05 chp1 and chp2 lecture

7/27/2019 05 Chp1 and Chp2 Lecture

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INTRODUCTION

In Engineering, carefully design experiments are

needed to: conceive and verify theoretical concepts develop new methods and products

evaluate performance and behavior of existing products etc.

Experimentation and the design of measurementsystems are major engineering activities. Why?

Applications of Engineering Experimentation

Measurement in Engineering Experimentation- new information is being sought

Measurement in operational devices

- for monitoring and process control


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INTRODUCTION (cont)

Measurement in Engineering Experimentation

Involves using the measurements process to seek newinformation and

Testing of existing products to determine maintenance

requirements

These falls broadly into three categories:

1. Research Exper imentat ion

2. Development Exper imentat ion3. Per fo rmance Tes t ing

Whats the main difference between 1 and 2?

In research, concepts for new products or processes are

being sought (often unsuccessfully), while in

development, known concepts are being used to

establish potential commercial products.Any examples?


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Measurement in Engineering Experimentation

Research experiments are frequently highly uncertain

and often leads to dead end.

Development programs usually have better define

goals than research programs and frequently result in an

operational product.

Performance testing is somewhat different from both

research and developmental experiments. How different?

It is done on products that have been developed and

in many cases are already on the market.It may be carried out to:

Demonstrate applicability for a particular application

To assess reliability or to determine product lifeline

Determine needs for maintenance (periodic testing).


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DIMENSIONS and UNITS

The physical world is described with a set ofdimensions.Length,

mass, time and temperature are dimensions. When a numericalvalue is assigned to a dimension, it must be done in a unit system.

The International System of Units (SI) is the most widely used unit

system in the world, due to its consistency and simplicity.

In the US here, a unit system based on the old British unit system is

still widely used., hence both units will be used in these class.


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Units Convertion


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Chapter 2

Topic: General Characteristics of

Measurements


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Generalized Measurement System

Figure 2.1 Generalized measurement system.

Measurands: Numerical values for physical variables.

The sensing element has a significant physical

characteristic that changes in response to changes in the

measurand.

The signal modification device changes the output of

the sensing element in some way to make it suitable for the

indicating or recording device.


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Example - Thermometer

Figure 2.2 Mercury-in-glassthermometer.

What is the physical dimension to be

measured?

Temperature

What is the sensing element in this

case?

Volume of mercury

What is the signal modification system?

By connecting the bulb to the stem.

(Note: the displacement mercury in the

bulb is proportional to the differentialthermal expansion between the mercury

and the glass envelop)

What is the indicating device?

The scale that is either next to the


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Measurement Error

Definition:

error = measured value true valueNote: Error in this technical usage does not imply there

is any mistake in the measurement process, although

mistakes can cause errors.i

If the true value of the measurand is known, there will be

no need to make measurements except in the case of..

Calibration!!!

What we can estimate is however the uncertainty

interval(or simplyuncerta inty)of the measurement.

The uncertaintyis an estimate (with some level of

confidence) of the limits of the error in the measurement.


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Example

Q. How will you interpret the error in the following

statement: with 95% confidence, the uncertainty ofvoltage measurement is +/- 1V.

This means the error will be greater than 1 V in less

than 5% of the cases.

Two Categories of Errors

1. Systematic Errors ( same as fixed or bias errors) -

Systematic or bias errors are consistent, repeatable

errors.

2. Random Errors (or precision errors) Random errors

are cause by lack of repeatability in the output of the

measurement.


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Sources of Systematic Errors (Bias

Errors)1. Error in the calibration of the measurement system.These are known as cal ibrat ion error s. One source

of this nonl ineari ty.

2. Errors obtained where the insertion of a measuring

device alters the measurand. This is known as

loading errors.

3. Measuring system is affected by variables other than

the measurand. Example related to this source of

error is spatial error.

If the measurand varies in a spatial region and yet a

single measurement or a limited number of

measurements are used to determine the average value

for the region, there will be a spatial error.


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Sources of Random Errors

1. Errors can originate from the measuring system itself,

from the experimental system or from theenvironment.

2. Uncontrolled variables in the measurement process.

E.g. not controlling temperature versus amplifier

performance.

3. Electrical noise. (electric and magnetic fields caused

by building wiring, local radio stations etc can affect

readings by altering voltages in measuring systems)

How to minimize Random Errors

Eliminating uncontrolled variables

Properly shielding or grounding the measurings stem


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Random and Systematic Errors

Random errors are cause by lack of repeatability in the output of the

measuring system.

Random error = reading average of reading

Systematic error = average of reading true value

Figure 2.3 Distinction between systematic and random errors.


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Example Problems

You attempt to determine the validity of a

bathroom scale by repeatedly placing 20 lb ofaccurate weights on it.

Ten readings were obtained with values of 20.2, 20.2,

20.6, 20.0, 20.4, 20.2, 20.0, 20.6, 20.0, and 20.2 lb.

Estimate (i) the systematic error and

(ii) the maximum random error

of the measurements.


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Example Problems

Determine whether the following measurements are

intrusive or nonintrusive.

(a) Measuring amperage of current in a wire using clamp - on

ammeter

(b) Measuring flow of fluid in a pipe by installing an orifice

meter in the pipe.(c) Measuring composition of gases in an exhaust using a

device which optically measures transmitted infrared radiation.

(d) Determining the surface temperature of a pipe using a

thermometer that measures the emitted infrared radiation.

(e) Rotational speed of a shaft indicated by astrobotachometer.

The strobotachometer measures rotational speed by shining a

pulsing light on a mark on the shaft and adjusting the pulsing rate

until the mark appears stationary. The light has negligible effect on

the rotation of the shaft and is non-intrusive.


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Example Problems

Determine if the following errors are of random or

systematic type. Justify your response.

(a) A digital scale, that always shows 0.2 lb when no

weight is applied.

(b) Vibration of the needle of an automobile

speedometer.

(c) Consistent temperature difference between two

sensors reading the air temperature in the same room.

05 chp1 and chp2 lecture

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