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.
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