An ‘el’ of a standard

The powers that be of the metric system are wrestling with the problem of defining the kilogram independently of an actual physical object (i.e. the very slowly degrading cylinder of platinum-iridium alloy kept at Sèvres, near Paris, known as “the international prototype of the kilogram”). Meanwhile, they are neglecting a longstanding question that urgently cries out for a solution. [Article contributed by Martin Clutterbuck].

At the 99th meeting of the Comité International des Poids et Mesures (CIPM) in October 2010, a draft specification for changes to the definitions of units was recommended to be submitted to the General Conference on Weights and Measures (CGPM) at its 24th meeting to be held in October 2011.

The SI brochure, which will contain all the revisions and is currently in its 8th edition, is managed by the Bureau International des Poids et Mesures (BIPM). The BIPM is supervised by the CIPM which is in turn is supervised by the CGPM!!

The draft states that the meeting will have considered “the international consensus on the importance, value, and potential benefits of a redefinition of a number of units of the International System of Units (SI)”*.

For example:

“the kilogram will continue to be the unit of mass, but its magnitude will be set by fixing the numerical value of the Planck constant to be equal to exactly 6.626 06X ×10-34 when it is expressed in the SI unit m2 kg s-1, which is equal to J s” **

[The symbol X represents one or more additional digits to be added at the time that revised definitions of the SI Brochure are finally adopted.]”

Well, yes, precisely, but is the average market trader or any other trader for that matter, going to rush to take any old imperial scales to the dump as a result? More likely, any down-to-earth Brit will say that all this proves is that metric is all about science – astrophysics and all that stuff so it doesn’t affect me!

Unfortunately, the CIPM and the BIPM can’t get their act together for one little thing that might just influence our old friend the ‘man-in-the-street’, the market trader or even ‘red-top’ newspaper journalists.

And what might that be? Well it’s another old friend, the symbol for the litre.

Surely the SI should not continue to offer the option of the use of either the lowercase ‘l’ or the uppercase ‘L’. Indeed the confusion caused and the understandable desire of catalogue and product labelling graphics artists to be as clear as possible has led to many examples of the use of ‘Ltr’ instead of either of the correct symbols.

Some products even mix uppercase ‘L’ with lowercase ‘l’ on the same label!

The CIPM, in 1990, considered that it was still “too early” to choose a single symbol for the litre… but how long do we have to wait for a decision?



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19 Responses to An ‘el’ of a standard

  1. Erithacus says:

    I am sure this wasn’t Martin’s intention, but we should not belittle the efforts of BIPM to define the kilogram more precisely since the kilogram underlies the definition of several base and derived units, including the ampere, newton, joule and watt (see the 4th bullet point of the CIPM proposal). So theoretically, if the international prototype of the kilogram decays by a few micrograms, so do the ampere, newton, joule and watt vary (and indeed the imperial pound, since even that is defined in terms of grams!).

    However, I do agree that the BIPM needs to come to a clear decision on the symbol for the litre. The present situation has resulted from the objections to the use of lower case “l” for “litre” because it can be confused with the number “1” (“one”) in some fonts (including Times New Roman) – especially in expressions such as “1 l” meaning “one litre”. Others have objected to “L” since, when it is used with a prefix, you get a somewhat ugly combination such as “mL” for “millilitre” (Note that a common error is to write the symbol for “kilowatt” as “kw” rather than the correct “kW”). Another objection has been that upper case symbols are reserved for units named after distinguished individuals such as Isaac Newton.

    There are in fact 3 possibilities for the combination of “litre” and “millilitre”:

    (a) “l” and “ml”
    (b) “L” and “mL”
    (c) “L” and “ml”

    (I have disregarded the combination “l” and “mL” as it is open to all the above objections).

    My preference is for (c), as it answers at least two of the objections. I know some will say that the symbol should be the same in both the base unit (litre) and when used with a prefix (millilitre), but I think we can tolerate the occasional anomaly (or compromise). After all, if SI were completely rigorously consistent, the base unit would be the gram – not the kilogram.

    I think a rule such as “use ‘L’ for the base unit and ‘l’ with prefixes” would be readily understood, and, more importantly, might find favour with journalists and those who label packages – and hence actually be used consistently.

  2. John Steele says:

    I don't see "mL" as a problem, as several other units have this problem when used with a prefix. I vote for one symbol, and think L and ml just continues the problem. The US recommendation, formally included in NIST SP 330 (the US version of the SI Brochure), is to state its preference for liter, and the symbol L, for the US. It accepts the alternates litre, and l, and mentions their use in other countries. [Please don't get into the question of spelling, which is an issue of language - not of symbols - and hence not a matter for BIPM - Editor]

    The SI Brochure has suppressed the script l for liter, formerly used in some countries. If suppressing l is too controversial, perhaps they could state that L is preferred but l is allowed.

    It is true that 1990 was judged "too soon" to decide this. However, the request for a recommendation on suppressing one symbol was made in 1979 when the two symbols were declared. If eleven years is "too soon" to decide which letter, I wouldn't expect speedy adoption of a "new kilogram." Perhaps they need reminding that 11 years has now grown to 32; surely that is NOT "too soon."

    I don't know whether any other coutries have a formal position on L vs l. Do others have a declared national preference? Are they rigid about it, or do all accept the alternate?

  3. John Frewen-Lord says:

    Many 'new world' countries have already adopted the L for the litre (Canada, US, for certain, maybe others). They also now use mL for millilitre. I can't see any problem with sticking with the upper case L for litre regardless of whether it has a prefix or not. What about kilolitre - kL seems logical and clear, kl looks like a misprint of something.

    This looks like one of those instances where the 'standards' can be defined by common usage (rather than the other way round), and all that remains is for the CIPM to get on board with this. There is only one reason to stick with the lower case 'l', and that is a misguided resistance from 'old Europe' to adapt to change - the very thing we in the UK are guilty of in resisting metric in the first place!

  4. Seares says:

    The trouble comes with the litre symbol in certain type faces. What is this- 2l, or I suppose more 'correctly' 2 l ? Could be a 'one' or an 'I' Not so bad when used here in Times New Roman, but try it in Arial! That's why signwriters use any alternative like Ltr... no mistake possible.
    There are in fact 3 possibilities for the combination of “litre” and “millilitre”:
    (a) “l” and “ml”
    (b) “L” and “mL”
    (c) “L” and “ml”

    So I'd go for (c) if I must make a choice. But as long as it's metric and understandable, who cares?

  5. Robert says:

    Maybe the answer is to use 'lt' or 'lr', as that preserves the concept of reserving upper case for units named after people whilst (AFAIK) not being confusable with anything. There is a precedence for using double letter units (eg Pa for Pascals).

  6. Mary says:

    Above, the main focus is on ‘L’ and ‘mL’. Consider all the twenty SI prefixes.
    I realise many of these aren’t used in non-technical everyday publications.
    And for measurements like absolutely enormous distances ‘km’ (or Light-years) will be used, instead of ‘Ym’
    Consider the following symbols:
    YL, ZL, EL, PL, TL, GL, ML, kL, hL, daL, dL, cL, mL, µL, nL, pL, fL, aL, zL, yL

    In the case of volumes, abbreviations for units like, ‘tea spoons’ (tsp) will unfortunately continue. I haven’t seen ‘OSP’ for ‘Olympic swimming pools’ and hope it doesn’t get used. Perhaps we need to publicise the volume of water in each of the pools going to be used for the London Olympics, giving the volumes in cubic metres (m3) and megalitres (ML).

  7. Peter Griffin says:

    At least we are rid of the other 'ell' of a muddle from years gone by.

    An 'ell' used to be a unit of length which must have been a source of enormous confusion, and frustration for consumers.

    Apparently an 'ell' was about 114cm (in England) but, amongst others, there was also a Scottish ell (~94cm), a Flemish ell (~69cm), a French ell (~137cm), a Polish ell (~79cm), and a Danish ell (~64cm).

    Fortunately most of the myriad different measurement units that were in use around the world have been abandoned and the more 'elementary' metric system adopted in their place.

  8. philh says:

    Here is a radical idea.

    When the scientists finally resolve the problem with the kilogram and find an agreeable way to define it without reference to a physical protoype, why not commemorate it with a new name for the basic unit of mass, so as to remove the anomaly of having the prefix kilo attached?

    I suggest the 'Einstein' with the symbol E - in honour of the great man.
    (Apart from his name the letter E is also famously associated with him through the equation:
    E = mc²
    Yes I know the letter E in this case refers to energy not mass, but the point of the equation is that mass and energy are interchangeable.)

    Given his outstanding contribution to our understanding of mass, energy, space and time, naming the basic unit of mass after him would seem entirely appropriate.

    As for the litre (or liter if you prefer) I suggest we drop it altogether. We don't really need it. Why not go back to calling it what it is, i.e. a cubic decimetre and use cubic centimetre for smaller volumes and so on. Apart from the unfortunate choice of letter conflicting with the number one it would remove the veil of obscurity created by an unnecessary special name.

  9. Ed the Yank says:

    Here in the United States of America where we love our traditional customary units, L and mL is the preferred use. The NIST guidelines for industry, science and trade to avoid confusion states L is to be used. For my science class I had to break my habit of using the lower case script l in my laboratory journal as training for Good Laboratory Practices for Canada and the U.S. The use of L is also consistent in food package labels and containers e.g. Coke 2L bottles. Where we have our metric problem is the food industry. For agricultural production reports it is common to use US bushels not Imp bushels, same for commodity trade. For government reports one has to convert to kilogram or Metric tonnes per acre or hectare. I hope the kilogram is defined more precisely soon as it is a base unit.

  10. Philip says:

    Phil's idea should be forwarded to BIPM, CIPM, and CGPM.

    More about this idea, a unit name 'einstein' with the symbol ‘E’; the symbol E is also used for exa the prefix for ten to the power eighteen. This should not cause a problem.
    The symbol capital letter T can refer to the unit tesla, and to tera the prefix for ten to the power twelve; TT is the symbol for teratesla.

    In the future, if we have the unit einstein the following symbols could be used:
    yE, zE, aE, fE, pE, nE, µE, mE, cE, dE, daE, hE, kE, ME, GE, TE, PE, EE, ZE, YE.

    [These are a few more for symbologists and symbolists to know for pub quizzes!]

  11. Raúl José Blanco says:

    I believe the root of the problem is the lowercase letter l, which can be confused with so many other symbols! It would be much harder to confuse with the capital I or the digit 1 if its elegant little tail wasn't stripped off by font designers at some point of the evolution of font faces. See , most fonts there have the tail and it looks just fine.

  12. michduncg says:

    Call me very old fashioned, but I still prefer to think of 1kg as the mass of 1 litre of water, and the metre as 1/10,000th of the distance from the equator to the North Pole on a meridian passing through Paris. There's a logic about it all then!

  13. A says:

    This article seems a lot less alarmist and more informed than the BBC article I have just read:

  14. John Frewen-Lord says:

    Not sure about the Einstein. Can we really imagine Mrs Smith from No. 42 Railway Mansions going into her butchers and asking for half an Einstein of minced beef, or 200 milli-Einsteins of smoked ham? Thought not. The kilogram is too well established, for better or for worse, throughout the world. It would be a step backwards to try and change it now.

  15. philh says:

    John Frewen-Lord said "Not sure about the Einstein. ..."

    OK how about the 'Planck' with symbol P? (Same symbol as prefix 'peta' but as Philip pointed out above no different to T for 'tera' and T for tesla).

    The logic in that case would be the fact that the proposed new definition ties the base unit of mass to Planck's constant.

    I realise that Mrs Smith would not appreciate the reason for the change but, in all probability, the kilogram would persist for some time in everday use even though deprecated in scientific circles.

    I don't see why it would be a step backward though. The idea is that it would be a refinement to improve the consistency of the SI.

  16. Philip says:

    einstein, symbol E
    planck, with symbol P
    * Both ideas should be forwarded to BIPM, CIPM, and CGPM.

    If one was approved within the next ten to twenty years, it would be a very long time before the new unit name would be taught in all schools and even longer before used in markets.
    Consider ISO recommendations like date format: 2011-01-29; it may be official and legal, but unfortunately it doesn't mean that its use is universal. People will still be encouraged to write the month in words to avoid confusion, example:
    01-10-10 (1st Oct 2010 or Jan 10th 2010)

  17. John Steele says:

    It would be better if the basic unit of mass didn't have a built in prefix; however I am not sure how I feel about the proposed names.

    Assuming the kilogram stays the same size but gets some new name (sans prefix), the mole (and Avogadro's number) also need to be reviewed -- remember it is supposed to be a coherent system. Actually, it isn't entirely coherent now, as moles lead to grams, and they have to be converted to kilograms before being passed to coherent calculations involving force, pressure, energy, and power.

    Just as it would be better if the kilogram had another name without a prefix, it would be better if a mole were 12 kg of C12, not 12 g. The mole was a holdover from the cgs system, just as the "gram" root of the kilogram is. They are both mistakes.

  18. Ezra Steinberg says:

    Excellent suggestion, Phil!

  19. ronniec says:

    I don't think that there would be any problem with using kL or mL. We have important precedents for the use of a lower-case prefix symbols with upper-case unit symbols. Common examples can be found such as kJ for kilojoules and kW for kilowatts.#

    I would standardise on the use of a capital L because a small l can be mistaken for the digit 1 (one) in handwriting.


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