6. Planning Laboratory Tests Experimental Measurements and Errors1

8/12/2019 6. Planning Laboratory Tests Experimental Measurements and Errors1

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FINAL PROJECT

(CE3216)

Planning Laboratory Tests, Experimental

Measurements and Errors

SCHOOL OF CIVIL ENGINEERING

Dr. DEEPAK T. J.


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Planning a Series of Laboratory

Tests

When Carrying out Laboratory or other Practical

Investigations, it is important to ensure that:

a) Appropriate Techniques are used

b) Meaningful Results are obtained

For this reason, it is essential that the test

programme is designed properly and that care istaken that the correct tests and test procedures

are adopted


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Ensuring that appropriate

techniques are used

In order to do this, the following questions must be

answered:

a) Is the correct equipment being used?

b) Is the equipment accurate enough?

c) Does the equipment have sufficient

capacity?

d) Do the tests actually measure what isintended?


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Ensuring that Results are

meaningful

Is the instrumentation being read correctly?

Does it make sense to measure the parameter

that is being investigated?

Are the resultsreproducible?


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Adopting a realistic approach to

Equipment Design

With particular reference to carrying out a final

year project, you will need to:

a) Check what equipment is available

b) If there are costs involved, check thatfunding is available

c) Modify your plan if necessary

d) Produce a Time Schedule for theinvestigation


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Equipment Design



e) Check whether materials are available.

Order materials if necessary. Find out whenthey will be available; allow extra time for

delivery in your schedule

f) If you require equipment to be specially

made, produce drawings or good sketches

including all dimensions, materials and

tolerances


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Equipment Design



g) Then discuss your requirements with the

laboratory techniciansh) Check how long it will take for the production

of the equipment and whether you can do

part or all of the work yourself. The

technicians usually have a long order book;

be prepared that it may take several months.


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An ExampleWill the tests do what

they are intended to?

You have been requested to carry out a load test on an

individual pile which has been designed to carry a working

load of 100 kN

You have obtained a hydraulic jack with a capacity of 120 kN,

with which you intend to apply a load to the pile You realise that you need something to jack against and so

get the JCB driver to come across and park his excavator (a

JCB 3CX) over the pile

You install the jack between the top of the pile and theunderside of the JCB and set up a frame and dial gauge on

the head of the pile to measure its settlement

Satisfied with the set-up, you start jacking to load the pile

How successful do you think your test will be?


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Monitoring the effectiveness of the

Test Procedure

In addition to ensuring that the test procedure is

correctly designed so that the tests provide the

appropriate information, it is also necessary to

monitor the results as the test programmeproceeds

Provided that the tests are undertaken with a

clear aim in mind and that the test hypothesis is

fully understood, it should be possible to judgewhether each of the individual test results seems

reasonable or whether the test procedure may

need revision


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ExampleAssessing Test

Effectiveness

You are testing a 1 metre long concrete plank by

clamping both ends and then applying a central

load as shown below:


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Effectiveness

Where would you expect the plank to fail?

In each of the first three tests, a failure occurs 10

cm from the left hand end.

Are you concerned? What steps might you taketo adjust your experimental technique?

In this case, the results are such that they would

cause some concern . In this respect, one of thefirst things that might be considered is to revise

the loading configuration.


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Effectiveness

In fact, for a bending test of this type, it is more

appropriate to apply two equal loads equidistant

from the centre of the test piece in order to apply

a uniform maximum moment across a widerdistance than for the original loading, which

results in a maximum moment at a single point:


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Some Practical Problems in

Designing Tests

In addition to the type of problems discussed

Previously, there are also some other practical

problems that need to be considered:

For example, you may decide that there areadvantages in assessing concrete strength using

smaller cubes than the standard 100 mm size

The advantages here are that you will require less

material and the failure loads will be lower, so thatyou might not require a particularly heavy testing

machine

However, can you think of any potential

disadvantages you may find???


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A Similar Type of Practical Problem

You have identified a research project to look at

the behaviour of fill materials to highway

structures. The fill in question usually comprises a

range of soil particle sizes from 5 mm up to 150mm. You are going to use a 60 mm square shear

box for the tests.

Can you identify some of the limitations that your

tests may suffer from?

Can you think of any ways in which these

limitations may be overcome?


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Checking that the Equipment works

correctly

You must check that all of the equipment is

working correctly. This includes:

a) Maintenanceall necessary maintenance

must be completedb) Cleanlinessnot least to prevent errors due

to contamination

c) Calibration should be up to date for allrelevant equipment

Double check all readings.


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Time Schedulingallowing enough

time for the Tests

Again, this is a particularly important consideration

for students own final year projects

Before you even start your project you will need

to produce a time schedule that takes account ofall the tasks you are going to carry out

The Golden Rule is that investigations will

almost certainly take longer than you expect.

You must ensure that you allow enough time for

all of the following:


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time for the Tests

a) Time to plan and make decisions on what you

are going to investigate and how you are going

to do it

b) Lead time for preparing, calibrating andchecking equipment, delivery and preparation

of materials

c) Time to carry out trial runs

d) Time to analyse results from trial runs

e) Time to review and if necessary repeat trial

runs


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time for the Tests

Again, this is a particularly important consideration

for students own final year projects

Time to carry out the actual practical investigation

Time to analyse results

Time to repeat some or all of the tests if

necessary

You must allow for any waiting time. For example,oven drying of soil takes 24 hours, concrete cube

curing takes 28 days.


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Using External Data

Again, considering a case primarily relevant to a

final year project, consider the following:

Your employer on your industrial placement is dueto carry out a full scale loading test on a structural

element comprising part of a reinforced concrete

bridge in three months time. She offers you the

chance to analyse some of the results and you

think that this would make a potentially interesting

final year project.


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Using External Data

Do you think it would be a good idea to

undertake such a project?

What potential problems do you think may occur

and can you do anything to avoid them?


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Potential problems when using

External Data


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Repeatability of Results

Repeatability is one of the most important

factors in any laboratory test programme

Individual tests should produce repeatable

results; you may need to check this out by:a) Repeating tests when you are carrying out your

preliminary testing programme

b) Designing your final test programme so that you

should be able to check the results of certain testsagainst each other

You MUSTprovide enough information in your

final report so that other researchers can repeat

your experiments in order to verify your findings


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Taking Experimental Measurements

(A Discussion of Errors)

All experimental measurements that are taken

will be subject to error of some sort

In order to judge how useful the test data is, it is

necessary to estimate how big (or small) theseerrors are likely to be

In addition, if the sources of error can be

identified, it may be possible to revise the design

of the experiment in order to reduce their effects


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Reporting Measured Values

Ideally, when any value is measured, it should

be reported in terms of both the value and the

expected error

Hence, values will be reported as:Best Estimate Uncertainty

(This will be discussed in more detail later on)


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Types of Error

In simple terms, errors may be broken

down into two distinct types:

a) Random Errors

b) Systematic Errors


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Random Errors

The main features of random errors are that:

a) They occur in a random fashion and change

with each measurement

b) As a result, it is possible to reveal thepresence of random errors by repeating

measurements


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Causes of Random Errors

Lack of Sensitivity in the Equipment or the

observer (this is the most common cause)

Background Noise to the measurement.

For example, natural variations in temperature oratmospheric pressure

Failure to define the quantity being measured

sufficiently accurately

Some physical processes are inherently random

(e.g. radioactivity)


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Dealing with Random Errors

Random errors are unavoidablethey

CANNOTbe completely eliminated, although

they may be reduced through good experimental

design The effects of random errors can generally be

accounted for by repeating the tests and

analysing the results statistically


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Systematic Errors

These are errors which are uniform and

reproducible and occur for every measurement

which is made

The effect of systematic errors is to cause a shiftaway from the true value being measured, as

opposed to random errors, which lead to a wider

range of readings either side of the true value


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Causes of Systematic Errors

Incorrect calibration of measuring instruments

(this will often lead to a zero error)

Faulty Equipment

Incorrect use of the equipment

Failure to account for a relevant effect, e.g. as a

result of making poor or incorrect assumptions

about the way in which the experiments work


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Dealing with Systematic Errors

These are more difficult to deal with than random

errors because they will NOTbe identified by

repeat testing. Consequently:

a) The major difficulties where systematic errorsoccur are in identifying that they are present

b) Once a systematic error has been identified, the

test procedure should be changed to eradicate

the source of the errorc) All test equipment should be properly

maintained and should be calibrated regularly


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Taking Experimental Measurements

All experimental procedures will require

measurements to be taken

When any quantity is measured, it is important to

understand that the measurement will never beEXACTLYthe same as the quantity being

measured

Consequently, we need to have some idea of

why the measurement is different to the reality

and also what implications this is likely to have


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Accuracy and Precision

These are two important terms which have

clear technical meanings

Accuracyis NOTthe same as Precision,

so that a measurement can be Precise,

but notAccurateand vice versa


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A Definition of Accuracy

A measurement is said to be Accurate whenit is similar to the true value of the quantity

being measured


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A Definition of Precision

A measurement is said to be Precise when anumber of repeated measurements of the

same quantity are similar to each other


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An example of a measurement being

Precise but not Accurate

Say, for example, that I decide to measure the

height of one of my students using a 3 metre tape

I do this five times and my measurements are:

1.35m, 1.34m, 1.35m, 1.35m, 1.36mI could then consider a value of 1.35m to represent a

precisemeasurement of the students height

However, having finished the measurement, I find that

the first 0.5 m of the tape has broken off. This means

that the answer I have is not accuratethe students

actual height will be around 1.85 m and my precise

measurement is not accurate


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An example of a measurement being

Accurate but not Precise

Having failed in my initial measurement of the

students height, I decide to use another tape measure

to repeat the process. However, I can only read this

one to the nearest 0.05m

My readings this time are:

1.90 m, 1.80m, 1.85m, 1.75m, 1.95m

This gives an average value of 1.85m

Provided that the student actually is 1.85 m tall, I nowhave an Accurate value, even though my

measurements here are less Precise than the first set


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Assessing the Accuracy of a given

measurement

One way of assessing the accuracy of a given

measurement is in terms of the percent of absolute

uncertainty of an individual measurement , which

is defined as:

Where M is a specific measurement that has been made and Msis the

actual value of the quantity being measured

The limitation here is that you need to know the

value of the quantity being measured.

100%xM

MMtyUncertainAbsolute%

s

s


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Assessing the Precision of a series

of measurements

In order to access the precision with which

measurements are carried out, it will be necessary to

carry out a series of measurements to determine the

percent relative uncertainty, which is defined as:

Where M is a specific measurement that has been made and Mavgis

the average value of a series of measurements

The above equation explains why it is necessary to

carry out repeat tests to check whether the test

method is sufficiently precise for the purposes

100%xM

MMtyUncertainRelative%

avg

avg


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Quoting the expected level of Error

As noted above, all experimental reports should

give an indication of the expected error in the

measurements being taken

This will often be discussed in an individualsection of the laboratory or research report,

although it will also be necessary to quote

values of measurements which will include an

assessment of the expected


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Reporting Measured Values

These should generally be reported to an

appropriate number of significant figures based on:

What is being measured

The level of error in the measurement

The second of these factors may require you to

assess the overall error in terms of:The accuracy of the equipment being used

The precision to which you can take

measurements


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Quoting Results and Errors

Where expected errors are quoted, the results

should be quoted to the same level of accuracy as

the errors

For example, if a persons height is measured as 1.87 mwith a tape where the expected error is 0.01m, then this

would be expressed as:

Height = 1.87 0.01 m

However, if the expected error in measurement is 0.05 mand the measured height is 1.87 m, it is not appropriate to

quote the value to the nearest 0.01 m, so the height would

be expressed to the nearest 0.05 m as:

Height = 1.85 0.05 m


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Levels of Precision and Civil

Engineering Design

One of the facets of civil engineering design that

students find difficult to understand is that

engineers can tend to arbitrarily round off

values in calculations The reason for this is that in many instances,

whereas high precision is encouraged in many

areas of science, this is not necessarily the case

in civil engineering


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Engineering Design

For example, an engineer using an I beam for a

design may have the choice between using two

beams of different consecutive sizes, for which:

Ixx = 408,000 cm4and Ixx = 481,300 cm4 In such a case, it is not necessary to find a very

precise value for the section requireda value

to the nearest 1,000 cm4will much more than

suffice. In addition, it is also rare that a choicesuch as this will be made on the basis of a single

value such as this


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Engineering Design

This factor should be taken into account when

designing laboratory tests, although it is also

important (particularly in the absence of design

experience) to follow the general rule ofcalculating individual values as precisely as

possible and only rounding up once the final

value has been evaluated


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Documents

6. Planning Laboratory Tests Experimental Measurements and Errors1