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1What is a Measurement?
Encyclopedia Encarta
In classical physics and engineering, measurement
generally refers to the process of estimating or
determining the ratio of a magnitude of a
quantitative property or relation to a unit of the
same type of quantitative property or relation.
Process of measurement involves the comparison
of physical quantities of objects or phenomena
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What is a Measurement? (contd) Wikipedia
Measurement is the estimation or determination of extent,
dimension or capacity, usually in relation to some standard
or unit of measurement.
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Comparison to a Standard
(Metrology)
Metrology is the study of measurement.
-In general, a metric is a scale of measurement defined in terms
of a standard: i.e. in terms of well-defined unit.
- If one says I am 5, that person is indicating a measurement
without
supplying an applicable standard.
-They could mean I am 5 years old or I am 5 feet high.
-Measurements are at best ambiguous, or at worst, meaningless,
with out units!
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Units of Measure What is a Unit of Measure?
-Act of measuring involves comparing the magnitude
of a quantity possessed by an object with a
standard unit by using an instrument
under controlled conditions.
-Examples of measuring instruments include:
Thermometer (Deg.)
Current Meter (Amps)
Pressure Sensor (psi)
What are
These gages
Reading?
Without
Prior
Knowledge
Of units
we have
No idea!4
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Units of Measure (contd)
Same quantity,
.. Different units
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Units of Measure (contd) Systems of Units
Imperial (English)
Before SI units were widely adopted around the world, the
British
systems of English units and later Imperial units were used in
Britain,
the Commonwealth and the United States. The system came to be
known
as U.S. customary units in the United States.
Sometimes called foot-pound-second systems after
the Imperial units for distance, weight (mass), and time.
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Units of Measure (contd) Systems of Units
Metric (MKS)
The metric system is a decimalised system of measurement based
on the
Meter (M), kilogram (K), and second (S).
The main advantage of the metric system is that is has a single
base unit for
each physical quantity. All other units are powers of ten or
multiples of ten
of this base unit.
Unit conversions are always simple because they will be in the
ratio of ten,
one hundred, one thousand, etc.
Also referred to as Systeme International (SI) Units
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Fundamental Units of
Measure
A system of measurement is a set of units which can be used
to specify anything which can be measured. Some quantities
are designated as fundamental units meaning all other needed
units can be derived from them.
Historically a wide range of units were used for the same
quantity; for example, in several cultural settings, length
was
measured in inches, feet, yards, fathoms, rods, chains,
furlongs,
miles, nautical miles, leagues, with conversion factors
which
are not simple powers of ten or even always simple
fractions.
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Fundamental Units of
Measure (contd)
This disagreement of units had serious military, cultural,
and
Fiscal impacts and eventually the British Royal Society headed
by
Michael Faraday adopted 3 fundamental Units,
distance (ft), weight (lb), and time (sec).
Later (1824) it was determined to be more
fundamental to substitute Mass (slug) for weight
(lb) as a fundamental unit of measure
F ma1lbf 1slug ftsec2
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Fundamental Units of
Measure (contd) In the 19th century, science developments showed
that either
electric charge or electric current must be added to
complete
the minimum set of fundamental quantities.
Mesures usuelles (French for customary measurements)
were a system of measurement introduced to act as
compromise between metric system and traditional
measurements.
This system of measures would eventually lead to the
Evolution of the modern SI system of measurements
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Fundamental Units of
Measure, SI system The SI system is founded on 8 fundamental
units. All other
Units can be derived from these quantities.
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Derived Units of Measure, SI
system Derived units are
algebraic combinations
of the eight base units
with some of the
combinations being
assigned special names
and symbols.
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Derived Units of Measure, SI
system (contd)
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Conversion of Units of Measure
Although the Imperial System of units is gradually being
Replaced by SI system, these units are still in common use
Amongst U.S. defense contractors , and NASA!
This use of the Imperial system is especially prevalent
For mechanical units like distance, force, moments of
inertia, pressure, and volume.
Accurate conversion from one system to another is
Essential!
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Conversion of Units of
Measure (contd)Not Important?!
The Mars Climate Orbiter (1998) was
destroyed when a navigation error caused the
spacecraft to miss its intended 150 km
altitude above Mars during orbit insertion.
Instead the spacecraft entered the Martian
atmosphere at about 57 km altitude.
The spacecraft was destroyed by
atmospheric stresses and friction at
this low altitude. 15
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Conversion of Units of
Measure (contd)Not Important?!
A review board found that thruster impulse data
were calculated on the ground in Imperial
units (pound-seconds) and reported that
way to the navigation team, who were
expecting the data in metric units
(newton-seconds)
Anticipating a different set of units, systems
aboard the spacecraft were not able to
reconcile the two systems of measurement,
resulting in the navigation error and loss of spacecraft!16
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Conversion of Units of
Measure (contd)Not Important?!
This calculation just saved $300 million dollars!
1 ntsec
1 kgmsec2
sec 1 kgm
sec
1
14.5939
slugkg
3.28083 ftm
1 lbfslug ft
sec2
3.28083
14.5939
kgmsec
slug
kg
ft
mlbf
slug ft
sec2
0.22481 lbf sec
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Conversion of Units of
Measure (contd)Careful with English units!
Pounds-mass (lbm) is not a fundamental unit of measurement!
Metric
English -- pounds mass (lbm) and pounds force (lbf) do not
cancel
F ma (1)nt kg msec2
F m
gca gc 32.174 lbm ft
lbf sec2
(1)lbf 32.174 lbm ftsec2
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Dimensional Analysis Most physical quantities can be expressed
in terms
of combinations of five basic dimensions. These are
mass (M), distance (D, L), time (t), electrical current (I),
and temperature (T)
Dimensions aren't the same as units. I.e. the
physical quantity, speed, may be measured in units of
meters per second, knots ; but regardless
of the units used, speed is always a distance
divided a time, so we say that the dimensions
of speed are distance divided by time, or instantaneously
dD/dt.
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Dimensional Analysis (contd)In same manner, dimensions of area
are D2 . area can
always be calculated as a distance in one direction times a
Perpendicular direction
. area of a circle --> r2 is really a result of the
integral
Acircle 1
2r rd
0
2
r2
22 r2
[1/2 Length of arc] x [height of triangle]
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Dimensional Analysis (contd)
Simple Dimensional Analysis Example
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Dimensional Analysis (contd)More Complex Dimensional Analysis
Example
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Dimensional Analysis (contd)In algebraic expression, additive
terms must have same dimensions.
--> each term on the left-hand side of an equation must have
the same dimensions as each term on the right-hand side.
"a" must have the same dimensions as the product "bc", and
"(1/2)xy"
must also have the same dimensions as "a" and "bc".
Equation is dimensionally correct when terms have consistent
dimensionality
Dimensional analysis is a valuable tool for validating the
correctness
of an algebraic derivation i.e. finding algebra errors
a b c 1
2x y
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Dimensional Analysis (contd)
Source:
http://www.physics.uoguelph.ca/tutorials/dimanaly/dimanaly_ans7.html,
Cited 12-22-06
More complex examples
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The Measurement Process:
Comparison to a Standard
Direct Comparison to a Standard
Length of a bar
Use a carpenters Rule
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StandardsAny time you measure anything, you are comparing it to
something whose
value you think you know. You assume your ruler is 1 ft long.
But who says
what a foot is?
A combination of several international agencies are responsible
for maintaining the
primary standard measures of various quantities. The standard
kilogram and the
standard second are maintained by the French. Others are kept
elsewhere. It
extremely important that these standards do not change with
time, even over
hundreds of years.
The National Institute of Standards and Technology in Maryland
is responsible for
keeping standards for the US.
http://www.nist.gov/public_affairs/standards.htm
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IST-F1 Cesium Fountain
Atomic Clock Primary Time and Frequency Standard for the United
States
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Hierarchy of Standards
The hierarchy of measurement standards starts from the
international standard at the apex, which is known with the
highest
precision and goes all the way down to working standards.
International measurement standards are standards recognized by
an
international agreement to serve internationally as the basis
for
assigning values to other standards of the quantity
concerned.
The oldest standard in use today is the International Prototype
of
the Kilogram, kept at the Bureau International des Poids et
Mesures
(BIPM) in Sevres.
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Hierarchy of Standards (contd)
These primary standards cant be
passed around to any entity that
wants to take some measurements
if we expect them to maintain their
values, so secondary standards are
kept which may be somewhat less
accurate, but much more accessible.
These are calibrated against the
primary standards. In this manner, a
hierarchy of standards exist.
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Hierarchy of Standards (contd)
A primary standard is designated or widely acknowledged as
having the
highest metrological qualities and whose value is accepted
without
reference to other standards of the same quantity.
Secondary standards are standards whose value is assigned by
comparison
to a primary standard of the same quantity. Primary standards
are usually used
to calibrate secondary standards. A working standard is a
standard that is used
routinely to calibrate or check material measures, measuring
instruments or
reference materials. A working standard is usually calibrated
with reference
to a secondary standard, and may be used to ensure that routine
measurements
are being carried out correctly - a check standard.
A reference standard is a standard generally having the
highest
metrological quality available at a given location or in a given
organization
from which the measurements made at that location are
derived.
Calibration laboratories maintain reference standards for
calibrating their
working standards. 30
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The Measurement Process:
Using Calibrated System
Using a Calibrated System
Length of a Big Snake
-- Naturalists measure length of the animal using in a string
following a imaginary
middle line of the body from head to tail.
-- then the length of the string is measured by laying it on a
ruler
-- allows recording of the actual length of the animal
regardless
of its position and without having to stretch the snake.
Hold still will ya!
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Calibration Example (2)
-10
-5
0
5
10
-2
-1.5
-1
-0.5
0
0.5
1
-0.3 -0.2 -0.1 0 0.1 0.2 0.3
P [
kP
a]
% fs erro
rVolts
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Calibration Example (3)Thermal Anemometry relies on property
many materials change resistance
with temperature. A hot-wire anemometer is a device that heats a
wire by
pumping current through it and keeps its resistance (and thus
its temperature)
constant. When air blow on the wire the current required to keep
the wire hot
goes up. . instrument is sensitive to velocity. Requires
calibration against
a known velocity.
0
10
20
30
40
50
60
-6
-4
-2
0
2
4
6
-3 -2 -1 0 1 2 3
Ve
locity[m
/s]
%e
rror
Volts
A1, V1, P1
A2, V2, P2
A1 is 100 times larger than A2, so V12 is
negligible compared to V22. So,
Bernoulli says that
P1
V1
2
2P2
V2
2
2
V2 2 P1 P2 /
Hot wire
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Sensors/Transducers (1)
A sensor is something that is sensitive to some phenomenon that
we
are interested in. It needs to respond to the phenomenon is some
way
that we can see or measure.
Examples: Mercury thermometer
Wind Sock
Thermocouple
A transducer is a sensor tied to stuff (very often electronics)
that
makes the output of the sensor readable. Most transducers output
a
voltage or a current. typically this is an element of a
sensor
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Generalized Measurement
System
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QuickTime and aTIFF (LZW) decompressor
are needed to see this picture.
QuickTime and aTIFF (LZW) decompressor
are needed to see this picture.
Pin outs
Scope to monitor analog
Out from PPT
PPT
Terminal Block
For RS-485 Bus
Honeywell PPT Interface Lab Engineering Development Unit
PC
Running Labview
7.1
Example I: Simple Lab
Measurement System
Sea level Systems:
USB to RS-422, RS-485 Serial Interface Adapter
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Example II: Aircraft Airspeed
Sensing System
Transducers are
Sub-elements of sensor
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Example III: Complex Sensor
System
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Transduction Transducers convert the physical phenomenon being
sensed
Into an alternative signal that can be more easily sensed
Pressure inputElectrical output
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Sensors/Transducers (2)
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Sensors/Transducers (3)
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Fiber Optic Sensors
Source: http://www.bluerr.com/papers/Overview_of_FOS2.pdf
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Fiber Optic Sensors (contd)
Source: http://www.bluerr.com/papers/Overview_of_FOS2.pdf
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Fiber Optic Sensors (contd)
Fiber Optic Sensor Examples
Source: http://www.bluerr.com/papers/Overview_of_FOS2.pdf
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Fiber Optic Sensors (contd)
Fiber Optic Sensor Examples
Source: http://www.bluerr.com/papers/Overview_of_FOS2.pdf
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Fiber Optic Measurement
System
Source: http://www.bluerr.com/papers/Overview_of_FOS2.pdf46
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Very Complex Example:
Remote Sensing Mission
Source: Sellers, Understanding Space 47
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Remote Sensing
Payloads Look at Target -- move sensor to point at subject
See the Target -- collect EM radiation from subject
Conversion -- Transform sensed EM radiation to useable data
Processing -- analyze data to produce useable information
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Telescopic Optical Sensing Systems
All remote sensors are basically one of two variations on a
Telescope Reflecting telescope (Hale (Mt. Palomar), Radar,
Radio telescopes, DSS)
Refracting telescope (very cumbersome and expensive)
Objective lens
Eyepiece
Eyepiece
Primary
Mirror
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Telescopic Optical Sensors (contd)
Catadioptric Telescope (hybrid)
Convex lens
Convex Mirror
Secondary Mirror i.e Hubble Space Telescope
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Hubble Space Telescope
2.5 m
Catadioptric
Design
2.4 m
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