Measurement & Uncertainty - Basic concept and its application

Sept. 11. 2012

JongOh CHOI (choijongoh@kriss.re.kr)

Head, Center of Standards and Quality management

Chair, Uncertainty committee of KRISS

Education

Ph. D. Analytical Chemistry, (1992) M. Eng. Chemical Engineering, (1983) B. Eng. Chemical Engineering, (1981)

Field of Interest

- Quality of Management (2005 -) - Vocabulary in Metrology (2004 -)

- Uncertainty in Measurement (2003 -) - Reliability of Chemical measurement (National Research Laboratory 1999)

- Analytical chemistry

More than 270 Lectures on

Measurement & Uncertainty, Reference Materials, PT, QMS, etc

Make it measurable !!

What is your interpretation?

The objective of a measurement is to determine the value of the measurand. (GUM, 1993)

- True value (error approach)

- Measured value (uncertainty approach)

kg)05.012.72(

Value ± something

GUM 2008 VIM

2007, 2nd ed.

SI 2006 (8th Ed.)

Unit Uncertainty Vocabulary

Obtained from a training course on uncertainty

?

Contents

1. Concept - measurement, traceability, uncertainty

2. Uncertainty evaluation 3. Error vs. Uncertainty

Vocabulary

• Quantity property of a phenomenon, body, or substance, to which a number

can be assigned with respect to a reference

• Value

number and reference together expressing magnitude of a quantity

• Measurand : quantity intended to be measured

• Measurement

process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity

• Measurement result information about the set of quantity values being attributed to a measurand

Who is going to win in the court?

Traceability of mass

The kilogram is the unit of mass; it is equal to the mass of

the international prototype of the kilogram.

NMI

BIPM

Traceability : after the measurement ???

Metrological traceability: VIM 3 (2007)

− property of measurement result whereby the result can be related reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty

Quantity

property of a phenomenon, body, or substance, to which a number can be

assigned with respect to a reference (VIM3)

Role of Measurement Standard and CRM !!!

It is the pre-requisite for your measurement !!!

Uncertainty evaluation

Weight using a balance at home,

72.45, 72.54, 72.53, 72.44, 72.39, 72.51, 72.33 kg

1) My weight?

2) Standard uncertainty ?

3) Expanded uncertainty ?

Calibration report

2,kg05.012.7200000.72 k

Uncertainty Evaluation

Sampling Analyst

extraction

stability

matrix

ability

experience

Laboratory Reagent Instrument

Temp

humidity

pressure

Reference m.

purity

impurity

resolution

automation

drift

Data treatment

Calibration

Assumption

Result

Variation of each parameter

Guide to the expression of Uncertainty in Measurement (GUM), 1993, BIPM,IEC,IFCC,ISO, IUPAC, IUPAP, OIML

Modeling the measurement

Identifying uncertainty components for each input quantity

Evaluating standard uncertainty Type A, Type B

Combining standard uncertainties of input quantities

Expanded uncertainty

Coverage factor

Sensitivity coefficient

5 steps in MU evaluation

n

su

nsu

sor

),...,,( 21 nxxxfy

ixu

iN

i i

xux

fyu 2

2

1

2

c )(

2k

)(c yukU

),...(),(),(

3,

2,

1,

i

i

i

xuxuxu

Type A evaluation of uncertainty

• Mean

• Standard Deviation

• S.D of the mean

• Degree of freedom

n

xx i

1

)( 2

n

xxs

i

n

s

1 n

Standard Uncertainty n

sui

Type B evaluation

• Previous measurement data

• Experience with or general knowledge of the

behavior and properties of relevant materials and

instrument

• Manufacturer’s specifications

• Data provided in calibration and other certificate

• Uncertainties assigned to reference data taken

from handbooks

Error vs. uncertainty

Conceptual changes of uncertainty

~ 1992 An estimate characterizing the range of values with which

the true value of a measurand lies (VIM 1)

~ 1984 A measure of the possible error in the estimated value of the

measurand as provided by the result of a measurement

~ 2007 Parameter associated with the result of a measurement, that

characterizes the dispersion of the values that could reasonably be

attributed to the measurand (VIM 2)

2008 ~ Parameter that characterizes the dispersion of the quantity

value that are being attributed to a measurand, based on the information

used. (VIM 3)

20

GUM E.5.4.

While the approach based on “true” value and

error yields the same numerical results as the approach taken in this Guide, this Guide’s concept of uncertainty eliminates the confusion between error and uncertainty. Indeed, this Guide’s operational approach, wherein the focus is on the observed (or estimated) variability of the value, makes any mention of error entirely unnecessary.

Reminder

1st

2nd

Range of true value (limit of error) vs. Uncertainty

Summary

1. Concept - measurement, traceability, uncertainty

2. Uncertainty evaluation 3. Error vs. Uncertainty