×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Strange SI prefixes
4

Strange SI prefixes

Strange SI prefixes

(OP)
Has anybody ever considered that the electron-volt (eV), a unit of energy as are the calorie and the Btu, equals 160.2 zJ ? The z is for zepto = 10-21.

Or that the fusion of pure Deuterium yields 0.33 PJ/kg ? Where the P stands for peta = 1015 ?

Strange SI prefixes, aren't they ? Does anybody know the etymological origin of the SI prefixes yetta (10E24), zetta (10E21), exa (10E18), peta (10E15), zepto (10E-21) and yocto (10E-24) ? Thanks.

RE: Strange SI prefixes

I do not know for sure, but I think that there is some connection between the number of zeroes and the name. On the "small" side - where prefixes were introduced earlier - there is an easy-to-follow etymology.

Everyone knows that micro means small and that nano means dwarf. And pico is small, again. Spanish pequeño is small, I think. Then, there are femto (10^-15) and atto (10^-18), which stem from the Danish language (fifteen = femtan and eighteen = atten). This is in hommage to Niels Bohr, the guy that introduced the modern atom theory.

There seems to be a similar connection between zetta and greek for seven, exa seems to be a variation of hexa (six) and so on. It seems that the naming of very large and very small numbers has a fundamental principle where the number of zeroes or decimal places is expressed as a multiple of three. Peta (penta in disguise?) for 15 zeroes and yocto (octo in disguise?) for 24 decimal places and so on.

RE: Strange SI prefixes

Thanks, IR!

I really wanted to know. Interesting.

RE: Strange SI prefixes

(OP)
Thanks to IRstuff and skogsqurra.

RE: Strange SI prefixes


That means that a very, very small unit of inductance is called a ChiciHenry.

Must go and talk to one of my electronics colleagues ......

RE: Strange SI prefixes

really good post IR, thanks

Cyril Guichard
Mechanical Engineer Consultant
France

RE: Strange SI prefixes

Have to agree. star. Always avoid Harpies

RE: Strange SI prefixes

Thanks CoryPad.  I had a little trouble with the website IRstuff posted, where the footnotes indicated that a lot of the prefixes I'd learned in physics class were "not SI approved".

BT

RE: Strange SI prefixes

ahhh, this nice French institute of the BIPM....I should have think about it

Cyril Guichard
Mechanical Engineer Consultant
France

RE: Strange SI prefixes

I had the (mis?)fortune of learning ft, inches lbs and pounds shillings and pence at junior school, and on going to high school having to be "decimalised" as the country went decimal (SI physical units and decimal currency) in quick succession.  It was soon drummed into us that the decimal units of metres, millimetres, litres etc were not necessarily strictly SI.  But all the unapproved units were still factors of ten removed from true SI.

So if you want to be pedantic, I suppose you could talk of "decimal" units rather than SI when you want to use decimetres or whatever.

Actually, it has proven very useful to me to be "multi-unitary" - I'm quite at home with most weird units (including megashekels per fortnight - actually my pay feels more like microroubles per fortnight sometimes!)

Bung
Life is non-linear...

RE: Strange SI prefixes

There is nothing non-SI about decimal units.  I still don't understand why everyone says that.  Deci- and deka- were adopted in 1960, as were centi- and hecto-, which people also seem to think are non-SI.  Just because they have prefixes only for every third power of ten *beyond* those levels doesn't mean that those levels aren't part of the system.

Powers of ten that the SI has prefixes for:
1,2,3,6,9,12,15,18,21,24 (and negatives thereof)

Litres, on the other hand, are non-SI (and so are hectolitres).

Hg

RE: Strange SI prefixes

HgTX,

According to NIST SP 811 Guide for the Use of the International System of Units (SI) , liter is a unit accepted for use with the SI, similar to minute, hour, day, and degree (plane angle).  This document is available at:

http://ts.nist.gov/ts/htdocs/200/202/mpo_pubs.htm

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Strange SI prefixes

Accepted for use with, but not part of.  I have nothing against litres (or degrees), but if people are going to talk about what is or isn't "strictly SI", they ain't it.  Has nothing to do with what units people will actually use in real life.

Hg

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources