Transcript What is a “Standard”? - University of Portland

Overview
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Measurement Standards
What are they?
 How are basic units of measure defined?
 How are measurement standards used?
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Calibration
Conversion Units
What is a “Standard”?
A standard is something established as a rule or
basis of comparison in measuring or judging
capacity. Webster’s New World Dictionary
Measurement Standards
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Before we can measure we need to define
the unit of measure.
 How long is 1 meter?
 How much is 1 kg?
 How long is 1 second?
 How hot is 50 degrees C?
Who defines standards?
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International Standards Organization, ISO
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National Institute of Standards and Technology,
NIST
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Maintains SI (metric) standards.
Mandated by the US Constitution
International Bureau of Weights and Measures,
BIPM (Bureau international des poids et
mesures) in Sevres, France.
Characteristics
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What are characteristics of a measurements
standard?
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They must have global availability
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They must be accessible and “usable”
They must be stable
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They must not change over time or location
What is the basic unit of time?
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What is a year?
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The time required for the earth to make one complete orbit about
the sun.
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What is a day?
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The time required for the sun to go from noon to noon.
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What is a second?
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Could be defined as: 1/(24X60X60) of a day
Just a second!
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Measurement standards must be stable.
But the length of a day is constantly changing
Earth’s rotation is slowing
 Slowing is inconsistent
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Defining a second as 1/(24X60X60) of a day is
no longer adequate!
Next option?
One Mississippi
Two Mississippi
Three Mississippi
Four ….
A “new” second?
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Since 1967 the second has been defined as:
“The duration of 9,192,631,770 periods of the
radiation corresponding to the transition
between the two hyperfine levels of the
fundamental state of the atom of cesium-133”.
Beckwith, et al.
“Atomic Clocks”
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Microwaves strike the cesium
The frequency that maximizes
the fluorescence is the natural
resonance frequency
The natural resonance
frequency is 9,192,631,770Hz
This defines the second.
Is there a problem?
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What happens when a day does not contain
24X60X60 seconds?
Sundials don’t match the atomic clock.
 Noon will shift, eventually occurring at night.
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Leap seconds are added as required
Keeps noon in the middle of the day
 Just like leap days keep June at the beginning of
summer
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How long is a meter?
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In 1889 the meter was redefined as the length of
the International Prototype Meter (a physical
artifact that defined a meter).
Since 1983 the meter has been defined as the
length light travels in a vacuum in 1/299,792,458
seconds.
Inversely, the speed of light is now, by definition,
299,792,458m/s.
What about US?
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Since 1959, the “international inch” is defined as
2.54 cm exactly!
However, the US Coast and Geodetic Survey,
(established in 1893) was based on the metric
system and defined an inch as 1/39.37 meter
(39.37inch = 1 meter, exactly). Beckwith, et al.
The “survey inch” is 2.54000508 cm (1/39.37)
Mass
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The kilogram was originally defined as the mass of 1
liter of water at 4ºC and 76 cm mercury atmospheric
pressure (1 liter contains 1000 cubic centimeter).
The kilogram was quickly replaced by a physical artifact.
The mass of this specific platinum iridium bar is by
definition, 1kg.
This is still the definition, and it is the only primary
standard based on a physical artifact.
It is maintained by the International Bureau of Weights
and Measures in Sèvres, France.
What about US?
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Since 1959 the pound-mass has been defined as
0.4535937kg
NIST maintains the US’s physical artifact, which
is secondary to the International artifact in
France.
Getting warm? How warm?
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Temperature is much more complex to define.
It requires definition at multiple points
Fixed point
Triple point of hydrogen
Triple point of neon
Triple point of oxygen
Triple point of argon
Triple point of mercury
Triple point of water
Melting point of gallium
Freezing point of lanthanum
Freezing point of tin
Freezing point of zinc
Freezing point of aluminum
Freezing point of silver
Freezing point of gold
Freezing point of copper
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Temperature (K)
13.8033
24.5561
54.3584
83.8058
234.3156
273.16
302.9146
429.7485
505.078
692.677
933.473
1234.93
1337.33
1357.77
Triple point: the temperature at which solid, liquid and gas phases coexist.
Interpolation between temperatures is defined by means of a platinum resistant
thermometer, calibrated at the above temperatures. Wheeler and Ganji
Triple Point video part I (in the video, watch at least from the 30 sec mark to the 4:30
mark) http://www.youtube.com/watch?v=qKDW1e3iNfs
Triple Point video part II (watch through the 1:30 mark)
http://www.youtube.com/watch?v=GafCi-3Xqko
What about US?
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Conversion:
F = 1.8C + 32
What about other things?
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The above units of measure are known primary or
fundamental standards.
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Time (second)
Length (meter)
Mass (kg)
Temperature
Others (electrical, luminosity, angles, mole)
All other measures are based off of the above basic
units. These are known as secondary or derived
standards.
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Velocity is distance per time
Volume is length cubed, etc.
NIST
(National Institute of Standards and Technology)
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Government agency mandated by the US
Constitution (Article I, Section 8).
Responsible for defining and regulating weights
and measures associated with US commerce.
All measurements for engineering should be
conducted using calibrated and certified
instruments and tools (traceable to NIST).
How does it work?
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NIST and other closely associated labs create
physical artifacts and determine their
characteristics (how long are they, etc.)
Physical artifacts = calibration standards
Calibration standards are used by labs to
calibrate measurement instruments and tools.
Calibration and Certification
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Calibration determines the uncertainty (error) of a measurement
device by comparing its measurement with the appropriate
measurement standard.
“Uncertainty” is a statistical description of the difference
between a measured value and the true value (the true value is
never known, it is only estimated).
Uncertainty is sometimes referred to as error.
Certification is the documentation of the calibration
“Unusual” Conversion Units…
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Ratio of an igloo’s circumference to its diameter: Eskimo Pi
2000 pounds of Chinese soup – Won ton
1 millionth of a mouthwash – 1 microscope
Time between slipping on a peel and smacking the pavement – 1
bananosecond
Weight an evangelist carries with God: 1 billigram
Time it takes to sail 220 yards at 1 nautical mile per hour: Knotfurlong
365.25 days of drinking low-calorie beer because it’s less filling – 1
lite year
16.5 feet in the Twilight Zone: 1 Rod Serling
Half of a large intestine: 1 semicolon
1000 aches: 1 megahurtz
Basic unit of laryngitis: 1 hoarsepower
Shortest distance between two jokes: a straight line
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454 graham crackers: 1 pound cake
1 million-million microphones: 1 megaphone
1 million bicycles – 2 megacycles
2000 mockingbirds: two kilomockingbirds
120 cards: 1 decacards
1 kilogram of falling figs: 1 Fig Newton
1000 grams of wet socks: 1 literhosen
1 millionth of a fish – 1 microfiche
1 trillion pins: 1 terrapin
10 rations: 1 decoration
100 rations: 1 C-ration
2 monograms: 1 diagram
8 nickels: 4 paradigms
2.4 statue miles of intravenous surgical tubing at Yale University
Hospital: 1 I.V. League
100 Senators: Not 1 decision
REFERENCES
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Beckwith, Marangoni, and Lienhard, Mechanical
Measurements, 5th ed., Addison-Wesley
Publishing, 1993.
Wheeler and Ganji, Introduction to Engineering
Experimentation, Prentice Hall, 1996)
DISCOVER, Vol. 25 No. 03, March 2004
Questions?