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1. Mechanical Measurements
& Instrumentation
1st Lecture
Nada Rikabi
nadarikabi@yahoo.com
22 April 2014

2. Good to know…
• One class per week – 2 hours
• Time: Tuesday 2:00-4:00 PM
• Location:
• E-Mail: nadarikabi@yahoo.com
22 April 2014 Mechanical Measurement - 3rd year 2

3. Course Grade Distribution (100 Points)
1. Term Work [40 Points]
1. Small quizzes : 20 Points
2. Mid-term exam () : 20 Points
2. Final Exam (): 60 Points
Note: A student less than 75% attendance will not be
allowed to sit for the final examination.
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4. Course Material
Lecture Slides
Reference:
Mechanical Measurements
Sirohi, R. S., Radha Krishna.
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5. Course Outlines
 Basic Concept of Measurements
 Inaccuracy of Measurement and its Analysis
 Pressure Measurements
 Flow Measurements
 Velocity Measurements
 Temperature Measurements
 Force Measurements
 Strain and deflection measurements
 Strain and Stress measurements
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6. Today’s Lecture
Basic Concept of Measurements:
 What is measurement ?
 Why measurement?
 Systems of Measurements.
 Types of Measurements
 Calibration
 Inaccuracy of Measurements
 Errors in Measurements
 Accuracy & Precision
 Standards organizations
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7. What is measurement ?
• A measurement tells us about a property of
something. It might tell us how heavy an object is, or
how hot, or how long it is. A measurement gives a
number to that property.
• Measurements are always made using by an
instrument (tool) of some kind. Rulers, stopwatches,
weighing scales, and thermometers are all measuring
instruments.
• The result of a measurement is normally in two parts:
a number and a unit of measurement, e.g. ‘How long
is it? ... 2 metres.’
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8. What is measurement ? ..Contd.
Act of measurement — the quantitative
comparison between a predefined standard
and a measurand to produce a measured result
Measurand : physical parameter or variable to
be measured.
Standard: basis for comparison of quantitative
value to measurand.
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9. Examples of Measurements
What kinds of measurements did you make
today?
You are making a measurement when you:
Check your weight.
Read your watch.
Take your temperature.
Read the speed indicator of your car.
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10. Why measurement?
 We all make measurements, every day of our lives.
 We are surrounded by the outputs of measurement
devices , from the wind speeds on weather forecasts to
the figures on electricity bills.
 In the case of process industries and industrial
manufacturing…
– To improve the quality of the product.
– To improve the efficiency of production.
– To maintain the proper operation.
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11. Systems of Measurements
British System of measurements
(BS)
Old system based on the inch
(length) and still in use in some
industries.
The International System of
Units (SI) is the modern form
of the metric system and is
generally a system devised
around the convenience of the
number 10.
It is the world's most widely
used system of measurement,
both in industry, science and
commerce.
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12. International System (SI)
Seven Basic Units of Measurements
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13. SI Derived Units
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14. Types of Measurements
• Direct comparison
– Easy to do but… less accurate
• e.g. to measure a steel bar
• Indirect comparison
– Calibrated system; consists of several devices to
convert, process (amplification or filtering) and
display the output ( ‫عرض‬‫الناتج‬ ).
• e.g. to measure force from strain gages(‫)معايير‬ located in
a structure.
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15. Calibration
• Calibration consists of comparing the output of the
instrument or sensor under test against the output of an
instrument of known accuracy (higher accuracy) when the
same input (the measured quantity is applied to both
instrument).
• The procedure is carried out for a range of inputs covering
the whole measurement range of the instrument or sensor.
• Ensures that the measuring accuracy of all instruments and
sensors used in a measurement system is known over the
whole measurement range, provided that the calibrated
instruments and sensors are used in environmental
conditions that are the same as those under which they
were calibrated.
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16. Calibration ..Contd.
Calibration involve a comparison of an
instrument with either:
1. A primary standard
2. A secondary standard
3. A known input
Short definition:
Comparison of an instrument against a reference
or standard, to find any errors in the values
indicated by the instrument.
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17. Inaccuracy of Measurements
 The purpose of any measurement is to describe some
physical property of an object or a system
quantitatively, exp. length, temperature, pressure,…
 All measured values are inaccurate to some degree.
 In fact, it is impossible to find the true value of a
physical quantity.
 The experimenters (we) aim is to find the most
probable value and assign an uncertainty to it.
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18. Errors in Measurements
Error may be defined as the difference
between the measured value and the true value.
No measurement can be made without errors at
all times i.e. 100% accurate measurements
cannot be made at all times.
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19. Error Classification
Error classified as following:
Systematic error
 Random errors
 Illegitimate errors
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20. Systematic Errors
 Generally they will be constant / similar form
recur consistently every time measurement is
measured.
 May result from improper condition or
procedures employed.
 This kind of error can be eliminated(‫)ازالة‬ by
calibration.
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21. Random Errors
Random errors are distinguishable(‫)متميز‬ by
their lack of consistency. ‫عدم‬‫تناسق‬) ).
The variations in temperature, vibrations(‫)اهتزاز‬
of external medium, etc. cause errors in the
instrument.
Errors of this type are normally of limited
duration( ‫مدة‬‫محدودة‬ ) & are inherent to specific
environment.( ‫اساسي‬‫في‬‫بيئة‬‫معينة‬ ).
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Mechanical Measurement - 3rd year 21

22. Illegitimate errors
Happens due to many reasons such as:
1. Mistakes
2. Computational errors( ‫اخطاء‬‫حسابية‬ ).
Illegitimate errors may be eliminated by careful exercise
& repetition of measurement.) ‫ممارسة‬‫حذرة‬‫وتكرار‬ ).
Note: If a measuring instrument is not calibrated
periodically it will lead to errors in measurement .
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23. Systematic & Random Error
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24. Sources of Error
1. Instrument calibration
Due to frequent use of a measuring instrument and also of
aging, the instrument may go out of calibration. Which leads
to systematic error. Therefore the instrument should be sent
for calibration at frequent intervals.
2. Instrument reproducibility
Even if the instrument has been calibrated under a set of
conditions, there may still be error in measurement due to the
difference in the calibration conditions.
3. Measuring arrangement
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25. Sources of Error .. Contd.
4. Workpiece
The nature of workpiece (hardness, roughness…, may lead to
an errors in measurement.
5. Environmental conditions
Temperature, air pressure, humidity and many other
conditions can affect the measuring instrument or the item
being measured.
6. Operator skill
some measurements depend on the skill and judgment of the
operator.
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26. Accuracy & Precision
 The terms precision and accuracy are used in
connection with the performance of the instrument.
 Precision is the repeatability of the measuring
process.
 Accuracy is the degree to which the measured value
of the quantity agrees with the true value.
 Error= true value – measured value [explained before]
 Less the error more accurate is the measurement.
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27. Distinction between Accuracy
and Precision
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28. Distinction between Accuracy
and Precision
28

29. Standards organizations
• ISO - International Organization for
Standardization
• SASO - Saudi Arabian Standards organization
• Others—ASME, NFPA, ASTM, etc.
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30. Question
• If you make several measurements and find
that they also agree closely with each other,
then they are precise. If they agree closely with
the actual value, then they are accurate.
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precise
accurate

31. Question
State the tool (s) and the Units you would use to
measure the following:
A. Temperature ____________________
B. Volume ____________________
C. Length ____________________
D. Time ____________________
E. Weight ____________________
F. Pressure ____________________
22 April 2014 Mechanical Measurement - 3rd year 31