The Changing Definition Of 'Kilogram'

DrLudicrous writes "The NYTimes is reporting that the platinum-iridium standard mass for the kilogram is shedding at an appreciable rate -- at least compared to other reference masses. The Pt-Ir cylinder is kept in France, and measured annually, and the slight discrepancy is important because the kg is an SI base unit- thus other quantities such as the Volt are based on it. A new standard is being sought- the two frontrunners are counting the number of atoms in a perfectly spherical single crystal of silicon, and another technique uses a device known as the Watt balance."

Kilogram?

[ihatewinXP (638000)]

Hey I live in America you insensitive clod! (but then again I alawys want to know how much they are lifting on Strongman Competition).

Re:Kilogram?

[PhlegmMaster (596165)]

Then maybe america should move out of the dark ages sometime.

Re:Kilogram?

[el-spectre (668104)]

It's not a matter of dark ages, it's a matter of infrastructure... while not the largest country in the world (the US is probably third or fourth, I'm not sure), we have by far the most technological infrastructure. It is not feasible to change all that in a short period of time.

A friend is in construction, and guestimates that it will take over 100 years to replace all failing/obsolete tech with the versions in metric equivalents. It just does not make any economic sense to replace a set of, say, water pipes with the metric standard if the current ones will last 20 years. It'll have to be a gradual thing.

Just to be difficult, though, I'd mention that most construction is done in 'tenths of feet', even the surveying equipment is marked this way. Has nothing to do with the metric system, it just makes the math easier...

Re:Kilogram?

[UniverseIsADoughnut (170909)]

" It's not a matter of dark ages, it's a matter of infrastructure... while not the largest country in the world (the US is probably third or fourth, I'm not sure), we have by far the most technological infrastructure. It is not feasible to change all that in a short period of time."

Bingo! this is why the US has been working on the process so long. Granted the push hasn't been very great but it's happening. If you're a country of a few million and only are the size of a small new england state, the change is pretty cheap and easy. When your huge, there is a massive infastructure change cost. and trying to re-wire 300 million peoples brains to a new way takes a lot more work.

I think places like Europe were also helped by war. They had to rebuild and start new with so much. So it was a perfect time to start fresh. The US is a pile of legacy ways. And nothing happens to change them.

With that said I wish we would try harder to convert. Get a dual system going now and run it for 20 years. let people adjust. Teach school in 95% SI ( only enough english units stuff so the comprehend them).

Re:Kilogram?

[radish (98371)]

You do know the metric system is many hundred of years old don't you? In fact it's older than your country. The point is the US has had 200 years and they haven't even started the process. There's nothing saying you can't run in parallel - the UK has been doing so for years. It's absurd to say you have to rip out all the imperial pipes and replace them - you just have to keep 2 sets of tools around until those old pipes get replaced naturally. It really isn't hard, it's just the US can't be bothered.

Re:Kilogram?

[radish (98371)]

I do get it. This already happens in the UK, it's not a problem at all. We have lots of houses which are older than the metric system (and the USA for that matter). They use imperial stuff. We have lots of new houses - they use metric. And yet I can still call a plumber and he can figure out how to fix my pipe, and my electrician is able to fix a light. Amazing.

If there was any will to do it you'd do it, which indicates there's no will. Which is fine, I don't give a toss what you measure your wooden houses in, but don't come over all "it's too haaaaaard" - you sound like a whinging kid.

Pipe-sizes are not that simple...

[Ashtead (654610)]

Threaded pipe dimensions in inches were based on the internal diameter at some point, just to make things even more interesting. Thus, a so-called 1/2" pipe actually has an external diameter of approximately 20 mm, which translates to somewhat more than 3/4". All the common Pipe Thread sizes are this way. There has been some attempt at metricising these, at least within Scandinavia, instead of referring to terms like 1/2" or 3/4" pipe threads, terms like R15 and R20 have been seen instead. That seems to be neither here nor there.

Just to confuse the matter more, in the 1970s, it was common to use metric sizes of threaded copper pipe, which had external diameters in sizes approximating common fractions of inches: 13mm = 1/2", 16mm = 5/8" and 19mm = 3/4" just to mention some of them. These appearently were all threaded with 1mm pitch threads.

Later, these were replaced by true metric pipe sizes with compression fittings or capillary solder fittings. Now the sizes changed again, common ones are 8, 10, 12, 15, 18, 22, and 28 mm. And of course, one needed compression fittings made for 16mm and 19mm also, so as to fit the older pipes...

That's Europe. What I have seen in the US are the commonly found so-called 1/2" copper pipes with solder fittings, this is about 16mm (5/8") in diameter, so I guess they are still using internal diameter measurements. Similarly, the so-called 3/4" pipes appear to have about 21mm outside diameter.

I guess the easiest way to turn these into metric sizes would be to redefine them as 16mm and 21mm and leave it at that. At least the traditional inch-units pipe thread sizes are roughly the same everywhere!

Re:Pipe-sizes are not that simple...

[Rolo Tomasi (538414)]

Sir, you are indeed a well-traveled man! You obviously have laid pipe across the globe.

Re:Kilogram?

[Tumbleweed (3706)]

> Then maybe america should move out of the dark ages sometime.

Yeah, tell it to the Queen.

Re:Kilogram?

[Spooky Possum (80044)]

Consider the facts:

Congress authorised the use of the metric system in 1866.

The US signed the Treaty of the Meter in 1875.

Congress passed the Metric Conversion Act in 1975.

So clearly the US *is* on the metric system :).

Re:Kilogram?

[SN74S181 (581549)]

Perhaps we don't have the coercive state apparatus necessary to 'make' the metric system the primary system that we use.

Me, I have been doing a lot of woodworking lately. It's convenient to use a unit (the Foot) that divides easily into subunits that are multiples of both 3 and 4, without having to get all mired in floating point arithmetic.

But we have this metric flamefest every time the metric system comes up on Slashdot, and the same crap comes up every time.

I'm just happy that pointy-head metric zealots don't seem to have much pull in the real world of regular people. Keep on ranting, dudes.

Re:Kilogram?

[by Anonymous Coward]

Moving to the metric system... inch by inch.

Re:Kilogram?

[sphealey (2855)]

Then maybe america should move out of the dark ages sometime.

Funny how when the topic is software or food supplies, everyone jumps in with comments about the dangers of monoculture and the value of diversity in supply, but when the topic is the metric system there can be no deviation from the ONE TRUE FAITH.

Personally, having gone through school at a time when the US was considering a change, and having spent some time in Europe, I have no problem with the metric system. It is more convenient from some tasks, particularly in the chem lab.

But there is nothing inherently superior about a measurement system based on powers of 10. For many tasks, such as woodworking, metric measurements are far more difficult to work with than inches and 1/16th. In fact I would argue that the most "natural" base for a measurement system is 12 as it is evenly divisible by 2, 3, and 4; whereras base 10 is only divisible by 2 and 5. Thirds and fourths are very common divisions of stuff; fifths are not, so a base 12 system is more user-friendly.

That's my 0.02 euro anyway.

sPh

Re:I Agree - We should go metric

[by Anonymous Coward]

It's not the arbitrariness, but the fact that metric is a decimal system.

The only countries left that don't use metric are the US and Bhutan. Bhutan is a fundamentalist islamic country that doesn't even have any phones yet. I guess we can see what the US' technical level is.

Re:I Agree - We should go metric

[The Cydonian (603441)]

Fundamentalist Islamic country [kingdomofbhutan.com] without any telephones [cia.gov]?

Can I have some of whatever your smoking please?

So is the US

[pommiekiwifruit (570416)]

IIRC the pound (or most versions of it anyway) is defined in terms of the kilogram these days, rather than the average weight of a grain of barley. Otherwise they would have problems if the US genetically modified barley to include jellyfish genes or whatever the seed companies reckon you should be forced to eat (and unless you grow your own food, you are eating it).

And the metre is defined properly these days (as is the second) in terms of wavelengths of radiation.

Re:I Agree - We should go metric

[Zork the Almighty (599344)]

A year is 365 days, 5 hours, 49 minutes (365.2424 Universal days)

Why don't we just get a big rocket, and alter the orbit so that it is exactly 365 days ? Or better yet 366 days, then we can give everyone a holiday (in rememberance of all of the species that were extinguished for our selfish ends).

Re:Kilogram?

[Wavicle (181176)]

Somebody tell those non-technical folk to stop making interplanetary satellite probes! [cnn.com]

Re:Kilogram?

[AftanGustur (7715)]

Hey I live in America you insensitive clod!

Ok, so for you it's "FreedomGram" then.

My wife changes her definition...

[craenor (623901)]

Everytime she steps on the scales...I would tell you what it was defined as last week, but kids may be reading this.

an excuse not to diet

[circletimessquare (444983)]

i'm not gaining weight, the kilogram is losing mass... so really, i stay the same weight, and they need more units to weigh me ;-)

I apply for the vacant job!

[q.kontinuum (676242)]

The kilo shall be defined to be 1/80 of my weight. In return for the honor I promis to make the worlds people slim down.

Solution?

[The_dev0 (520916)]

Why couldn't they just take it down the shops and measure it against, say, 1kg of carrots or a kg of sugar?

Counting Si

[brokenbeaker (267889)]

The problem with the single crystal of silicon method, a few years ago, was that there were all these lattice vacany defects cropping up. The formation of such point vacancies is so entropically favoured that I don't think they can ever eliminate them...

Re:Counting Si

[porp (24384)]

I think I can speak for everyone and say

HUH?

porp

Re:Counting Si

[miracle69 (34841)]

I only went as far as Chem 1 for majors, so don't try to save the earth with this information.

Can I use the information to destroy the earth?

Re:Counting Si

[dhovis (303725)]

Vacancies are not necessarily a problem. As you say, vacancies are entropically favored, but there is also a formation energy associated with a vacancy. So thermodynamics tells us there will be a balance between the energy required to create a vacancy with the entropy gained by creating one.

Thus, there is an equilibrium number of vacancies in any crystal. As long as you know what the equilibrium value is for a given temperature and you maintain that temperature, then you will also know how many vacant sites you will have on the crystal lattice. I don't have any of my texts handy, but I'm sure someone can chime in with the numbers for silicon.

To sum up. All crystals will have vacancies because vacancies are thermodynamically favored. However, the number of vacancies will tend towards an equilibrium value which allows them to be accounted for.

Re:Counting Si

[neodymium (411811)]

If you really would try to build such a crystal, vacanies could very well be the problem. As you said, there is an equilibrium value of defects in any crystal. This equilibrium value is temperature dependant with a exp(-Eform/kT) law, where Eform is the formation enthalpy. High temperature means high rate of defects.

Si single crystals are usually prepared at very high temperatures out of molten Silicon (1414C, Czochralsky method). Essentially, this will lead to a freezing of the defect structure at temperatures close to the melting point, because the lattice reorientation kinetics (point diffusion) also are thermally activated.

You would have to temper the crystal for _very_ long times at temperatures of i.e. 300C to get a thermal equilibrium of defects at this temperature. These times could be >>years !

reproducibility

[nthomas (10354)]

Although it was mentioned in the article, I think it should be emphasized that the SI definition of the kilogram, unlike their definitions of the meter and second, cannot be reproduced -- or rather, reproduced exactly. This is quite important, as it is neccessary for the standards governing body in each country to have a very precise reference weight of their own.

Since there is only one reference object for the kilogram, everything else is just a copy -- and even if it is a first generation copy, errors are bound to creep in.

The redefinition of the kg is long overdue, mad props to the scientists working on this.

Re:reproducibility

[vrt3 (62368)]

I think we should digitalize it and put in Kazaa. There will be thousands of perfect copies in no time.

Re:reproducibility

[troc (3606)]

Except for the madonna version which would have

"Stop stealing Mass, you fucks"

written through it

Troc

Best units of measure

[Alien54 (180860)]

I sort of like the idea of a universal unit of measure.

One nominee that is amusing is to have the basic unit of distance based on the speed of light.

One light nanosecond = roughly 11.1 inches, kinda close to a foot.

I remember how Grace Hooper used to pass out wires that were that long, just to make the point.

Any other nominees?

So that's why it takes more.

[DanThe1Man (46872)]

And all this time I just thought I was just getting used to cocaine.

Can someone help me convert here??

[Helpadingoatemybaby (629248)]

give me imperial measurements any day

Darn right! After all, it's easy enough to convert fortnights to stone with a Mayan calendar.

We're going to in the future eventually. It's inevitable.

I know it's 60 firesticks per 100 Watts, and 3000 Volts per staticy tomcat, but it might just be easier if we all just jumped in and switched to metric 144%.

I mean picture doing 100 on the highway! Wouldn't that be great? And dozens of future Mars landers would actually land on Mars, instead of digging ideal tree planting holes and landscaping future martian neighbourhoods. ("Zyphod! Incoming! It's the Americans!")

No more two sets of wrenches and lost sockets! Now you can have one set of sockets with half the sockets missing, instead of two sets of sockets with half the sockets missing. And no more asking for an 5mm and trying to make a 1 3/4" fit, rounding off the edges and carving a perfect turkey slice off your hand and gushing gallons of blood. It would be litres, which is less.

And you get to tell women that you, sir, are endowed with twenty-two centimeters of man!

Of course, the loss of the 25 cent piece will be a negative, since we'll have to pay for everything in dimes. But it's worth it dammit.

Seriously, we all know this is going to happen. When are we on board? Are we that stubborn?

Why not use diamond?

[Jeremy Erwin (2054)]

Is there any physical reason (other than that small matter of cost ) that crafting a new kilogram (or more likely, gram) out of diamond would not be an ideal solution?

BTW, theNational Physical Institute [npl.co.uk] has a FAQ on its Pl-Ir standard kilo.

In other news

[djupedal (584558)]

The only known quantity of Unobtainium (UB238) has gone missing.

The 1 kilo square block was being held in Brussels awaiting return to Brazil, where it was originally unearthed.

It was determined that the physical stability of the material was being affected by being moved from it's original location, that of being south of the equator. Investigators are anxious to reclaim the material in hopes of stabalizing it's rumored flux in mass. The UB238 was being packaged for transit, when it suddenly dissapeared from the shipping room counter. The rumor that it had created, and subsequently fallen into, a 'portable black hole' was discounted by investigators on the scene.

Once the Unobtainium is recovered, and returned to Brazil, it can be weighed and certified as a replacement for the Pt-Ir cylinder that is kept in France [slashdot.org], and measured annually, representing the kilo standard for the world.

MPEG at 11.

Volt is no longer defined by Kilogram

[Dhrakar (32366)]

The posted article, while interesting, is wrong about the volt being based on the Kilogram. Since about 1990, the volt is defined to be the voltage applied to a Josephson junction that produces a frequency of 483,597.9 GHz. This new standard was implemented in order to get away from relying on 'artifact' standards (such as the Kg cylinder). One quick source page on Josephson junctions (which completely revolutionized the field of Metrology back when I was a calibration tech in the AF) is:
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ squid.html
If I recall correctly, the eventual goal of the international standards organization was to find ways to define everything in terms of frequency/time since we can measure time so accurately/precisely.

Re:Volt is no longer defined by Kilogram

[vrt3 (62368)]

If the volt was based on the kilogram (and therefore a relationship exists between the two), and now volt is based on frequency, isn't it possible and wouldn't it be a good idea to base the kilogram on the volt? Then we don't need those perfect references anymore.

huzzah!

[madmarcel (610409)]

So I now weigh 75kg...give or take a bit :o

Wait till I tell my fiance that her weight
fluctuates on a weekly basis!

Alternative Physics

[SEWilco (27983)]

a perfectly spherical single crystal

These pseudoscience concepts are getting out of hand.
I don't think we need "feel-good" physics.
Now they want to base a standard on a crystal ball?

Wrong subject... yet...

[Qender (318699)]

"'It's certainly not helpful to have a standard that keeps changing,' says Peter Becker, a scientist at the Federal Standards Laboratory..."

Wow, someone should tell the computer industry that.

"Some of the metal plugs were issued to countries that later vanished, including Serbia and the Dutch East Indies. The Japanese had to surrender theirs after World War II. Germany has acquired several weights, including the one issued to Bavaria in 1889 and the one that belonged to East Germany."

SURRENDER YOUR KILOGRAM!

Re:Annually

[sould (301844)]

Why exactly does it have to be measured annually......Anyone care to enlighten me?

It doesn't exactly have to be measured. They just do that to check it's still right. Go read about the history of the Systeme International the NIST site [nist.gov] and the definition of a kilogram at the same place [nist.gov]

But essentially, its part of a way of ensuring that the measuring units Scientists use around the world are the same, not slightly different.

For instance, anyone around the world can reproduce (in a well equipped lab anyway) the definition for time (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 cesium 133 atom).

There are only 7 base SI units (meter, kilogram, second, ampere, kelvin, and candela) from which many more units [abdn.ac.uk] are derived. Hence, if kilo is out/changing many of these are changing too.

and why should I care if it detoritates?

Presuming you're American, you would use feet, pounds, find metric too complicated, etc, etc - so probably wont care if it does.

Re:The mystery unit?

[sould (301844)]

Is it the mole?

I can't reveal its identity for this precise reason.

Yes - there is a mole in the base S.I. units - but I can't tell you it's name. Its been on a secret long term sleeper mission - to liberate the S.I. units and term them into "Freedom Units"

Re:Please Splain Something to Me?

[Yosho (135835)]

The unit of "volt" can also be expressed as m^2kgs^-3A^-1.

yeah it's a mess

[Trepidity (597)]

The various prefixes -- kilo, Mega, Giga, and so on -- are very precisely defined SI prefixes that have been in common use in the sciences for quite some time now. In computing though, 1024 bytes was originally termed a "kilobyte" because it was very close to an actual "kilo" of bytes (1000 bytes), and so was a convenient term to use. In other computer-related disciplines though, in particular engineering, the correct SI usage prevailed, so your 128 kbps mp3s actually have 128000 bits per second, not 128 * 1024.

The big problem is that 2^(10x) and 10^(3x) diverge as x increases: 1024 is 2.4% more than 1000, 1048576 is 4.9% more than 1000000, 1073741824 is 7.4% more than 1000000000, and so on. So obviously the "close enough" thing is getting less and less true -- when there's a 10% difference between the two measurements they're not even close enough for everyday colloquial speech.

So the solution of both the SI and the IEEE is to reassert the original meanings of the SI prefixes (kilo = 1000, Mega = 1000000, etc.), but to add new base-2 prefixes in recognition of their usefulness in computing. These are kibi, Mebi, Gibi, etc. (basically the same as the SI prefixes but with the last two letters replaced by "bi"). Their standard abbreviations are the same as for the SI prefixes, but with a lowercase 'i' appended (so ki, Mi, Gi, etc.).

The conversion is obviously nowhere near complete, and irritates some computer people who don't want to change the terms we've been using for decades, but this seems to be the only really reasonable way of doing things. The only other two options are to either force the rest of the sciences to change to use the base-2 definitions (which is obviously not going to happen, and they got there first anyway), or to maintain the current ambiguity, which is also obviously undesirable.

Re:yeah it's a mess

[Waffle Iron (339739)]

I think that these base-2 units are confusing because their names look too much like the base-10 units and because their magnitudes are too close.

A better way would be to invent an all new imperial-style system for measuring computer storage. That way, there would be no chance for confusion with any base-10 system. For example:

korb = 3 bytes
fleb = 12 korbs
splin = 20 fleeb
fnit = 6 splins
Fnit = 6000 splins
frush = 48 fnits
watz = 18 frushes (19.5 frushes in the U.K.)
spoff = 480 watzen
nurm = 320 spoffs
long nurm = 80 nurm
munnel = 24 long nurm

This system easily covers storage capacities up to today's confusingly named "petabyte". Plus, there's no ambiguity about what you're measuring. Any of these units implies bytes of storage, which is a much cleaner solution.

The computer I'm using now has 71+29/32 watzes of system memory and 44+10/16 spoffs of disk space. There's no confusion about fuzzy definitions of "mega" with that measurement.

Re:Why not...

[wass (72082)]

The answer is another question : How would you define a standard energy?

c is a constant, of course. In fact, it's used to define the meter as how far light travels in a vacuum in 1/(299,792,458) of a second. Second is defined as the time for a certain number of vibrations of a Cesium atom to occur. As per your question of relating mass to Joules, note that high-energy physicists do this all the time. They usually refer to masses of particles as MeV/c^2. And they usually work in units where c=hbar=1, thereby making distance, time, and energy all essentially the same units (easier to do calculatins that way).

One thought that jumps to mind for a standard energy interval is the lyman alpha energy width (the jump of the electron in a hydrogen atom from n=2 to n=1 where n is the energy quantum number). Or, for mass, use a standard mass of a well-defined particle such as an electron. In fact, I'm surprised that NIST doesn't do this. It might be that isolating electrons for mass measurements are too difficult (gravity is weak), but electron mass does show up in many other calculations (specific heat of degenerate electron gases, for instance). Or isolating ultra-pure hydrogen gas and spectroscopically measuring Lyman alpha is more difficult than it seems. I guess NIST wants [relatively] easy methods for measuring these quantities.

Okay, I just found this site [unc.edu] which answers the question. They quote

This one physical standard is still used because scientists can weigh objects very accurately. Weight standards in other countries can be adjusted to the Paris standard kilogram with an accuracy of one part per hundred million. So far, no one has figured out how to define the kilogram in any other way that can be reproduced with better accuracy than this. The 21st General Conference on Weights and Measures, meeting in October 1999, passed a resolution calling on national standards laboratories to press forward with research to "link the fundamental unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram." The next General Conference, in 2003, will surely return to this issue.

It all boils down to ability to measure the standard units to the highest precision possible. I'm actually stunned that the mass of that bar can be weighed to that precision.

As a side note, if you can come up with a better way of measuring fundamental constants, you might win a Nobel Prize. The guys that discovered the integer quantum hall effect initially published their results as a better way to measure some of the fundamental constants.

Re:Change in Gravitational Constant?

[LastToKnow (449735)]

a kilogram is, however, a measure of mass, not weight. it is therefore unaffected (if properly measured) by the force of gravity.

Re:Define Plank's Constant?

[Dr. Spork (142693)]

What? We don't define the speed of light. Definitions are up to us, and the speed of light is fortunately not. We define how long a second is, and we define what a meter is in terms of seconds and the speed of light. But we don't define the speed of light. That's just given by nature, and that's why it's so useful.

The same deal with Plack's constat. It's value is not up to us, but up to nature. "Defining" it would be like defining pi as 3.

Re:Aaargh.

[2nd Post! (213333)]

Except that no one has actually counted/created a structure composed of a mol of any particular element.

The minute you can do that, then you can reliably and predictably create a fixed metric by which any one in any place can measure mass.