction
 Potential source of noise.
Electrical/electronics Instruments
Now a days electrical/electronic techniques are being
increasingly applied to measurements in many fields.
64

Advantages
 Instruments are light in weight
 Instruments are compact
 Good frequency to transient response
 Feasibility of remote indication and recording
 Less power consumption and less load on the system
being measured.
 Possibility of non contact measurement
 Possibility of mathematical processing of signals like
summation, differentiating and integration.
 These instruments are used to build analog and digital
computers.
65

IMPORTANT QUESTIONS
1. Describe the function performed by each element of
generalized measurement systems.
2. Identify and explain functional elements of bourdon
tube pressure gauge with neat diagram.
3. What are the applications of measurement system ?
Discuss.
4. Draw the input-output configuration for the
measurement system & explain the various types of
inputs with suitable examples.
5. What are primary, secondary & tertiary measurements?
Explain each with practical examples.
66

6. Analyze the following devices as generalized
measurement system.
a) Pressure actuated thermometer
b) Bourdon tube pressure gauge
7. Explain the function of pressure actuated thermometer.
Draw its schematic diagram.
67

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