New edition of SI brochure

One of our readers has drawn attention to the recent issue for a comment of a revised edition of the SI brochure.

He writes:

“The draft 9th edition of the SI brochure is available at:

Some of you may also be interested in BIPM committee minutes at:

Changes of interest to me include:
* Version 8 describes many non-SI units, organised in four tables (tables 6,7,8, and 9). Version 9 describes fewer non-SI units, all listed in one table (table 8).
* Both version 8 and version 9 have a table of ‘non-SI units accepted for use with …’ However, the number of units in the table has increased from nine to fifteen. This is an increase in explicit status for those particular units.

In both versions 8 and 9, I noticed a comment against the tonne, ‘In English speaking countries this unit is usually called metric ton‘. I believe this to be false. In the UK, I don’t think I ever hear or read it called that. If it’s not too late, we may have an opportunity to challenge that statement.”

9 thoughts on “New edition of SI brochure”

  1. I have read ‘metric ton’ in Commons debates before now, but usually in a context where it should be perfectly clear that ‘tonnes’ are meant, e.g. in discussion of government procurement. I have always put it down to our elected members perhaps not always fully realising that, say, the NHS and the armed forces operate exclusively in metric units and wishing to emphasise that they mean a metric ‘tonne’, especially as there is no difference between the pronunciation of the words ‘ton’ and ‘tonne’.


  2. I looked up “tonne”, “metric tonne”, “metric ton” and “ton” in the on-line version of Hansard (which appears to catalogue both the House of Lords and the House of Commons from 2010 to date). “Tonne” appeared in 274 spoken references and 11 written statements, “metric tonne” in 4 spoken references and no written statements, “metric ton” in no spoken reference or written statement and “ton” in 40 spoken references and 2 written statements. It appears therefore that “tonne” is by far the preferred form.


  3. I think they mostly mean “the Americans.” The US edition (NIST SP 330) states, “Metric ton is the name to be used for this unit in the United States.” It then goes on to state the original BIPM text for the tonne. I have noticed Canada seems to use metric ton occasionally and India usually in print. The short ton (2000 lbm) is still in wide use here, so spoken ton and tonne would be very confusing. In the US, an unqualified ton is normally short, metric and long tons need the qualifier; the proper symbol is still “t” whether tonne or metric ton is written out or spoken.


  4. Are Commons debates’ alerts an unedited form of what finally appears in Hansard? If no reference to ‘metric ton’ actually ever gets published, so much the better.


  5. I believe the correct pronunciation of tonne is ‘tonn’, as opposed to ‘tun’. If that was properly established, that might clear up the intended meaning when spoken.


  6. On the status of non-SI units used with the SI, there is no real change in status, only how the table is organized. In the 8th edition, certain units, although in tables 7 and 8, had explicit footnotes or text stating they were approved for use with the SI. These are the astronomical unit, electron volt, dalton, neper and bel (decibel). Note the astronomical unit was elevated to table 7 in a 2014 Supplement to the 8th edition.

    However, the bar, mmHg, angstrom, barn, nautical mile and knot, formerly of table 8, are not mentioned at all, nor are the derived cgs units of table 9. Are they as “unmentionable” as Imperial/Customary units?


  7. @John Steele:

    Yes, it is probably intended as a tidying exercise rather than a change of status—the new version is considerably shorter and much has been completely re-written. Given the rather fundamental change to the method for defining the unit of mass which is pending, it is a bit surprising that they did not extend their tidying and make the gram the base unit instead of the kilogram and remove the `special case’ for its prefix. AFAICT, that wouldn’t change anything else—particularly the historical use of kilograms in the definitions of other units—and would result in slightly less text overall. Indeed, the last sentence/ paragraph of the new definition does seem to imply that someone has considered reference masses other than kilograms…

    Either way, it has always struck me as inconsistent that 1 Mg (and 10? g) is not listed as the SI equivalent for the tonne, in the same way as 1 hm² is for the hectare and 1 dm³ is for the litre. With any luck, those three will also become `unmentionable’ eventually.


  8. Our USA-educated (FSVO) MP has even been slipping non-imperial tons into Hansard as tonnes, so you have to take it with a fairly large pinch of salt…


  9. Looks like we still have a ways to go to standardize all of the units in actual usage.

    For example, a torque driver made by Sumake in Taiwan does not list the range of torque available on their tool in Newton-meters but rather kgf/cm. Yikes!!!


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