“to measure is to know” but how to measure? · vkrt meeting 8 feb 2007 - 1 nmi - the art of...
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NMi - the Art of MeasurementVKRT meeting 8 Feb 2007 - 1
“to Measure is to Know”
but how to measure?
Gert RietveldNederlands Meetinstituut
Delft
GRietveld@NMi.nl
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Nederlands Meetinstituut
• Legal inspection• Testing and certification• Scientific metrology
(national standards)
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Overview
• Measurement in daily life… and in your profession
• How to make a good measurement?– What is a measurement?– Uncertainty– Effects affecting measurements– Units & the role of NMi– Pass/fail testing– ISO requirements
• Summary
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You wake up…
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Make a cup of tea…
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On the road…
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Fill up with petrol…
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Complement breakfast…
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Oops!
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Do your shopping…
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Make a small bet…
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Take a taxi home…
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Measuring in your job
Production and testing of materials:• Weight• Temperature• Force• Time• Hardness• Length• Volume• Strength• Particle size• ……
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Measurement
• What do you want to know?• How are you doing the measurement?• How good is the measurement?
“De Geograaf” – Jan Vermeer
“The art of measurement”
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Metrology and standaards
• Metrology:the science of measurement
• Why measurement standards and calibrations?
“We do know the length of a meter”
⇒ You can not thrust measurementequipment without verification!
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“to Measure is to Know”
What is a measurement?
Comparison to a reference
Result:
Value = number x unit
± ?
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Uncertainty
Uncertainty the qualification of the result of the measurement
In which band can I expect the “true value”, related to the indication of the instrument
Simply said:
Uncertainty is lack of information
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Uncertainty
Precise Accurate
•
• •
•••
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Uncertainty sources
• Noise, repeatability (imprecision)• Systematic effects:
– Environment (air, oil, water, etc)– Temperature of test setup and of environment– Sample dimensions– Time, drifts– Resolution– Offsets, biases– Human factor (operator)– Errors in analysis (Arrhenius plot)– Sample preparation– Reference equipment (calibration!)– Etc….
“the less you know, the better you measure”
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Sample preparation
• Batch reproduction
⇒ how stable is production?
• Sampling within batch
⇒ how homogeneous is production?
⇒
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“to measure is to know”
What is a measurement?
Comparison with a reference
Result:
Value = number x unit
Task of NMi
- SI units!
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History of the meter4000 BC Egypt Cubit Forearm Pharaoh1130 England Yard Nose to thumb King1790 France Meter Circumference earth1799 France Meter Pt rod1960 CGPM Meter Wavelength Kr line1983 CGPM Meter Speed of light
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The ultimate kg … Grand K
BIPM:scientific centre for metrology,depository of the Grand Kmass reference
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Compare measurements
Industry requires traceability!
Unbroken chain of measurements to (inter)nationalmeasurement standards
NL DE
?
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Compare measurements (2)
Via traceabilityComparable measurements!
NL UK DE
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1 kg Key comparison
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Pass / fail
PASS UNDEFINED FAIL
mar
gin
s
Nominal
value
: measurement value with 95% uncertainty interval (k=2)
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Doping - guilty or not?
Player 1 – 7.3 ng/ml
Player 2 – 2.3 ng/ml2 ng/ml
zero
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ISO requirements test & cal labs
ISO 17025 requirements for test and calibration laboratories:
• Management system – equivalent to ISO 9001
• Technical competence– Personnel– Test procedures– Measurement uncertainty– Test equipment– Cross-checks– Quality control – interlaboratory tests
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ISO standards for testing
• Sample characteristics• Preparation of samples• Describe test method• Critical attention points• Uncertainty contributions
– Minimum number of contributions:repeatability, reproducibility + systematic effects
– Methods for determining uncertainty– Typical uncertainty budget
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To Measure is to Know…
• A measurement value without uncertainty is meaningless
• Precise ≠ accurate• Behing each measurement there is a
standard(preferably a quantum standard)
• Only traceable measurement values can be compared with each other
Knowing = measuring = our profession!
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Nederlands Meetinstituut
“The art of measurement”
“De Geograaf” – Jan Vermeer
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