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Reflections on the NPL-2013
Estimate of the Boltzmann Constant
Michael de Podesta
Fundamental Constants
February 2015
Also my wonderful NPL colleagues
Gavin Sutton, Robin Underwood,
Gordon Edwards, Graham Machin,
Richard Rusby, David Flack,
Andrew Lewis, Michael Perkin,
Stuart Davidson, Kevin Douglas,
Rob Ferguson, David Putland,
Anthony Evenden, Louise Wright,
Eric Bennett, Alan Turnbull,
Gareth Hinds, Phil Cooling,
Gergely Vargha, Martin Milton,
Michael Parfitt, Peter Harris, Leigh Stanger
and others
Also my colleagues outside NPL
Paul Morantz, Cranfield
Darren Mark and Fin Stuart, SUERC
Laurent Pitre, LNE-CNAM
Roberto Gavioso, INRiM
Inseok Yang, KRISS
1. Background
2. History
3. The NPL uncertainty estimate
4. NPL Update February 2015
5. The NPL Analysis
6. Summary
Reflections on the NPL-2013
Estimate of the Boltzmann Constant
The NEW International System of Units
c h
kB
∆𝜈
Cs133
K
m kg
s
Energy
(joule)
Molecular
Motion Thermometer
How do we relate the number produced by a
thermometer (e.g. 20 ºC) to the basic physics
describing the jiggling of molecules?
The Challenge
Primary thermometers are based on gases • Molecular motions are simple
• We can approach ‘ideal gas’ conditions at low pressure
• In an ideal gas the internal energy is just the kinetic energy of
the molecules
• Per molecule 𝟏
𝟐𝒎 𝒗𝒙
𝟐 + 𝒗𝒚𝟐 + 𝒗𝒛
𝟐 = 𝟑 ×𝟏
𝟐𝒌𝐁𝑻
𝟏
𝟐𝒎 𝒗𝒙
𝟐
𝟏
𝟐𝒎 𝒗𝒚
𝟐
𝟏
𝟐𝒎 𝒗𝒛
𝟐
Ar
=𝟏
𝟐𝒌𝐁𝑻
=𝟏
𝟐𝒌𝐁𝑻
=𝟏
𝟐𝒌𝐁𝑻
He
𝟗
𝟓𝒔𝒑𝒆𝒆𝒅 𝒐𝒇 𝒔𝒐𝒖𝒏𝒅 𝟐
𝒌𝐁 =𝒎
𝟑𝑻 𝒗𝒙
𝟐 + 𝒗𝒚𝟐 + 𝒗𝒛
𝟐
The big idea…
𝟏
𝟐𝒎 𝒗𝒙
𝟐 + 𝒗𝒚𝟐 + 𝒗𝒛
𝟐 =𝟑
𝟐𝒌𝐁 𝑻 𝒌𝐁 =
𝟑𝒎
𝟓𝑻 𝒔𝒑𝒆𝒆𝒅 𝒐𝒇 𝒔𝒐𝒖𝒏𝒅 𝟐
Find mass of
molecule
Measure the
speed of sound Carry out experiment
at TTPW
Measure the speed of sound in a spherical resonator
Speed of Sound =𝑅𝑒𝑠𝑜𝑛𝑎𝑛𝑡 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 × 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑅𝑎𝑑𝑖𝑢𝑠
constant
0
0.001
0.002
0.003
0.004
0.005
7526 7530 7534 7538
Sig
na
ls /
V
Frequency / Hz
Centre Frequency
7532.512 234 Hz
± 0.000 070 Hz
Temperature
20.000 000°C
±0.000 005 °C
?
Measure the Average Radius using Microwaves
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑅𝑎𝑑𝑖𝑢𝑠 =𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 × 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑅𝑒𝑠𝑜𝑛𝑎𝑛𝑡 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦
Microwave resonance
in a perfect sphere
0
20
40
60
80
100
2109 2110 2111 2112
Sig
na
l
Frequency (MHz)
TM11 Resonance
F0 is inversely proportional
to the radius
Microwave resonance
in a nearly perfect sphere
0
20
40
60
80
100
2109 2110 2111 2112
Sig
na
l
Frequency (MHz)
TM11 Resonance
F0 is in error – but not
possible to say by how much!
Microwave resonance
in a triaxial ellipsoid
0
20
40
60
80
100
2109 2110 2111 2112
Sig
na
l
Frequency (MHz)
TM11 Resonance
Measuring F0, F1 and F2
•Average radius
•Shape
•Uncertainty
Radius 62 mm
0.031 mm eccentrcity
Infers kB from measurements of the speed of sound in a
combined microwave and acoustic resonator
Acoustic Gas Thermometery Microwaves Acoustics
SPL
Frequency (kHz)
(0,2) (0,3) (0,4)
Frequency
Corrections
(Boundary Layer)
Theoretical
S12
Frequency (GHz)
TM12 Triplet
Radius
Frequency
Corrections
(Dielectric)
TM13 Triplet
Pressure
MWf
MWξ
Pressure
Af2
Aξ2
Microwaves Acoustics
SPL
Frequency (kHz)
(0,2) (0,3) (0,4)
Frequency
Corrections
(Boundary Layer)
Theoretical
S12
Frequency (GHz)
TM12 Triplet
Radius
Frequency
Corrections
(Dielectric)
TM13 Triplet
Pressure
MWf
MWξ
Pressure
Af2
Aξ2 Radius
Pressure
ξ
Δf0
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,5)
(0,7)
(0,8)
(0,9)
Spe
ed o
f S
oun
d S
qua
red
c2
(m2 s
-2)
Pressure (kPa)
Isothermal data for c2
𝑐𝐸𝑋𝑃2
𝑐02
0
0.001
0.002
0.003
0.004
0.005
0.006
0 2000 4000 6000 8000 10000
Argon T = 30 C
Am
plit
ud
e (
V)
Frequency / Hz
(1,1)
(2,1)
(0,3)
(0,2)
kB is inferred from 𝒄𝟎𝟐
1. Background
2. History
3. The NPL uncertainty estimate
4. NPL Update February 2015
5. The NPL Analysis
6. Summary
Reflections on the NPL-2013
Estimate of the Boltzmann Constant
2005:
…and just 4 years later…
2009:
2010:
2010:
𝑢𝑅 = 3.1 × 10−6
Highlighted Issues
• Variability of Molar Mass
• Need for precise pressure measurement
• ‘Negative’ Excess Half-Widths
• Line narrowing
2010:
-3
-2
-1
0
1
2
3
0 1 2 3 4 5 6 7 8 9 10 11
Measurement
Ch
an
ge i
n M
(Ar)
/10
6
Repeat
Repeat Repeat
-2%
-1%
0%
1%
2%
3%
4%
5%
6%
1 2 3 4 5 6 7 8 9 10
Measurement
Ch
an
ge
Fro
m M
old
over'
s S
tan
da
rd
Change in R36/40
Change in R38/40
Repeat
Repeat
Repeat
2011:
2011:
2010:
CMM
Microwaves Pyknometry
2011:
Microwaves u(k = 1) = 11.7 nm,
uR(k = 1) = 0.17× 10-6.
Pyknometry
CMM u(k = 1) = 114 nm,
uR(k = 1) = 1.8× 10-6.
2012:
Pyknometry u(k = 1) = 37 nm,
uR(k = 1) = 0.60 × 10-6.
Microwaves u(k = 1) = 11.7 nm,
uR(k = 1) = 0.17× 10-6.
CMM u(k = 1) = 114 nm,
uR(k = 1) = 1.8× 10-6.
2013:
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,5)
(0,7)
(0,8)
(0,9)
Spe
ed o
f S
oun
d S
qua
red
c2
(m2 s
-2)
Pressure (kPa)
Isothermal data for c2
𝑐𝐸𝑋𝑃2
𝑐02
kB is inferred from 𝒄𝟎𝟐
𝑢𝑅 = 0.71 × 10−6
Highlighted Issues
• Variability of Molar Mass
• Need for precise pressure measurement
• ‘Negative’ Excess Half-Widths
• Line narrowing
Since 2013
Acoustic Thermometry
c2 kB = g
M
T NA
-10
-5
0
5
10
-250 -200 -150 -100 -50 0 50
T-
T9
0 (
mK
)
t90
(°C)
-10
-5
0
5
10
-250 -200 -150 -100 -50 0 50
T-
T9
0 (
mK
)
t90
(°C)
WG4-
WG4+
-10
-5
0
5
10
-250 -200 -150 -100 -50 0 50
T-
T9
0 (
mK
)
t90
(°C)
PRELIMINARY
NPL High-Temperature Cylindrical Resonator
Hea
ter
He
ate
r
Insu
lati
on
Air
Gap
1. Background
2. History
3. The NPL uncertainty estimate
4. NPL Update February 2015
5. The NPL Analysis
6. Summary
Reflections on the NPL-2013
Estimate of the Boltzmann Constant
Working Equation
𝑘𝐵 =𝑀/𝛾
𝑁A𝑇TPW×
2𝜋𝑎 𝑓 0,𝑛 + Δ𝑓 0,𝑛
𝜉 0,𝑛
2
• Measure f(0,n) in the limit of low pressure
1. Resonator radius, 𝑎
2. Frequency Corrections Δ𝑓 0,𝑛
3. Eigenvalues 𝜉 0,𝑛
4. Pressure
5. Temperature
6. Molar Mass
1: How wrong could the radius estimate be?
1. Resonator radius, 𝒂
2. Pressure
3. Eigenvalues 𝜉 0,𝑛
4. Frequency Corrections Δ𝑓 0,𝑛
5. Temperature
6. Molar Mass
Microwave Radius Estimates in Vacuum
-60
-50
-40
-30
-20
-10
0
10
1 2 3 4 5 6 7 8
(a (
21
.5 °
C)
- 6
2 0
32
60
0),
nm
Mode (TM1n)
-60
-50
-40
-30
-20
-10
0
10
1 2 3 4 5 6 7 8
(a (
21
.5 °
C)
- 6
2 0
32
60
0),
nm
Mode (TM1n)
±3.5 nm
±9 nm
nanometres
With Microphones
No microphones
2: How wrong could the pressure be?
1. Resonator radius, 𝑎
2. Pressure
3. Eigenvalues 𝜉 0,𝑛
4. Frequency Corrections Δ𝑓 0,𝑛
5. Temperature
6. Molar Mass
11.7 nm 0.38 ppm
u(k=1) u(kB)
Low Pressure Measurements
• Affects thermal boundary layer correction
• Affects estimate of p-1 term
94,750
94,755
94,760
94,765
94,770
94,775
94,780
0 20 40 60 80 100
-50
0
50
100
150
200
2500 10 20 30 40
Spe
ed o
f S
oun
d S
qua
red
(m
2 s
-2)
Pressure (kPa)
Data including
the p-1 term
App
roxim
ate
Fra
ctio
nal C
han
ge (p
arts
in 1
06)
Molar density (mol m-3)
Microwave Radius Estimates at Pressure
62.0104
62.0105
0 100 200 300 400 500 600 700
aeq /m
m
Pressure /kPa
62.0104
62.0105
0 100 200 300 400 500 600 700
aeq /m
m
Pressure /kPa
Data has been corrected for
dielectric constant of gas
(which depends on pressure)
Look at the residuals of a
straight-line fit to this data for
systematic pressure errors
100 nm
Pressure
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
0 100 200 300 400 500 600 700
Isotherm 3Isotherm 4Isotherm 5
-30
-20
-10
0
10
20
30
Re
sid
uals
(nm
)
Pressure (kPa)
Re
sid
uals
(Pa)
Traceable pressure
meter calibration
(before and after)
3: How wrong could the eigenvalues be?
1. Resonator radius, 𝑎
2. Pressure
3. Eigenvalues 𝝃 𝟎,𝒏
4. Frequency Corrections Δ𝑓 0,𝑛
5. Temperature
6. Molar Mass
11.7 nm 0.38 ppm
6.3 Pa 0.11 ppm
u(k=1) u(kB)
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,5)
(0,7)
(0,8)
(0,9)
Spe
ed o
f S
oun
d S
qua
red
c2
(m2 s
-2)
Pressure (kPa)
Data for c2
𝑐𝐸𝑋𝑃2
𝑐02
From high pressure
studies
Common to all modes
Low Pressure Speed of
Sound Squared
Common to all modes
‘Accommodation’
Correction to Boundary
Layer
Common to all modes
Experimental
Estimates
Function of
pressure P
and mode, n
Virial Correction
Common to all modes
‘Shell’ Correction
Varies with mode
Virial Correction
Common to all modes
Data Model
𝑐𝐸𝑋𝑃2 −𝐴3𝑃3= 𝑐0
2 + 𝐴−1𝑃−1 + 𝐴1𝑛𝑃 + 𝐴2𝑃2
After correction for Boundary Layer
-4
-3
-2
-1
0
1
2
3
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
l
Pressure (kPa)
96% of data
within ±2uR
Residuals of all data to fits
expressed in terms of standard uncertainty
Low Pressure Speed of
Sound Squared
Common to all modes
Data Model
𝑐𝐸𝑋𝑃2 −𝐴3𝑃3= 𝑐0
2 + 𝐴−1𝑃−1 + 𝐴1𝑛𝑃 + 𝐴2𝑃2
𝑢(𝑐02) = 0.017 m2 s−2
𝑢𝑅(𝑐02) = 0.18 × 10−6
4: How wrong could the Frequency Corrections be?
1. Resonator radius, 𝑎
2. Pressure
3. Eigenvalues 𝜉 0,𝑛
4. Frequency Corrections 𝚫𝒇 𝟎,𝒏
5. Temperature
6. Molar Mass
11.7 nm 0.38 ppm
6.3 Pa 0.11 ppm
0.18 ppm
u(k=1) u(kB)
Central frequency
0
0.001
0.002
0.003
0.004
0.005
Sig
na
ls /
V
Frequency / Hz
Half-Width should be
exactly what we expect
When f0 =3548.8095 Hz
expected width = 2.864 Hz
measured width = 2.858 Hz
-2
0
2
4
6
8
10
0 100 200 300 400 500 600 700
10
6 x
g/
f (0,n
)
P / kPa
(0,5)
(0,2)
(0,3)
(0,4)
(0,7)
(0,9)
(0,8)
-2
0
2
4
6
8
10
0 100 200 300 400 500 600 700
10
6 x
g/f
(0,n
)
Pressure (kPa)
(0,7)
(0,5)
(0,4)
(0,8)
(0,2)
(0,3)
(0,9)
Half-Width (Experiment – Theory) Parts per million of resonance frequency
f0 =3548.8095 Hz
expected width = 2.864 Hz
measured width = 2.858 Hz
Half-Width (Experiment – Theory) Shell Interaction
Shows the effect of mode and pressure
0
2
4
6
8
10
2 3 4 5 6 7 8 9
Excess H
alf-W
idth
(
g/f
) (p
pm
)
Radial Mode Index
Pre
ssure
(0,2) (0,9)
(0,6)
-2
0
2
4
6
8
10
0 100 200 300 400 500 600 700
10
6 x
g/
f (0,n
)
P / kPa
(0,5)
(0,2)
(0,3)
(0,4)
(0,7)
(0,9)
(0,8)
-2
0
2
4
6
8
10
0 100 200 300 400 500 600 700
10
6 x
g/f
(0,n
)
Pressure (kPa)
(0,7)
(0,5)
(0,4)
(0,8)
(0,2)
(0,3)
(0,9)
Half-Width (Experiment – Theory) Parts per million of resonance frequency
f0 =3548.8095 Hz
expected width = 2.864 Hz
measured width = 2.858 Hz
Half-Width
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0 50 100 150 200
(0,2)(0,3)(0,4)(0,5)(0,7)(0,8)(0,9)
10
6 x
g/
f (0,n
)
P / kPa
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0 50 100 150 200
(0,2)(0,3)(0,4)(0,5)(0,7)(0,8)(0,9)
10
6 x
g/f
(0,n
)
Pressure (kPa)
f0 =3548.8095 Hz
expected width = 2.864 Hz
measured width = 2.858 Hz
Experiment – Theory
Part
s p
er
mill
ion o
f
reso
nance f
requency
Experiment – New Theory
5: How wrong could the Temperature be?
1. Resonator radius, 𝑎
2. Pressure
3. Eigenvalues 𝜉 0,𝑛
4. Frequency Corrections Δ𝑓 0,𝑛
5. Temperature
6. Molar Mass
11.7 nm 0.38 ppm
6.3 Pa 0.11 ppm
0.18 ppm
~0
u(k=1) u(kB)
Temperature Gradient
Temperature gradient was
±91 µK about equator
Temperature
0
2
4
6
8
10
-150 -100 -50 0 50 100 150
Perc
en
tag
e o
f S
urf
ace
Are
a o
r V
olu
me
Deviation from Mean Temperature (K)
Volume
Surface
Average of Equatorial
Thermometers
We modelled the
temperature at the inner
surface of the sphere, and
then in the gas.
6: How wrong could the Molar Mass be?
1. Resonator radius, 𝑎
2. Pressure
3. Eigenvalues 𝜉 0,𝑛
4. Frequency Corrections Δ𝑓 0,𝑛
5. Temperature
6. Molar Mass
11.7 nm 0.38 ppm
6.3 Pa 0.11 ppm
0.18 ppm
~0
0.364 ppm 0.099 mK
u(k=1) u(kB)
Relative Molar Mass determined acoustically
-6
-5
-4
-3
-2
-1
0
1
0 2 4 6 8 10
2
f/f
wrt
Isoth
erm
5 g
as /10
-6
Gas Sample
Iso
the
rm 5
Iso
the
rm 3
&4
-6
-5
-4
-3
-2
-1
0
1
0 2 4 6 8 10
2
f/f
wrt
Isoth
erm
5 g
as /10
-6
Gas Sample
Iso
the
rm 5
Iso
the
rm 3
&4
Iso
the
rm 6
&7
-6
-5
-4
-3
-2
-1
0
1
0 2 4 6 8 10
2
f/f
wrt
Isoth
erm
5 g
as /10
-6
Gas Sample
Iso
the
rm 5
Iso
the
rm 3
&4
Iso
the
rm 6
&7
Iso
the
rm 8 B
OC
Ar 4
19
3
Sp
ectra
Ga
se
s
Ar 6
27
1
Ar 6
27
2
-6
-5
-4
-3
-2
-1
0
1
0 2 4 6 8 10
2
f/f
wrt
Isoth
erm
5 g
as /10
-6
Gas Sample
Iso
the
rm 5
Iso
the
rm 3
&4
Iso
the
rm 6
&7
Iso
the
rm 8 B
OC
Ar 4
19
3
Sp
ectra
Ga
se
s
Ar 6
27
1
Ar 6
27
2
Molar Mass Differences from Isotherm 5 Gas
kB gas Further Investigations
6: How wrong could the Molar Mass be?
1. Resonator radius, 𝑎
2. Pressure
3. Eigenvalues 𝜉 0,𝑛
4. Frequency Corrections Δ𝑓 0,𝑛
5. Temperature
6. Molar Mass
11.7 nm 0.38 ppm
6.3 Pa 0.11 ppm
0.18 ppm
~0
0.364 ppm 0.099 mK
0.390 ppm 0.390 ppm
Traceable to isotopic composition
of atmospheric argon
u(k=1) u(kB)
Uncertainty
-4
-3
-2
-1
0
1
-2
-1
0
1
2
k
B -
NP
L (
part
s in 1
06)
k
B - CO
DA
TA
(parts
in 1
06)
LNE2011
NIST1988
ThisWork
uR(k =1) = 0.71 ×10-6
Have we learned anything since 2013
that could shed light on the LNE-CNAM-
NPL discrepancy?
1. Background
2. History
3. The NPL uncertainty estimate
4. NPL Update February 2015
5. The NPL Analysis
6. Summary
Reflections on the NPL-2013
Estimate of the Boltzmann Constant
CODATA November 2015
NPL Update
February 2015
1. New estimate for thermal conductivity
2. Temperature Gradients
3. Argon Isotopic Molar Mass
16.30
16.40
16.50
16.60
16.70
0 100 200 300 400 500 600 700
(m
W m
-1 K
-1)
P (kPa)
New estimate for thermal conductivity of argon
• Δ𝜆 (-0.11% ) is close to
estimated uncertainty
• (u = 0.1% ) NPL paper
New uncertainty in 𝜆
(u = 0.02% ) is a factor 5
lower than we estimated
16.30
16.40
16.50
16.60
16.70
0 100 200 300 400 500 600 700
(m
W m
-1 K
-1)
P (kPa)
16.30
16.40
16.50
16.60
16.70
0 100 200 300 400 500 600 700
(m
W m
-1 K
-1)
P (kPa)
LNE
New Results
94,680
94,700
94,720
94,740
94,760
94,780
94,800
0 20 40 60 80 100-800
-600
-400
-200
0
200
400
0 10 20 30 40
Sp
ee
d o
f S
oun
d S
qu
are
d (
m2 s
-2)
Pressure (kPa)
(0,2)
As Measured
(0,3) (0,4)
(0,5)
(0,6)
(0,7)
(0,8)
(0,9)
Ap
pro
xim
ate
Fra
ctio
na
l Ch
an
ge
(parts
in 1
06)
Molar density (mol m-3)
94,680
94,700
94,720
94,740
94,760
94,780
94,800
0 20 40 60 80 100-800
-600
-400
-200
0
200
400
0 10 20 30 40
Sp
ee
d o
f S
oun
d S
qu
are
d (
m2 s
-2)
Pressure (kPa)
(0,2)
As Measured
Corrected
(0,3) (0,4)
(0,5)
(0,6)
(0,7)
(0,8)
(0,9)
Ap
pro
xim
ate
Fra
ctio
na
l Ch
an
ge
(parts
in 1
06)
Molar density (mol m-3)•ΔkB = -0.19 ppm.
•uR ~ 0.69 x 10-6
• Excess half-widths
increased
• Δg/f ~ + 0.1 x 10-6
@100 kPa
New estimate for thermal conductivity of argon
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0 50 100 150 200
(0,2)(0,3)(0,4)(0,5)(0,7)(0,8)(0,9)
10
6 x
g/f
(0,n
)
Pressure (kPa)
Part
s p
er
mill
ion o
f
resona
nce f
requency
Experiment – New Theory
• Thermal conductivity
revision will increase the
low-pressure end of these
curves by ~0.1 parts in 106.
New estimate for thermal conductivity of argon
NPL Update
February 2015
1. New estimate for thermal conductivity
2. Temperature Gradients
3. Argon Isotopic Molar Mass
Temperature Gradient
• Temperature gradient was
±91 µK about equator
Since 2013
• Replaced microphones
• Moved the pre-amplifier.
• Two additional thermometers
added to sphere (6 in all).
• No systematic gradient:
(max – min) is ±58 µK
• No ‘change in kB’ (u~0.3 ppm).
0.00980
0.00985
0.00990
0.00995
0.01000
0 2 4 6 8 10 12 14 16
bottom L&Nbottomequatorequatortop L&Ntop
Therm
om
ete
r R
ead
ing (
°C)
elapsed hours
Temperature Gradient
NPL Update
February 2015
1. New estimate for thermal conductivity
2. Temperature Gradients
3. Argon Isotopic Molar Mass
NPL update#3: Molar Mass
• IRMM and KRISS have made gravimetrically traceable isotopic analyses
• Comparison between KRISS (2014) and IRMM (2009) analysis of the same samples
Figure 7. The observed variability of the speed-of-sound squared in argon compared with gas used in
Isotherm 5. The data were estimated by measuring the resonant frequency of the (0,3) resonance with
the gas close to the temperature of the triple-point of water and at pressure of 200 kPa. The graph
shows twice the fractional shift in the resonant frequency expressed as parts in 106. Plotting the data
in this way means the vertical axis is equivalent to the fractional change in molar mass of the gas.
• No clear pattern of agreement or disagreement between KRISS and IRMM
1. Background
2. History
3. The NPL uncertainty estimate
4. NPL Update February 2015
5. The NPL Analysis
6. Summary
Reflections on the NPL-2013
Estimate of the Boltzmann Constant
Self-Consistent Analysis
•Makes the significance of fits to data meaningful.
•Data
•Type A uncertainty of Data
•Model
•Fit the model to the data
•Look at residuals (data - model)
•Show data and model are self-consistent
•We published our data and analysis scripts.
•These have been independently checked
Data
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,7)
(0,8)
(0,9)
(c0)2
(m
2 s
-2)
Pressure (kPa)
Isotherm 3
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,7)
(0,8)
(0,9)
(c0)2
(m
2 s
-2)
Pressure (kPa)
Isotherm 5
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,7)
(0,8)
(0,9)
(c0)2
(m
2 s
-2)
Pressure (kPa)
Isotherm 4
From high pressure
studies
Common to all modes
Low Pressure Speed of
Sound Squared
Common to all modes
‘Accommodation’
Correction to Boundary
Layer
Common to all modes
Experimental
Estimates
Function of
pressure P
and mode, n
Virial Correction
Common to all modes
‘Shell’ Correction
Varies with mode
Virial Correction
Common to all modes
𝑐𝐸𝑋𝑃2 −𝐴3𝑃3= 𝑐0
2 + 𝐴−1𝑃−1 + 𝐴1𝑛𝑃 + 𝐴2𝑃2
Type A Uncertainty
• Estimated from repeats of a resonance acquisition.
• Use pooled uncertainty to weight the data used in the fit.
• Inflate uncertainty estimate
0.00
0.01
0.10
0 100 200 300 400 500 600 700
Typ
e A
Unce
rtain
ty u
(c2)
(m2 s
-2)
All modes0.00
0.01
0.10
0 100 200 300 400 500 600 700
Typ
e A
Unce
rtain
ty u
(c2)
(m2 s
-2)
All modes
0.00
0.01
0.10
uPooledInflated
0 100 200 300 400 500 600 700
Typ
e A
Unce
rtain
ty u
(c2)
(m2 s
-2)
All modes
Normalised Residuals from Global Fit: Isotherm 3
• 96% of residuals fall within ±2u of best fit.
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rmalis
ed R
esid
uals
(0,2)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rmalis
ed R
esid
uals
(0,9)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rmalis
ed R
esid
uals
(0,8)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rmalis
ed R
esid
uals
(0,7)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rmalis
ed R
esid
uals
(0,4)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rmalis
ed R
esid
uals
(0,3)
Pressure (kPa)
Normalised Residuals from Global Fit: Isotherm 4
• 96% of residuals fall within ±2u of best fit.
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,2)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,3)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,4)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,7)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,8)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed R
esid
uals
(0,9)
Pressure (kPa)
Isotherm 4
Normalised Residuals from Global Fit: Isotherm 5
• 96% of residuals fall within ±2u of best fit.
Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed R
esid
uals
(0,9)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,8)
Pressure (kPa)
Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,7)
Pressure (kPa)
Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,4)
Pressure (kPa)
Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,3)
Pressure (kPa)
Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rmalis
ed R
esid
uals
(0,2)
Pressure (kPa)
Isotherm 5
Residuals by Mode
Normalised Residuals from Global Fit: (0,2)
• 96% of residuals fall within ±2u of best fit.
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rmalis
ed R
esid
uals
(0,2)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lised
Resid
ua
ls
(0,2)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lised
Resid
ua
ls
(0,2)
Pressure (kPa)
Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rma
lised
Resid
ua
ls
(0,3)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lised
Resid
ua
ls
(0,3)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed
Resid
ua
ls
(0,3)
Pressure (kPa)
Isotherm 5
Normalised Residuals from Global Fit: (0,3)
Normalised Residuals from Global Fit: (0,4)
• 96% of residuals fall within ±2u of best fit.
Normalised Residuals from Global Fit: (0,7)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rma
lised
Resid
ua
ls
(0,4)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed
Resid
ua
ls
(0,4)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lised
Resid
ua
ls
(0,4)
Pressure (kPa)
Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rma
lised
Resid
ua
ls
(0,7)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lised
Resid
ua
ls
(0,7)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed
Resid
ua
ls
(0,7)
Pressure (kPa)
Isotherm 5
Normalised Residuals from Global Fit: (0,8)
• 96% of residuals fall within ±2u of best fit.
Normalised Residuals from Global Fit: (0,9)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rma
lised
Resid
ua
ls
(0,8)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lised
Resid
ua
ls
(0,8)
Pressure (kPa)
Isotherm 4
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lised
Resid
ua
ls
(0,8)
Pressure (kPa)
Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700
Isotherm 3
No
rma
lised
Resid
ua
ls
(0,9)
Pressure (kPa)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lised
Resid
ua
ls
(0,9)
Pressure (kPa)
Isotherm 4Isotherm 5
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed
Resid
ua
ls
(0,9)
Pressure (kPa)
Alternative Analysis
Data
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,7)
(0,8)
(0,9)
(c0)2
(m
2 s
-2)
Pressure (kPa)
Isotherm 3
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,7)
(0,8)
(0,9)
(c0)2
(m
2 s
-2)
Pressure (kPa)
Isotherm 5
94,750
94,800
94,850
94,900
94,950
95,000
0 100 200 300 400 500 600 700
(0,2)
(0,3)
(0,4)
(0,7)
(0,8)
(0,9)
(c0)2
(m
2 s
-2)
Pressure (kPa)
Isotherm 4
𝑐𝐸𝑋𝑃2 −𝐴3𝑃3= 𝑐0
2 + 𝐴−1𝑃−1 + 𝐴1𝑃 + 𝐴2𝑃2
• Do this 18 times
• Average the 18 estimates of 𝑐02
• Treat each of 18 isotherm/modes independently
• Gives 18 estimates for 𝑐02 instead of 1
• Gives 18 estimates for 𝐴1 instead of 6
• Gives 18 estimates for 𝐴2 instead of 1
Normalised Residuals of Individual Fits: Isotherm 3
• Type A uncertainty estimated from 10 – or more – repeats of a resonance acquisition.
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 3 (0,2)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 3 (0,9)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 3 (0,8)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 3 (0,7)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 3 (0,4)
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed
Re
sid
ua
lsPressure (kPa)
Isotherm 3 (0,3)
Normalised Residuals of Individual Fits: Isotherm 4
• Type A uncertainty estimated from 10 – or more – repeats of a resonance acquisition.
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 4 (0,9)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 4 (0,8)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 4 (0,7)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 4 (0,4)
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed
Re
sid
ua
ls
Pressure (kPa)
Isotherm 4 (0,3)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 4 (0,2)
Normalised Residuals of Individual Fits: Isotherm 5
• Type A uncertainty estimated from 10 – or more – repeats of a resonance acquisition.
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 5 (0,7)
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed
Re
sid
ua
ls
Pressure (kPa)
Isotherm 5 (0,9)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 5 (0,8)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 5 (0,4)
-4
-2
0
2
4
0 100 200 300 400 500 600 700
No
rma
lise
d R
esid
ua
ls
Pressure (kPa)
Isotherm 5 (0,2)
-4
-2
0
2
4
0 100 200 300 400 500 600 700N
orm
alis
ed
Re
sid
ua
ls
Pressure (kPa)
Isotherm 5 (0,3)
NPL Strengths#3: Alternative Analysis
• Fit each isotherm individually and produce 18 estimates for 𝑐02.
• Average value is ~+ 0.25 ppm higher than ‘global’ estimate (+1.4u)
94755.9
94756.0
94756.1
94756.2
94756.3
94756.4
94756.5
94756.6
-3
-2
-1
0
1
2
3(c
0)2
(m
2 s
-2)
Mode/Isotherm
Sh
ift from
glo
ba
l estim
ate
(pp
m)
Isotherm 3 Isotherm 4 Isotherm 5
1. Background
2. History
3. The NPL uncertainty estimate
4. NPL Update February 2015
5. The NPL Analysis
6. Summary
Reflections on the NPL-2013
Estimate of the Boltzmann Constant
Summary
1. NPL-2013 estimate of kB has uR = 0.71 x 10-6
2. Differs significantly from LNE-CNAM-2011
3. Significant differences in analytical
assumptions and estimated sensitivity to errors
4. Possible reconciliation is through a molar mass
error by either NPL or LNE-CNAM.
5. Time will tell.