measure 1st lec m7md abashar

M e c h a n i c a l M e a s u r e m e n t s & I n s t r u m e n t a t i o n

1st Lecture

Nada Rikabi

[email protected]

22 April 2014

Good to know…

• One class per week – 2 hours

• Time: Tuesday 2:00-4:00 PM

• Location:

• E-Mail: [email protected]

22 April 2014 Mechanical Measurement - 3rd year 2

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.


Course Material

Lecture Slides

Reference:

Mechanical Measurements

Sirohi, R. S., Radha Krishna.


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 measurements22 April 2014 Mechanical Measurement - 3rd year 5

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


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.’


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.


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.


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.


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.


International System (SI)

Seven Basic Units of Measurements


SI Derived Units


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 toconvert, process (amplification or filtering) anddisplay the output ( الناتجعرض ).

• e.g. to measure force from strain gages(معايير) located ina structure.


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.22 April 2014 Mechanical Measurement - 3rd year 15

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.22 April 2014 Mechanical Measurement - 3rd year 16

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.


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.


Error Classification

Error classified as following:

Systematic error

Random errors

Illegitimate errors


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.


Random Errors

Random errors are distinguishable(متميز) by

their lack of consistency. (تناسقعدم ).

The variations in temperature, vibrations(اهتزاز)

of external medium, etc. cause errors in the

instrument.

Errors of this type are normally of limited

duration( محدودةمدة ) & are inherent to specific

environment.( معينةبيئةفياساسي ).

22 April 2014

Mechanical Measurement - 3rd year 21

Illegitimate errors

Happens due to many reasons such as:

1. Mistakes

2. Computational errors( حسابيةاخطاء ).

Illegitimate errors may be eliminated by careful exercise

& repetition of measurement.) وتكرارحذرةممارسة ).

Note: If a measuring instrument is not calibrated

periodically it will lead to errors in measurement .


Systematic & Random Error


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 reproducibilityEven 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


Sources of Error .. Contd.

4. WorkpieceThe nature of workpiece (hardness, roughness…, may lead to

an errors in measurement.

5. Environmental conditionsTemperature, air pressure, humidity and many other

conditions can affect the measuring instrument or the item

being measured.

6. Operator skillsome measurements depend on the skill and judgment of the

operator.


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.


Distinction between Accuracy

and Precision


Distinction between Accuracy

and Precision

28

Standards organizations

• ISO - International Organization for

Standardization

• SASO - Saudi Arabian Standards organization

• Others—ASME, NFPA, ASTM, etc.


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.


precise

accurate

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 ____________________


measure 1st lec m7md abashar

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