Precision Measurement Techniques

Precision Measurement Techniques

Murray EarlyMeasurement Standards Laboratory of

New Zealand,

Industrial Research Ltd

2

Overview

1. A bit about MSL and IRL MSL: Measurement Standards Laboratory of New Zealand IRL: Industrial Research Limited

2. The International System of Units (the SI) Describe the base units

3. Precision Measurement Techniques Some of the methods used to realize the SI electrical quantities

4. Future Developments of the SI Replacing the kg

3

0. Wellington, NZ

LatitudeNZ: 41 SJapan: 36 N

AreaNZ: 266 000 km2

Japan: 378 000 km2

PopulationNZ: 4.2 MJapan: 127 M

4

Progress


2. The International System of Units (the SI) describe the base units

3. Precision Measurement Techniques some of the methods used to realize the SI electrical quantities


5

1. MSL, IRL, CRI, NMI ….!!

MSL Measurement Standards Laboratory of New Zealand About 30 staff, mainly scientists (15 PhD’s in Physics, 2 PhD’s in Chemistry) Maintains the national physical measurement standards in New Zealand Has a legislative role to define the values of physical units

used in New Zealand Is part of a bigger organization: IRL

IRL Industrial Research Limited One of 8 Crown Research Institutes

(government owned companies) About 300 staff (~ 200 scientists) Major groups:

Carbohydrate Chemistry (patents formaterials used in cancer therapy)

High Temperature Superconductors (patent for the discovery of BSSCOor Bi-2223, one of the most usefulHTS materials)

6

1. National Metrology Institutes (NMI’s)

New Zealand’s NMI is MSL in Wellington (http://msl.irl.cri.nz/) Part of IRL, one of 8 government research institutes

Japan’s NMI is NMIJ in Tsukuba (http://www.nmij.jp/) Part of AIST, the main research institutes in Japan

Nearly all industrialised countries have an NMI Good links via collaboration, long term relationships Strong overlap of problems of interest

http://www.nmij.jp/

7


More formal links through international arrangements Global groupings called Regional Metrology Organisations Japan and New Zealand are in APMP (Asia Pacific

Metrology Program) Global agreement about the recognition of

measurements made in other countries Based on a set of internationally accepted and published

measurement capabilities (see BIPM website) Rigorously verified by international measurement

comparisons

8

1. NMI’s are Globally Connected

Regional MetrologyOrganizations

9


What do NMI’s do for their nation? Provide traceability to the SI

research capability maintain and develop primary standards calibration services training and advice

Provide the science behind the national quality infrastructure

standardisation, metrology, testing, certification, accreditation

10

1. National Quality Infrastructure

11

Progress





12

2. Why is this needed?

A measurement system becomes important when people exchange information:- Important for trade - stable

society demands fair trade- Important for scientific

communication

Measurement systems and units are not fundamental – chosen for convenience to humans. e.g. theory h = c = 1

13

2. History

A long history Egypt 2500BC – pyramids built to accuracy of

0.05%

“Do not use dishonest standards when measuring length, weight or quantity” Leviticus 19.35 (Bible)

14

2. A Confusing History

By 1800’s, there was a very large number of measurement systems in use around the world

Complicated, inefficient, not trustworthy A need for a revolution…

15

2. A Revolution in Measurement

Around 1800 the French developed a decimal system based on the properties of the planet: The meter: 10-7 of the length

of the quadrant from the North Pole to the equator via Paris

The kilogram: the weight of 1 cubic decimeter of water

With the discovery of electrical phenomena, it was eventually realized that power and energy should be consistent between mechanical and electrical units (Maxwell, 1863) Led to the units ampere, volt,

coulomb, ohm and also joule and watt

16

2. A Revolution in Measurement

Advantages of the metric system: uniform measures

across a nation and across the world

a scientific basis time invariant and reproducible

a decimal scale offers convenience of

calculation

But note - there are many possible metric systems

17

2. The Metric System

Metric scale: ratios are meaningful (the scale has a natural zero) time interval: 6 seconds/2 seconds = 3 time of day: 6pm/11am =…..?? time of day is only an interval scale: 6pm-11am = 7

hours Key point: only 1 standard defines the entire scale

Need accurate scaling methods to build up the scale Metrologist talk in ratios! e.g. ppm, 10-6 , parts in 106 all mean the same

Metric property allows the measurement scales of different quantities to be combined (quantity calculus): mass in kg acceleration in m/s2 = force in

newtons Want a coherent system: no conversion factors

1 watt = 1 volt 1 ampere Conversion factors appear in formulae

221

04

1

r

qqF

00000342.12

1 V

V

is 3.42 ppm

18

2. History of the Metric System

1875: The Metre Convention signed (now including the second) a diplomatic treaty originally 17 nations Japan signed 1881, New Zealand 1991! 2008: 51 member states, 27 associate states

Other base units added: 1946: the ampere 1948: the kelvin and candela 1971: the mole

19

2. The beginning of the SI

1960: standardised on the MKSA base units: Meter, Kilogram, Second, Ampere + Kelvin, Candela (+

Mole in 1971) Named in French: Le Système international d'unités (SI)

or in English: The International System of Units Base units are definitions – require practical methods to

implement them

Weights and Measures Legislation(1846)

Metre Convention(1875)

Dominion Physical

Laboratory(1939)

Measurement Standards Legislation(1946)

Earthquake building standard

(1932)

Laboratory Accreditation

(1972)

MSL(1992)

CIPM MRA(1999)

1840 2008

20

2. Structure of the Metric System Formal structure from the top level body

(CGPM) down to internationally representative technical committees (see http://www.bipm.org/) For example both Japan and New Zealand

have a representative on the Consulative Committee for Electricity and Magnetism (CCEM)

BIPM – laboratory where the primary artefacts were held (meter and kilogram)

http://www.bipm.org/

21

2. Anticipating Needs

The SI sytem is not static Accuracy improves on average about a factor

of ten every 15 years

10-4

10-5

10-6

10-7

10-8

1980 1990 2000

Year

Rel

ativ

e A

ccu

racy

Present Time

Present industrial‘Best’

instrument / method

Next generationinstrument / method

Nextgenerationstandard

Accuracy Limit

AccuracyOf NationalStandard

Calibration Limit Refinement Region

Region not yet accessible

Regionaccessible to all

(courtesy of Brian Petley of NPL, London)

22

2. The Measurement Arms Race

Continual scientific and technology advances lead to constant improvements in accuracy

Pressure on NMI’s to make continual improvements some accuracy improvements actually simplify the

standard Watt Meters

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1970 1980 1990 2000 2010

Year

Rel

ativ

e A

ccu

racy

Industry need

MSL capability

Self-regulation

Research

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2. Measurement Uncertainty

A measurement without an uncertainty is meaningless

The physics of the measurement process is contained in the uncertainty

The 1993 publication of the ISO “Guide to the Expression for Uncertainty in Measurement” has led to much greater international consistency in the calculation of uncertainty

Quite a lot of research presently being done to ensure uncertainty calculation is based on rigorous statistics

Metrologists take uncertainty seriously!

24

2. SI definition of the Meter

“The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.”

Realized by counting wavelengths of an iodine stabilized He-Ne laser

25

2. The Meter

Initially made equal to one ten-millionth of the distance from the equator to the North Pole Hence earth’s

circumference is ~ 40,000 km

Speed of light was fixed in 1983 by the SI definition

Can resolve physical distances of 1 nm on macroscopic objects

Can achieve uncertainties of 10-12

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2. SI definition of the kilogram

“The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.”

The international prototype kilogram made from Pt-Ir is held at the BIPM in Paris

27

2. The kilogram

The mass of a cubic decimeter of water at the ice point

Concern over undetectable drift ~ 50 g over 100 years

(the mass of a dust particle of 0.4 mm diameter)

To precious to use! (only used for comparisons 3 times in more than 100 years)

The scale is disseminated with stainless steel masses microscopically messy

Very large air buoyancy correction

28

2. SI definition of the Second

“The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.”

Caesium Atomic Clock- best clocks have uncertainties ~ 10-16

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2. The Second

Improvements in clock performance have been a consequence of the very productive research in atomic physics linked to ~ several Nobel prizes

(Ramsey, Phillips, Hall etc) cooled ion and atom clocks,

laser frequency combs General Relativistic corrections

are required NIST time and frequency lab in

Boulder is at an altitude of 1.6 km

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2. History of the Second

Was 1/86400 of the mean solar day but the earth is not a good clock:

From NMIJ website

31

2. Leap Seconds

leap second on 0h 1 Jan 2009

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2. SI definition of the Kelvin

“The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.”

Realised by a triple point cell Water in the form of vapour, liquid

and solid in equilibrium (you can try this at home..)

In practice true thermodynamic temperature is very difficult to measure ideal gas law, Johnson noise

Instead a practical scale based on the temperature of a platimum resistor is used (ITS 90) The resistance ratio is defined at ~

six fixed points corresponding to melting transitions of pure metals (gallium, gold etc)

33

2. The Kelvin

Triple Point Cells are consistent internationally to ~ 40K

A recent finding has been the need to define isotopic composition of the water used in the cell (variations in 2H, 17O and 18O can cause shifts of 100K)

34

2. The Kelvin

In practice true thermodynamic temperature is very difficult to measure ideal gas law, Johnson

noise Instead a practical scale

based on the temperature of a platimum resistor is used (ITS 90) The resistance ratio is

defined at ~ 8 fixed points corresponding to melting transitions of pure metals (mercury, gallium, silver, gold etc)

35

2. SI definition of the Candela

“The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.”

Realised by a Cryogenic Radiometer

36

2. The Candela

Cryogenic Radiometer is actually a measurement of electromagnetic power

Use in an electrical substitution mode Compere the heating

effects of a beam of light with that generated by a current through a resistance heater

The most difficult part is accounting for the loss of light in enetrng the cryostat

Limits accuracy to ~10-5

37

2. SI definition of the Mole

“1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.

2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.”

This defines Avogadro’s constant NA = 6.02214179 x 10-23 /mol

Resolved differences between chemistry and physics But is a counting base unit

necessary?

38

2. SI definition of the Ampere

“The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.”

The present definition of the ampere fixes the magnetic constant 0 (the permeability of vacuum) at 4 x10-7 H/m

Realised by force balances

d

IlengthunitperForce

2

20

I

Id

39

2. MSL’s Base Units in Summary

Mass: three stainless steel 1kg - calibrated at the BIPM

Length: I2 stabilised He-Ne Laser- internationally agreed lines

Time: three HP Cesium Clocks- contribute to global average maintained at

BIPM

Electricity: 10 V Josephson Array, Calculable Capacitor,

Quantum Hall Resistor Temperature: Various Fixed points

- agreed practical temperature scale (IPTS 90)

Radiometry: Cryogenic Radiometer

As well as many derived units and scales (power, impedance, humidity, pressure etc)

40

Progress





41

3. General Comments

In practice the SI definition of the ampere has proved to too difficult to implement at sufficient accuracy For many years only ~ 0.2 ppm

It is possible to make electrical measurements with more precision than can be shown to be consistent with the rest of the SI units e.g. Capacitance ~ 0.001 ppm

Two important discoveries of quantum phenomena have enabled the electrical quantities of voltage and resistance to be obtained with precision better than 0.01 ppm current is a more difficult quantity to measure directly

Field of quantum metrology The standards for electrical quantities have progressed in

a self consistent way well beyond their SI traceability“The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.”

42

3. The Josephson Effects

The ac Josephson effect Junction between two

superconductors Apply ac current at frequency f

(microwave frequency) Get constant-voltage steps

h/2e 2 V/GHz Josephson constant KJ 483 597.9

GHz/V

V

I

n = 0

+1

-1

V

IIc

R

nfe

hVn 2

43

3. Quantum Metrology – the Josephson Volt

Brian Josephson (1973)

"for his theoretical predictions of the properties of a supercurrent

through a tunnel barrier, in particular those phenomena

which are generally known as the Josephson effects"

mVfeh

0.22

Initially realized by a single Josephson junction radiated by microwaves

The key technical advance in the 1990’s was to put thousands of them in an array – and out pops 10 V

About 20 systems now in use by industry and the military in North America

44

3. The Impact of the Josephson Effect

Artifact Quantum

Cell/Array Uncertainty: Parts in 106 Parts in 109

Reproducibility: Parts in 105 Parts in 1010

10-9

10-8

10-7

10-6

10-5

10-4

1920 1940 1960 1980 2000

Between LabsWithin LabsC

hang

e in

Vol

tage

/ V

olta

ge

Year

Weston Cells

SingleJunctions

Arrays

From Bachmair, 1988 and Hamilton, 1998

45

3. Aside: Counting

Note that the most accurate SI quantities involve counting (time and length)

The most accurate because counting is a simple and robust measurement method - can count with very high precision

Ideally would like to convert all measured quantities into a frequency (or vice versa – the Josephson Volt converts a frequency into a voltage)

46

3. Quantum Metrology – the Quantum Hall Effect

2 dimensional electron gas formed at the boundary of

a hetrostructure At high magnetic fields

electrons condense into Landau levels

At certain field values there is an energy gap between levels which cannot be overcome at low temperatures (< 4 K)

The Hall resistance (transverse voltage/ longitudinal current) becomes quantised

Klaus von Klitzing (1985)

“for the discovery of the quantized Hall effect”

2

1

e

h

iRH

47

3. Quantum Metrology – the Quantum Hall Effect

keh

8.252

Laughlin, Stomer and Tsui (1998)

“"for their discovery of a new form of quantum fluid with fractionally charged excitations" ”

48

3. Measuring the QHR

How do we relate the QHR to real resistors without losing accuracy?

Use a Cryogenic Current Comparator (CCC) precise dc current ratio device uses a SQUID (superconducting quantum

interference device) as a null detector

Used in a Bridge configuration bridge Techniques are powerful because they

cancel out common effects (e.g. variation in applied voltage)

matched components, bridge balance sensitive to any differences (e.g. Wheatstone Bridge)

Used in many transducer applications (e.g. strain gauges)

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3. Cryogenic Current Comparator CCC – an almost ideal

current ratio device Set of ratio windings

(1,2,4,8,….4001 turns) Carefully shielded by a

superconductor (lead) in an overlapping tube construction (“a snake swallowing its tail”)

Net flux is coupled to a SQUID (resolution is 10-5 of a flux quantum 0)

Can achieve current ratios of ~10-11

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3. CCC Bridge

Two balances to ensure definition is accurate: Voltage balance

across resistors with high impedance null detector

Current balance via flux balance in CCC

SQUID senses total flux in CCC

51

3. An HTS Cryogenic Current Comparator?

52

3. Quantum Metrology – Quantum Current Source

Promising because it is a counting measurement

International research thrust to increase current and accuracy

SET (single electron tunneling) R pump is promising Analysis of co-tunnelling errors of

various devices by Iwabuchi-san and Bubanja etc

strong relevance to future electronic devices (e.g. memory) and new physics (e.g. qubits for quantum computing)

SAW (surface acoustic wave) Charge offset problems

CCC (cryogenic current comparator) to amplify current

pAef 6.1

U 1 U 2

V/2 -V/2

53

3. Classical Metrology – the Calculable Capacitor

Thompson-Lampard Theorem (1956) A capacitance that depends

only on a single length dimension

Can achieve ~ 10-9

Otherwise geometry insensitive

Measure the cross capacitances C1 and C2 between opposite bars

Since c and 0 are fixed, so is 0

1expexp0

2

0

1

L

C

L

C

20

0

1

c

C

C

l

54

3. Capacitance Bridge

High accuracy is achieved by coaxial bridge techniques and careful measurement definition (four port)

Built around multistage transformer ratios Electronics affected by

drift, material properties Transformers work on a

more fundamental principle (Faraday’s Law)

Probably not taught by any university in the world!

55

Note the scale of these constants – great for the ‘nanoscopic’ world of atomsbut lousy for the macroscopic world we experience .…

1. However: which is nice for a resistance.

2. Also: but combine this with a high electronic frequency,

(say f ~ 100 GHz) then: which is sort of OK for a voltage.

3. But: and combining this with the highest practical frequency,

(say f ~ 10 MHz) then: which is way too small for a nice current.

3. Quantum Metrology Summary

119106.1 HzAe

1151005.22

HzVeh

keh

8.252

mVfeh

0.22

pAef 6.1

56

Quantum Metrology – Summary

Q H R C C C

Ratio Effect Scaling Standard Accuracy MSL

100 2eh

10 -9 90%operational

JV A rray 10 Veh2

10 -8 operational

Q C S A rray/C C C ~ 1 Ae 10 -8 ? research

ac Q H R

ac JV

one day?10 -7 ?100 ,2 kH z

ac B ridges

10 -7 ?1 V ,

2 kH zone day?

57

3. Also AC Electrical Units

Built around simple thermal devices (compare heating of ac and dc)

AC Josephson systems (>$1M) struggle to match the performance of a $100 thermal device (~10-6)

simple fundamental principles

58

Progress





59

4. Future Development of the SI

The kilogram artefact must go! Concern over possible non-

detectable drift Limits the accuracy of SI

electrical quantities Cannot be realised by

anyone else (need direct comparisons back to the BIPM)

Use the existing quantum electrical standards to define the kg

Electrical quantities gain SI accuracy

The kilogram loses SI accuracy

0 becomes measureable!

60

4. But can the electrical units be trusted?

Quantum Metrological Triangle a demanding test of the quantum effects

French NMI has the most advanced project on the verge of getting useful results

f

V I

Josephson Effect

Quantum Hall Effect

SET

2

hV n f

e ( 1,2)I me f m

,...)2,1(,2

nIne

hV

61

Realized by the Watt Balanceexperiments in progress(NPL, NIST, METAS, BIPM, LNE)

moving mode

weighing mode

Electrical Power:

or in terms of constants:

Mechanical Power: (to move a mass m in a gravitational field g at velocity v)

Insist that:

Watt balance is a measurement of h in the SI

4. Watt Balance

h

ehe

hPelec ~2~

2

2RV

Pelec2

mgvPmech

mechelec PP

62

4. Watt balance theory & terms

Mechanical versus electrical energy Two modes: weighing and calibration (static & dynamic) Weighing current I, induced voltage U, coil velocity Geometric factor , magnetic field B

Uv

B

(b)

I

US

mgRB

F

R

(a)

F I dl B I m g YYYYYYYYYYYYYY

U B dl dl B v v YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY

Hence m g U

I v and

U Im

g v

Calibration mode

Weighing mode

63

4. Watt Balance

There are two measurements seeking to allow the redefinition of the kg

Watt Balance (under developemnt at NPL since 1975 now achieving a few parts in 108 )

Silicon Sphere (dimension + lattice spacing) Can make almost perfectly

spherical Problem: these

measurements are inconsistent!

64

4. Inconsistency in kg replacement

NP

L W

att

20

07

CO

DA

TA

20

02

NP

L g

am

ma

-p 1

97

9

NIS

T F

ara

da

y 1

98

0

NIS

T W

att

19

89

NM

L v

olt

19

89

NP

L W

att

19

90

PT

B v

olt

19

91

NIM

ga

mm

a-p

19

95

NIS

T W

att

19

98

CO

DA

TA

19

98

Av

og

ad

ro 2

00

1

Av

og

ad

ro 2

00

3

NIS

T W

att

20

05

CO

DA

TA

20

06

NIS

T W

att

20

07

0.60

0.65

0.70

0.75

0.80

0.85

[h/1

0-3

4 J

s -

6.6

260]

x 1

04

1.1

pp

m

65

4. Time for a change to the SI

physica l a rte facts"p roperties o f our p lane t" -

no t invarien t

atom ic param etersinvarian t, em p irica l ra ther

than fundam enta l

quantumconstants

invarian t and fundam enta l

The trend in the In ternationa l S ystem of U n its (S I):

The ra lly ing cry: "ava ilab le to anyone, anyw here , a t anytim e"(as accura te ly as best poss ib le m easurem en ts requ ire )

The next F rench (m easurem ent) R evo lu tion : to base the S I on 5fundam enta l constants o f defined va lue (by 2011 m aybe)... the P lank constant: h ~ 6.6x10 -34 J s "the cen tra l constan t o f quan tum m echan ics"

the e lem entary charge: e ~ 1.6x10 -19 C

the speed of light: c ~ 3.0x10 8 m s -1 "the cen tra l constan t o f re la tiv ity"

the Boltzm ann constant: k ~ 1.4x10 -23 J K -1

the Avogardro constant: N A ~ 6.0x10 23 m ol-1

66

Summary

The international measurement system works very well for nearly all practical applications

Accurate physical standards have resulted from discoveries in physics

Standards based on rigorous physical laws can achieve high accuracy: CCC – Ampere’s Law SQUID - Flux quantisation QHR - gauge invariance AC Bridges – Faraday’s Law Counting is best

Watch out for significant changes in the SI system coming soon (2011/2015?)

67

Thank you…

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Precision Measurement Techniques

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