MODULE 4
DEFINITIONS
MEASUREMENT :
It is the act of result of quantity comparison between a pre-
determined standard and unknown magnitude. For comparison we
should have proper operation and procedure and it must be
reliable.
Measurement provides us with a means of describing various
phenomenon in quantitative terms. The result is expressed in terms
of numerical value. In order that the result of a measurement are
meaningful and these are two basic requirements
1. The standard used for comparison purpose must be accurately
defined and should be universally and commonly accepted.
2. The apparatus used and method adopted for the purpose of
comparison must be provable.

SIGNIFICANCE OR AIMS OF MEASUREMENT
(RESEARCH & DEVELOPMENT (R &D)
In the engineering field R & D program, measurement and
its interpretation is of great importance. Measurement is a
vast field which embraces detection, acquisition, control
and analysis of data. It involves the measurement of
physical, electrical, mechanical, optical and chemical
quantities and plays a very significant role in every branch
of scientific research and engineering processes, which
include control systems, process instrumentation and data
reduction. Measurement and Instrumentation in every field
from machine to satellite.

Mechanical Measurements
The measurement involved in the field engineering and technology are
classified into 2 broad categories
1. Mechanics type of measurements
2. Power type of measurements
1. Mechanics type of measurements:
Mechanics type of measurements are used as a design tool for experiment,
research and developmental programs. Complete solution of any
mechanical engineering design problem of any mechanical engineering
design problem requires the following 3 methods
(i) Empherical method
(ii) Rational method
(iii) Experimental method

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 The Empherical design method is based upon
INTUTION and good engineering judgement
on the part of the designer. The designer
usually makes this judgement, that is an
outcome of his or other designer’s past
experience of working similar designs. Since
this method of design is mainly based upon
the judgement of the designer, which may
prove wrong, it cannot be relied upon
especially in the case of design of complete
systems.

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The rational design method is strictly based upon well
established scientific laws and relationships. These laws
are mainly available in the areas of mechanics and
thermodynamics. While in other areas of mechanical
engineering their availability is rather therefore in areas
othrerthan mechanics and thermodynamics. The use of
rational design method has a very limited scope.
In the experimental method, the product based upon an
initial design tested. Accurate and corrected
measurements of all physical quantities involved are
done. The results are scientifically analyzed and process
are interpreted. Then by TRIAL AND ERROR method the
parameters are adjusted till the product or system design
gives the desired performance.

2. Power type of measurement
These are operational measurements which are usually
a part of a control system for example: in order that a
steam power plant operates reliably and economically
many quantities have to be continuously monitored,
some of the quantities are steam pressure,
temperature, flow rate along with speed of the turbine,
quantities of coolant etc., these are compared with the
corresponding reference quantities and the error signal
that are generated.

FUNDAMENTAL METHOD OF MEASUREMENT
The fundamental method of measurement is classified into two types
1. Direct Method :
In these methods the unknown quantity is directly compared against the standard. The result is
expressed as a numerical number or unit. Direct methods are common for the measurement of
length, mass and time. Although the measurement of length and time is easier than
measurement of mass.
2. Indirect Method :
Measurement of direct methods are not always possible, feasible and practicable which may be
inaccurate because they involve human factors.
A measuring system consists of a transducing element which converts the quantity to be
measured into analogous signal. The analogous signal is then proceeded by some intermediate
means and it then fed to the end devices which present the results of the measurement.

1. CALIBRATION:
Every measuring system must be provable i.e it must prove its ability to measure reliably. The
procedure for this is called calibration. It consists of determining the system’s scale, at some
point during the preparation of the system for measurement, known magnitude of the basic
input quantity must be fed into the detector-transducer and the system behavior must be
observed.
Calibration is usually carried out by making adjustments such that readout device produces
zero-output for zero-measured input and similarly it should display an output equivalent to the
known measured input near the full scale input value.
It is important that any measuring system calibration should be performed under environmental
conditions that are as close as possible to those conditions under which actual measurements
are to be made.

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It is also important that the reference measured input
should be known to much greater degree of accuracy,
usually the calibration standard for the system should
be atleast one order of magnitude more accurate
than the desired measurement system accuracy i.e
accuracy ratio of 10:1. The static and dynamic
calibration systems are used in calibration of
measuring system.

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TRANSDUCERS

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GENERALIZED DESCRIPTION OF A MEASURING
SYSTEM

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