precision measurement techniques
DESCRIPTION
Precision Measurement Techniques. Murray Early Measurement Standards Laboratory of New Zealand, Industrial Research Ltd. Overview. A bit about MSL and IRL MSL: Measurement Standards Laboratory of New Zealand IRL: Industrial Research Limited The International System of Units (the SI) - PowerPoint PPT PresentationTRANSCRIPT
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
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
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
7
1. National Metrology Institutes (NMI’s)
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
1. National Metrology Institutes (NMI’s)
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
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
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)
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
23
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
26
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
29
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
30
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
32
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
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
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)
49
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
50
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
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
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…
68