Measurements & Measurement .Systems.pptx

MANIPAL INSTITUTE OF TECHNOLOGY
Department of Mechanical & Manufacturing Engineering, MIT, Manipal 1
Measurements
&
Measurement Systems

MEASUREMENT SYSTEM
• Measurement means determination of anything that exists in some
amount.
• If those things that exist are related to mechanical engineering, then
the determination of such amounts are referred to as mechanical
measurements.
• An engineer is not only interested in the measurement of physical
variables but also concerned with their control.
• These two functions are closely related because one must be able to
measure a variable such as temperature, or flow in order to control
it.

Definition of Measurement :
Measurement is defined as the process or the act of obtaining a
quantitative comparison between a predefined standard and an
unknown magnitude.

Requirements of Measurements

SIGNIFICANCE OF MEASUREMENT SYSTEM
Measurement provides the fundamental basis for research and
development. Development is the final stage of the design
procedure involving the measurement of various quantities
pertaining to operation and performance of the device being
developed.
• Measurement is also a fundamental element of any control
process, which requires the measured discrepancy between the
actual and the desired performances.

• Many operations require measurement for proper performance .
For example :In modern central power stations, temperatures, pressures, vibrational
amplitudes etc., are monitored by measurement to ensure proper performance .
• Measurement is also a bias of commerce, because the cost of the products are
established on the basis of amounts of materials, power, expenditure of time and labour
, and other constraints .

Methods of Measurements
• Direct Methods
In these methods, the unknown quantity (measurand) is directly
compared against a standard. The result is expressed as a numerical
number and a unit. The standard in fact is a physical embodiment of a
unit.
E.g.: measurement of length, mass , time.

• Indirect Methods
A measurement system consists of a transducing element which
converts the quantity to be measured in to an analogous signal.
The analogous signal is then processed by some intermediate
means and is then fed to the end devices which present the results
of the measurement.

Primary, Secondary & Tertiary Measurements
• Primary Measurement
It is one that can be made by direct observation without involving
any conversion of the measured quantity in to length where the
observer can sense the change directly.
E.g.: matching of two lengths, two colours, counting of strokes of
a clock chime to measure the time.

• It is clear that there are two distinct categories of length measurement.
1. A primary measurement of the length of the quantity itself
2. The length of travel of an indicator over a calibrated scale where length unit may
represent any quantity under measurement ( temperature, pressure, speed etc.). This
type of length measurement may be secondary or tertiary.

• Secondary Measurement
It involves only one translation (conversion) to be done on the
quantity under measurement to convert it in to a change of length.
E.g.: to measure pressure of gas it requires
1. An instrument which translates pressure changes in to length
changes
2. A length scale or a standard which is calibrated in length units
equivalent to known changes in pressure.
Therefore in a pressure gauge the primary signal (pressure) is
transmitted to a transducer and the secondary signal (length) is
transmitted to observer’s eye.

• Tertiary Measurement
It involves two translations.
E.g.: measurement of temperature of an object by thermocouple.
1. The primary signal (temperature) is transmitted to a translator
which generates a voltage which is a function of the
temperature.
2. The voltage in turn is applied to a voltmeter through a pair of
wires to convert voltage in to length. The tertiary signal
(length change) is transmitted to the observer’s brain.

Measurement of Rotating shaft

Generalized Measurement System

A generalized Measurement system consists of the
following:
1. Basic Functional Elements
2. Auxiliary Functional Elements

Most measuring systems fall within the framework of a general
arrangement consisting of three phases or stages:
• Stage 1. A detection-transduction, or sensor -transducer, stage
• Stage 2. An intermediate stage, which we shall call the signal
conditioning stage
• Stage 3. A terminating, or readout-recording, stage

1. Basic Functional Element
They are those that form the integral parts of all instruments.
They are the following
1. Transducer Element that senses and converts the desired input
to a more convenient and practicable form to be handled by
the measurement system.
2. Signal Conditioning or Intermediate Modifying Element for
manipulating/processing the output of the transducer in a
suitable form.
3. Data Presentation Element for giving the information about
the measurand or measured variable in the quantitative form.

Examples of the three stages of a generalized
measurement system

2.Auxiliary Functional Element
They are those which may be incorporated in a particular system depending on the
type of requirement, the nature of measurement technique etc.
1. Calibration Element to provide a built-in calibration facility.
2. External power Element to facilitate the working of one or more of
the elements like the transducer element, the signal conditioning
element, the data processing element or the feedback element.
3. Feedback Element to control the variation of the physical quantity
that is being measured. In addition feedback element is provided in
the null-seeking potentiometric or Wheatstone bridge devices to
make them automatic or self- balancing.
4. Microprocessor Element to facilitate the manipulation of data for
the purpose of simplifying or accelerating the data interpretation. It
is always used in conjunction with analog-to-digital converter
which is incorporated in the signal conditioning element.

Basic & Auxiliary Functional Elements of a Measurement System

Functional Element of the instruments
Bourdon Tube Pressure Gauge

Bourdon Tube Pressure Gauge with electrical read-out

Electro-dynamic Displacement Measuring Device

Transducer Element

Desirable Characteristics of Transducer Element
• The transducer element should recognize and sense the desired
input signal and should be insensitive to other signals present
simultaneously in the measurand.
• It should not alter the event to be measured.
• The output should preferably be electrical to obtain the advantages
of modern computing and display devices.
• It should have good accuracy
• It should have good reproducibility(precision)
• It should have amplitude linearity

• It should not induce phase distortions (i.e. should not induce
time-lag between the input and the output transducer signals)
• It should be able to withstand hostile environment without
damage and should maintain the accuracy within acceptable
limits.
• It should be easily available reasonably priced and compact in
shape and size (preferably portable).
• It should have good reliability.

Signal Conditioning Element
• The output of the transducer element is usually too small to
operate an indicator or a recorder. Therefore, it is suitably
processed and modified in the signal conditioning element.
• The transducer signal may be fed to the signal conditioning
element by means of either mechanical linkages (levers, gears,
etc.), electrical cables, fluid transmission through liquids or
through pneumatic transmission using air. For remote
transmission purposes, special devices like radio links or
telemetry systems may be employed.
• The signal conditioning operations that are carried out on the
transduced information may be one or more of the following:

The term amplification means increasing the amplitude of the signal
without affecting its waveform. A suitable amplifying element is incorporated
in the signal conditioning element which may be one of the following
depending on the type of transducer signal.
1. Mechanical amplifying elements such as levers, gears or a combination
of the two designed to have a multiplying effect on the input transducer
signal.
2. Hydraulic/Pneumatic amplifying element employing various types of
valves or constrictions, such as venturimeter/ orificemeter, to get
significant variation in pressure with small variation in the input
parameters.
3. Optical amplifying elements in which lenses, mirrors and combinations
of lenses and mirrors or lamp and scale arrangement are employed to
convert the small input displacement into an output of sizable magnitude
for a convenient display of the same.

4. Electrical amplifying element employing transistor circuits,
integrated circuits, etc. for boosting the amplitude of the
transducer signal. In such amplifiers we have either of the
following

Data Presentation Element
This element gathers the output of the signal conditioning
element and presents the same to be read or seen by the
experimenter.
This element should:
1. Have as fast as response as possible
2. Impose as little drag on the system as possible
3. Have very small inertia, friction, stiction, etc. (hence using
light rays and electron beams is advantageous)

• This element may be either of the visual display type, graphic
recording type or a magnetic tape. In the visual display type element,
devices such as pointer and scale / panel meter, multi-channel CRO,
storage CRO, etc. may be employed.
• The graphic recording type element gives a permanent record of the
input data. The device in this element may be pen recorders using
heated stylus, ink recorders on paper charts, optical recording systems
such as mirror galvanometer recorders or ultraviolet recorders on
special photosensitive paper. Further a magnetic tape may be used to
acquire input data which could be reproduced at a later date for
analysis.
• In case the output of the signal conditioning element is in the digital
form, then the same may be displayed visually on a digital display
device. Alternatively, it may be suitably recorded either on punched
cards, perforated papers tape, magnetic tape, typewritten page or a
combination of these systems for further processing.

Performance Characteristics of Instruments
The detailed specifications of the functional characteristics of any
instrument are termed as its performance characteristics. These are in
general, indicative of the capabilities and limitations of the instrument
for a particular application.
Therefore, the knowledge of the performance characteristics is quite
important as it enables us to have quantitative estimates of the positive
as well as negative points of various commercially available
instruments.
Consequently, one can select the optimum type of instrument for the
given application.
Instrument performance characteristics can be broadly classified as:
• Static Characteristics (For time-independent)
• Dynamic Characteristics (For time-dependent)

• In a number of situations, the desired input to the instrument
may be constant or varying slowly with respect to time. In
these situations, the dynamic characteristics are not important.
• Therefore, the various static performance parameters like
accuracy, precision, resolution, sensitivity, linearity,
hysteresis, drift, overload capacity, impedance loading, etc. are
usually good enough to give meaningful quantitative
descriptions of the instrument.

Static Calibration
• Calibration is a comparison between measurements – one of
known magnitude or correctness made or set with one device
and another measurement made in as similar a way as possible
with a second device.
• Static Calibration refers to a procedure where an input, either
constant or slowly time varying, is applied to an instrument
and corresponding output measured, while all other inputs
(desired, interfering, modifying) are kept constant at some
value.
• Instruments are so constructed that the signal conversion they
perform have the property of irreversibility or directionality.
This implies that a change in an input quantity will cause a
corresponding change in the output.

Static Performance Characteristics of
Instruments

• Linearity
• Static Sensitivity
• Repeatability
• Hysteresis
• Threshold
• Readability
• Range / Span
• Accuracy
• Precision
• Resolution
• Dead Band
• Backlash
• Drift
• Impedance Loading and
Matching

Linearity
• If the relationship between input and output.
• Achieving linearity is practically not possible. The variation
from ideal line is known as linearity tolerances. Independent
Linearity and Proportional Linearity are two form of linearity
• In Independent linearity, the value of the output will lie within
the specified tolerance lines as shown in fig (a).
• In proportional linearity, the value of output will lie within the
specified proportion of the recorded value as shown in the fig
(b).

• The instruments which do not possess linearity may be linear
over a range of usage, i.e. the deviation of the output may not
vary with the input for a part of range and may show
proportional variation for the rest of the range as shown in the
figure below

Static Sensitivity
• Static Sensitivity: - The static sensitivity is defined as the slope
of the calibration curve
• That is given by
• If the input-output relation is linear, the sensitivity is constant
for all values of input. The sensitivity of nonlinear instrument
depends on the input quantity. The two curves for both linear
and nonlinear are shown in the figures below

When the instruments sensitivity to its desired input is of primary
importance its sensitivity to interfering and modifying inputs may
also be considered.

Repeatability
• If an instrument is used to measure same or an identical input
many times and at different time intervals, the output is not the
same but show a scatter. This scatter or deviation from the
ideal static characteristics in absolute units or a fraction of the
full scale, is called repeatability error.

• Resolution is defined as the smallest increment of the
measured value that can be detected by certainty by the
instrument. The least count of any instrument is taken as the
resolution of the instrument.
• Accuracy of an instrument can be defined as the ability of the
instrument to respond to true value of the measured variable
under reference condition. In other word it is the closeness
with which the instrument reading reaches the true value of
quantity being measured.
• Precision is a measure of reproducibility of the measurement,
given a fixed value of quantity or the degree of exactness for
which instrument is designed or intended to perform. In other
words, precision is measure of degree of agreement within the
group of measurement.

Hysteresis
• It can be defined as the different readings taken down when an
instrument approaches a signal from opposite directions.
• That is the corresponding value taken down as the instrument
moves from zero to midscale will be different from that
between the midscale and full scale reading.
• The reason is the appearance of stresses inside the instrument
material due to the change of its original shape between the
zero reading and the full scale reading.
http://www.instrumentationtoday.com/

Threshold
• If the input to the instrument is gradually increased from zero,
a minimum value of that input is required to detect the output.
• This minimum value of the input is defined as the threshold of
the instrument.
• The numerical value of the input to cause a change in the
output is called the threshold value of the instrument.
• Both threshold and resolution are not zero because of various
factors like friction between moving parts , play or looseness
in joints (backlash), inertia of the moving parts, length of the
scale, spacing of graduations, size of the pointer, parallax,
effect etc.

Readability - The readability depends both on the instrument and the
observer.
Refers to the ease with which the readings of a measuring instrument
can be read.
Reproducibility - It can be defined as the ability of an instrument to
produce the same output repeatedly after reading the same input
repeatedly, under the same conditions. The span refers to the range of
the instrument.
Range - It can be defined as the measure of the instrument between
the lowest and highest readings it can measure. A thermometer has a
scale from −40°C to 100°C. Thus the range varies from −40°C to
100°C.
Span - It can be defined as the range of an instrument from the
minimum to maximum scale value. In the case of a thermometer, its
scale goes from −40°C to 100°C. Thus its span is 140°C.

Backlash
• It is defined as the maximum distance or angle through which any
part of the mechanical system may be moved in one direction
without causing motion of the next part. The output-input
characteristics of an instrument system with backlash error is similar
to hysteresis loop due to Coulomb’s friction. Backlash error can be
minimized if the components are made to very close tolerances.

• Drift: - Drift is the change in the reading of an instrument of
a fixed variable with time.
There are many environmental factors which causes drift. These may be stray
electric/ magnetic fields, thermal emf changes in temperature, mechanical
vibrations, wear and tear and high mechanical stresses developed in some parts of
instruments and systems.

TYPES OF ERRORS
Error can be defined as the difference between the measured and the
true value (as per standard).
Systematic or Cumulative Errors
Such errors are those that tend to have the same magnitude
and sign for a given set of conditions. Because the algebraic sign is
the same, they tend to accumulate and hence are known as
cumulative errors.
Since such errors alter the instrument reading by a fixed
magnitude and with same sign from one reading to another, therefore
the error is commonly termed as instrument bias.
These types of errors are caused due to the following:

Instrument Errors
Certain errors are inherent in the instrument systems.
These may be caused due to poor design/ construction of the
instrument. Errors in the divisions of graduated scales, inequality
of the balance arms, irregular spring tension, etc. cause such
errors.
Instrument errors can be avoided by:
I. Selecting a suitable instrument for a given application
II. Applying suitable correction after determining the amount of
instrument error
III.Calibrating the instrument against a suitable standard

Environmental error
These types of errors are caused due to variation of conditions
external to the measuring device, including the conditions in the
area surrounding the instrument. Commonly occurring changes in
environmental conditions that may affect the instrument
characteristics are the effects of changes in temperature,
barometric pressure, humidity, wind forces, magnetic or
electrostatic fields, etc.
For example, change in ambient temperature causes errors
due to expansion of the measuring tape. Similarly, buoyant effect
of the wind causes errors on weights of a chemical balance.

Loading Error
Such errors are caused by the act of measurement on the
physical system being tested.
Common examples of this type are:
I. An obstruction type flow meter may partially block or absorb
the flow conditions and consequently the flow rate shown by
the meter may not be same as before the meter installation.
II. Introduction of a thermometer alters the thermal capacity of
the system and thereby changes the original state of the
system which gives rise to loading error in the temperature
measurement.

Accidental or Random Errors
These errors are caused due to random variations in the parameter
or the system of measurement. Such errors vary in magnitude and
may be either positive or negative on the basis of chance alone.
Since these errors are in either direction, they tend to compensate
one another. Therefore these errors are also called chance or
compensating type of errors.
The presence of such errors is detected by a lack of
consistency in the measured values when the same input is
imposed repeatedly on the instrument.
In a well designed experiment very few random errors
occur but they become important in a high accuracy work.

1.Inconsistencies Associated with Accurate Measurement of
Small Quantities
The outputs of the instruments become inconsistent when very
accurate measurements are being made. This is because when
the instruments are built or adjusted to measure small quantities,
the random errors (which are of the order of the measured
quantities) become noticeable.
For example - if one wishes to measure the weight of a bucket of
water to the nearest kilogram, there would be no excuse of one
observation differing from another, no matter how many times
the measurements are made.
However, if one attempts to determine the same weight to the
nearest milligram, individual observations are sure to differ
unless the are performed with extreme care and without
prejudice.

2.Presence of Certain System Defects
System defects such as large dimensional tolerance in mating
parts and the presence of friction contribute to errors that are
either positive or negative depending on the direction of motion.
The former causes backlash error and the latter causes
slackness in the meter bearings. One way of detecting and
correcting such errors is to measure the quantity first while
increasing and then decreasing the magnitude. This procedure is
based on the method of symmetry.

3. Effect of Unrestrained and Randomly Varying Parameters
Chance errors are also caused due to effect of certain
uncontrolled disturbances which influence the instrument output.
Line voltage fluctuations, vibration of the instrument supports,
etc. are common examples of this type. The experimenter should
try to reduce the influence of such randomly varying parameters
to the minimum. But , in spite of this there are always certain
residual contributions due to these random perturbations.

Miscellaneous Type of Gross Errors
• Personal or Human Errors
These are caused due to the limitations in the human senses.
For example, one may sometimes consistently read the observed
value either high or low and thus introduce systematic errors in
the results. While at another time one may record the observed
value slightly differently than the actual reading and consequently
introduce random error in the data. Therefore, it becomes
necessary to exercise extreme care with mature and considered
judgement in recording the observation so as to reduce such
errors.

• Errors due to Faulty Components/ Adjustments
Sometimes there is a misalignment of moving parts, electrical
leakage, poor optics, etc. in the measuring system. These may
simultaneously cause, for example, zero shift coupled with zero
drift which are systematic and random errors respectively.
In such cases, it becomes necessary to determine the magnitude
of the systematic and random errors from the overall gross error.
The procedure usually consists of repeating a measurement a
sufficiently large number of times by feeding a ‘standard’ signal
to the instrument. The difference between the mean value of
signal and the standard signal gives the best estimate of
systematic error. Further, the estimate of uncertainty which
represents the random errors in measurement is evaluated from
the dispersion of the data

• Improper Application of the Instrument
Errors of this type are caused due to the use of instrument in
conditions which do not conform to the desired design/ operating
conditions.
For example, extreme vibrations, mechanical shock or pick-up
due to electrical noise could introduce so much gross error as to
mask the test information. In such cases it becomes necessary to
stop the experiment until the disturbing elements causing the
chaotic type of gross errors are eliminated.

Nakra B.C. and Chaudry K.K., Instrumentation, Measurement & Analysis, Tata McGraw Hill, New Delhi, 2002.

Alan S Morris, Measurement and Instrumentation Principles Butterworth-Heinemann, 09-Mar-2001 - Technology & Engineering -
512 pages

• A dial gauge is used to measure the pressure in the
vessel. The pivot is not exactly centered and as a result
the readings are subject to systematic error. It was found
that the imperfection makes the readings too large in a
linear fashion from state 1, i.e. 6.895kN/m2 for a dial
readings of zero to state 2, i.e. 27.58kN/m2 for a dial
reading of 150. What would be the value of pressure for
a dial reading of 100?

Linear Interpolation
Linear interpolation involves estimating a new
value by connecting two adjacent known values
with a straight line.
If the two known values are (x1, y1) and (x2, y2),
then the y value for some point x is:

The length of line is measure with a 50m metallic tape and found to be 935.12m at
45C. The tape was standardized at 20C and coefficient of thermal expansion is
0.0000065/C. what is the correct length of the line to the nearest hundredth of a
meter?

Correction =
935.12 [ (50 + 50 x 0.0000065 x 25) −50]
50
= 0.151957 m
Actual Length = Lm + correction = 935.12 + 0.151957
= 935.272m

• A thermometer is calibrated 150 C to 200 C. The
accuracy is specified within +/- 0.25 percent of instrument
span. What is the maximum static error.
Sawhney A.K., Mechanical Measurement & Instrumentation, Dhanpat
Rai & Co, New Delhi, 2002.

Measurements & Measurement .Systems.pptx

More Related Content

What's hot

Similar to Measurements & Measurement .Systems.pptx

More from happycocoman

Recently uploaded

Measurements & Measurement .Systems.pptx

Editor's Notes