Standard Kilogram

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Ian Macmillan - 19 Sep 2007 17:31 GMT

The recent report about a 50ug discrepancy in the comparison of the standard
1 Kg mass with its copies led me to wonder how such precision is attained,
50ug being 5 x 10^-9 Kg.

Can anyone describe the method of making such a comparison, and how the
degree of confidence in the result is estimated?

All the best
Ian Macmillan

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REMOVE - 20 Sep 2007 03:56 GMT

"Ian Macmillan"

> 50ug being 5 x 10^-9 Kg.

50 ug = 5 x 10^-8 kg !!

Aage

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Ian Macmillan - 21 Sep 2007 16:28 GMT

> "Ian Macmillan"
>
[quoted text clipped - 3 lines]
>
> Aage

How true, how very true. But 5 x 10^-8 Kg is still 4/5 of a very small
amount compared to a kilogram, and I am no wiser about the difficulties of
measuring this. Your turn!

All the best
Ian Macmillan

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enders@ekp.uni-karlsruhe.de - 21 Sep 2007 00:42 GMT

> The recent report about a 50ug discrepancy in the comparison of the standard
> 1 Kg mass with its copies led me to wonder how such precision is attained,
[quoted text clipped - 5 lines]
> All the best
> Ian Macmillan

I assume such a precision is possible by converting a measuring of
mass into a measuring of length.

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Uncle Al - 21 Sep 2007 16:28 GMT

> > The recent report about a 50ug discrepancy in the comparison of the standard
> > 1 Kg mass with its copies led me to wonder how such precision is attained,
[quoted text clipped - 8 lines]
> I assume such a precision is possible by converting a measuring of
> mass into a measuring of length.

Measuring a small signal within a big background is not clever. Null
the background to leave the net signal remaining, as with a balance
rather thana scale. The necessary mass differential sensitivity is
not unreasonable to obtain in hardware,

http://prola.aps.org/abstract/PRL/v89/i16/e161102
"Determination of the Gravitational Constant with a Beam Balance "
Phys. Rev. Lett. 89 161102 (2002)

Or, e-mail the folks and ask them how they determined the change.

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Ian Macmillan - 22 Sep 2007 03:40 GMT

> > > The recent report about a 50ug discrepancy in the comparison of the standard
> > > 1 Kg mass with its copies led me to wonder how such precision is attained,
[quoted text clipped - 19 lines]
>
> Or, e-mail the folks and ask them how they determined the change.

I appreciate the advantage of a balance, but it seems to me that there are
special difficulties measuring a tiny differential between relatively large
masses. Equality of temperatures, volumes and barometric pressure, symmetry
of pivot friction, magnetic and electric fields, convection, possibly even
tidal effects, and so on must give rise to significant uncertainties
difficult to quantify.

I assume that obtaining the required sensitivity is a given. My query is
directed more to the management of the uncertainties and determining the
degree of confidence in the result.

I suppose in the end one has to say that the Standard is correct by
definition and that the copies have drifted off. It does not seem very
satisfactory. Is there a possibility of a standard of mass other than the
mass of a particular lump of metal?

Incidentally the "Determination of the Gravitational Constant with a Beam
Balance " link would have been interesting but seems to require a paid
subscription for access.

I'd be interested to email the folks who determined the change, but as it
was basicly a media report I have no address.

All the best

Ian Macmillan

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Jonathan Thornburg [remove -anminal to reply] - 22 Sep 2007 20:52 GMT

"Uncle Al" <UncleAl0@hate.spam.net> pointed out a recent measurment
of G by Schlamminger et al,

> http://prola.aps.org/abstract/PRL/v89/i16/e161102
> "Determination of the Gravitational Constant with a Beam Balance "
> Phys. Rev. Lett. 89 161102 (2002)

Alas, as Ian Macmillan <iandmac@tpg.com.au> pointed out, the PRL
web site

> seems to require a paid
> subscription for access.

For anyone else who's interested, Schlamminger's Ph.D thesis covers
a lot of similar material, and google scholar found this on the web at
http://faculty.washington.edu/schlammi/pdf/diss.pdf

Google scholar on "Schlamminger G measurement" also turns up a number
of other related papers...

ciao,

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Dan Riley - 24 Sep 2007 19:20 GMT

> > > > The recent report about a 50ug discrepancy in the comparison of the
> standard
> > > > 1 Kg mass with its copies led me to wonder how such precision is
> attained,
> > > > 50ug being 5 x 10^-9 Kg.

[...]

> I appreciate the advantage of a balance, but it seems to me that there are
> special difficulties measuring a tiny differential between relatively large
> masses. Equality of temperatures, volumes and barometric pressure, symmetry
> of pivot friction, magnetic and electric fields, convection, possibly even
> tidal effects, and so on must give rise to significant uncertainties
> difficult to quantify.

There are various publications from the government standards offices
on cleaning procedures, temperature equilibration, etc. Some of the
factors you list cancel out in a differential measurement, and the
weights of the replicas is always stated as an offset from the
standard 1 kg. The Wikipedia article on the kilogram

http://en.wikipedia.org/w/index.php?title=Kilogram&oldid=159737378

suggests that unintentional variations in the cleaning procedures may
be a significant factor in the variability of the primary standard.
(That link is to the current version as of Sep. 23, 2007.)

> I suppose in the end one has to say that the Standard is correct by
> definition and that the copies have drifted off. It does not seem very
> satisfactory. Is there a possibility of a standard of mass other than the
> mass of a particular lump of metal?

http://www.nist.gov/public_affairs/releases/electrokilogram.htm
"NIST Improves Accuracy of Watt Balance Method for Defining the Kilogram"

Some other approaches are listed in the Wikipedia article.

-dan

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robert bristow-johnson - 21 Sep 2007 16:28 GMT

> The recent report about a 50ug discrepancy in the comparison of the standard
> 1 Kg mass with its copies led me to wonder how such precision is attained,
> 50ug being 5 x 10^-9 Kg.

50 ug = 5 x 10^-8 kg

> Can anyone describe the method of making such a comparison, and how the
> degree of confidence in the result is estimated?

the source page that i can find is: http://www.bipm.org/en/scientific/mass/verifications.html

dunno what scale they use to compare the standard mass to the copies.
i wonder if there is something about cleaning of the main standard and
the copies that contributes to the variations.

i personally think that they should make this go away (sorta) by
defining the kilogram in such a way as to set Planck's Constant to a
defined constant.

r b-j

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J. J. Lodder - 24 Sep 2007 19:20 GMT

> > The recent report about a 50ug discrepancy in the comparison of the standard
> > 1 Kg mass with its copies led me to wonder how such precision is attained,
[quoted text clipped - 10 lines]
> i wonder if there is something about cleaning of the main standard and
> the copies that contributes to the variations.

> i personally think that they should make this go away (sorta) by
> defining the kilogram in such a way as to set Planck's Constant to a
> defined constant.

Unfortunately, that is metrologically impossible,
(for the forseeable future)
for you cannot realise the kilogram on basis of such a definition.
What is (almost) possible is to define Avogadro's number.
(it might have happened already, but unexplained systematic differences
between different sets of measurements delayed things)

This will ultimately change the kilogram into:
One kilogram is the mass of (defined value of Avogadro's number here)
atoms of isotope ###/atomic weight. (C12/12 for example)

Best,

Jan

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Tom Roberts - 25 Sep 2007 10:41 GMT

>> i personally think that they should make this go away (sorta) by
>> defining the kilogram in such a way as to set Planck's Constant to a
[quoted text clipped - 3 lines]
> (for the forseeable future)
> for you cannot realise the kilogram on basis of such a definition.

Richard Steiner of NIST gave a colloquium at Fermilab on precisely this
subject:

"How Measuring the Planck Constant gets to an Electronic Kilogram
Standard"
http://www-ppd.fnal.gov/EPPOffice-w/colloq/colloq_06_07.html
(scroll down to 1-AUG-2007, Abstract, PPT, and video)

Tom Roberts

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robert bristow-johnson - 25 Sep 2007 10:41 GMT

> > i personally think that they should make this go away (sorta) by
> > defining the kilogram in such a way as to set Planck's Constant to a
[quoted text clipped - 3 lines]
> (for the forseeable future)
> for you cannot realise the kilogram on basis of such a definition.

really? i thought Peter Mohr and Barry Taylor (both of NIST, at least
at some time they were) had published multiple times about this.
instead of using the Watt-balance to measure Planck's constant (in
terms of the kg and the other SI defined units for time and length and
current), they use it to determine how much stuff is a kg in terms of
the defined h and the other previously defined units. whatever are
the instruments that are used to accurately measure h (in terms of a
kg standard) can instead measure out what a kg is (in terms of a fixed
h). it's just changing what the one degree of freedom leftover is.

> What is (almost) possible is to define Avogadro's number.
> (it might have happened already, but unexplained systematic differences
[quoted text clipped - 3 lines]
> One kilogram is the mass of (defined value of Avogadro's number here)
> atoms of isotope ###/atomic weight. (C12/12 for example)

yup. that's another proposed re-definition for the kg. there is
another that fixes the mass of the electron (why not some other
fundamental particle?) and another that fixes the elementary charge
(while still not changing the core definition of the Ampere). it
might have gotten over-edited since i put in these quotes of these re-
definitions (from sources in the lit, that i cited) but there are a
few alternate proposed definitions at the wikipedia site (sad how good
articles can be edited into crap over there - i finally gave up
there, ... well, i was sorta kicked out.)

> Best,

and also to you.

r b-j

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Dr J R Stockton - 22 Sep 2007 20:52 GMT

>The recent report about a 50ug discrepancy in the comparison of the standard
>1 Kg mass with its copies led me to wonder how such precision is attained,
>50ug being 5 x 10^-9 Kg.
>
>Can anyone describe the method of making such a comparison, and how the
>degree of confidence in the result is estimated?

By using a very very good balance.

When national copies are compared with the Standard at BIPM, great care
is needed in transporting them there. ISTR hearing that, for those
occasions, a Rolls-Royce was used to transport the UK standard, together
with the person in direct charge and the head of his establishment; but
I cannot vouch for the truth of that.

These may be of interest
<http://www.tmcnet.com/usubmit/2007/09/21/2957865.htm>
<http://technology.newscientist.com/channel/tech/mg19526221.500-perfect-units-kis
sing-the-kilo-goodbye.html>
<http://www.bipm.org/>, search for kilogram watt avogadro

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Uncle Al - 24 Sep 2007 19:20 GMT

> > enders@ekp.uni-karlsruhe.de wrote:
> > >
[quoted text clipped - 8 lines]
> the
> > > > degree of confidence in the result is estimated?

[snip]

Any physical artifact is deficient. The upcoming monocrystal silicon
ball will suffer surface oxidation and hydrogen diffusion.

> Incidentally the "Determination of the Gravitational Constant with a Beam
> Balance " link would have been interesting but seems to require a paid
> subscription for access.

Academic library. E-mail the authors for a reprint.

> I'd be interested to email the folks who determined the change, but as it
> was basicly a media report I have no address.

http://www.wtopnews.com/?nid=220&sid=1246362
<http://www.cnn.com/2007/TECH/science/09/12/shrinking.kilogram.ap/index.html?eref
=rss_mostpopular>

"Physicist Richard Davis of the International Bureau of Weights and
Measures in Sevres, France."

<http://www.zoominfo.com/people/Davis_Richard_53355399.aspx>
<http://www.bipm.org/en/scientific/mass/members.html>

Mankind is a toolmaker. Use your tools.

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Uncle Al
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(Toxic URL! Unsafe for children and most mammals)
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Natural Science Forum / Physics / Research / September 2007

Standard Kilogram