Debre Tabor University
Department of Electro-Mechanical Engineering
By
Simachew G.
simagas@dtu.edu.et
Chapter-2 standards of measurment
Overview
Measurement Standards
What are they?
How are basic units of measure defined?
How are measurement standards used?
Calibration and Certification
What is a “Standard”?
A standard is something established as a rule or
basis of comparison in measuring or judging
capacity.
Measurement Standards
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?
The basic requirements for getting
meaningful results of measurements are
i).The standard employed for comparison purpose must
be accurately defined & should be commonly accepted.
ii). The standard must be of the same character as the
measurand & usually is prescribed and defined by a
legal/recognized agency or organization. eg.ISO
iii).The apparatus used & method adopted for the
comparison purpose must be provable.
Who defines standards?
International Standards Organization, ISO
Maintains SI (metric) standards.
National Institute of Standards and Technology,
NIST
Mandated by the US Constitution
International Bureau of Weights and Measures,
BIPM (Bureau international des poids et
mesures) in Sevres, France.
Characteristics
What are characteristics of a measurements
standard?
They must have global availability
They must be accessible and “usable”
They must be stable
They must not change over time or location
What is the basic unit of time?
What is a year?
The time required for the earth to make one complete orbit about
the sun.
What is a day?
The time required for the sun to go from noon to noon.
What is a second?
Could be defined as: 1/(24X60X60) of a day
Just a second!
Measurement standards must be stable.
But the length of a day is constantly changing
Earth’s rotation is slowing
Slowing is inconsistent
Defining a second as 1/(24X60X60) of a day is
no longer adequate!
A “new” second?
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”.
How long is a meter?
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?
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).
The “survey inch” is 2.54000508 cm (1/39.37)
Mass
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?
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?
Temperature is much more complex to define.
It requires definition at multiple points
Fixed point Temperature (K)
Triple point of hydrogen 13.8033
Triple point of neon 24.5561
Triple point of oxygen 54.3584
Triple point of argon 83.8058
Triple point of mercury 234.3156
Triple point of water 273.16
Melting point of gallium 302.9146
Freezing point of lanthanum 429.7485
Freezing point of tin 505.078
Freezing point of zinc 692.677
Freezing point of aluminum 933.473
Freezing point of silver 1234.93
Freezing point of gold 1337.33
Freezing point of copper 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.
What about other things?
The above units of measure are known primary or
fundamental standards.
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.
Velocity is distance per time
Volume is length cubed, etc.
NIST
(National Institute of Standards and Technology)
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.
How does it work?
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
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
There are different types of standards of measurement
on the basis of their function & hierarchy of standards
with different level of accuracy
i).International standards(IS)
Defined by international agreement
They represent certain units of measurements to the
closest possible accuracy that the production &
measurement technology allows.
ii).primary standard(PS)
Maintained by IS laboratories in different parts of the
world.
It’s main function is the verification & calibration of
secondary standards
iii). Secondary standard(SS)
The values of secondary standard quantity is less
accurate than primary standard.
Pre-calibration is required, without calibration the
result given by this instrument is meaningless.
Calibration can be made by comparing the results with
a primary standard instrument or/with an instrument
having high accuracy.
Questions?
