[ARC5] The International System of Units (SI)

[Richard Knoppow]

     There are a lot of not quite correct conventions especially in some 
old schematics. For instance mmf for micro-micro farad now more commonly 
pf and the prefix m for one thousand ohms usually with "meg" for 
megohms.  The latter can be quite confusing if you are not familiar with 
it. Vacuum tube diagrams were all over the place.

On 2/14/2016 6:54 AM, J Mcvey via ARC5 wrote:
> That's all well and good, but who has a greek character font that they 
> can readily inject into their text? Thus the convention of using "u" 
> for mu?
> I never had a problem understanding the usage in that context. It's 
> better and less confusing than m which is more universally used to 
> denote milli.
> Since that was the case, why did they ever use mfd for microfarads 
> when ufd kind of looks like mu and is now universally understood as such?
> Where you can get tripped up with mHz (milli hertz) vs MHz (megahertz) 
> , but that too can usually be figured out in the context.
>
> There are lots of backwards conventions, like current flow 
> diagrams,etc, but at the end of the day, it does the same thing so it 
> doesn't matter.
>
> New or old way.I can get through it OK.
>
>
>
>
>
> On Sunday, February 14, 2016 8:42 AM, Roy Morgan <k1lky68 at gmail.com> 
> wrote:
>
>
>
> On Feb 14, 2016, at 5:50 AM, Leslie Smith <vk2bcu at operamail.com 
> <mailto:vk2bcu at operamail.com>> wrote:
>
> Re: [ARC5] The BC-221 low frequency tank circuit puzzle.
>
> > ... - the 10.4 uH vs 10.4mH difference nicely illustrates one of
> > my pet peeves.  This is  a failure to distinguish corretly between unit
> > designators.  By this I refer to those pesky prefixes - such as "u"
> > (properly mu, not "u") or 10e-6 and "m" (milli, one one-thousandth),
> > Mega (x10e6)  and so on.
>
> (From an earlier post I made on the Collins list, and expanded for 
> this post):
>
> > …  By the way GHz is spelled GHz and not Ghz!
>
> For the authoritative standard way to use abbreviations of this sort, see:
>
> http://www.nist.gov/pml/div684/fcdc/si-units.cfm
> International System of Units (SI)
> "The International System of Units (SI) provides definitions of units 
> of measurement that are widely accepted in science and technology …”
>
> Guides to the SI:
> ...
> A practical description of the SI is Guide for the Use of the 
> International System of Units (SI), 2008 ed. (U.S. Government Printing 
> Office, Washington, DC, 2008) [NIST Special Publication 811].
> http://www.nist.gov/pml/div684/fcdc/upload/sp811.pdf
>
> Wherein you will find:
>
> Table 3. The 22 SI coherent derived units with special names and symbols.
>
> Derived quantity  Special name  Special symbol
> ...
> frequency  hertz  Hz
>
> (Note: Table 3 contains many of the units named after famous persons 
> such as Newton and Pascal.  I was under the impression that most, but 
> not all symbols for such units are capitalized but that at least one 
> was not.  I cannot find that example however. The “special names” in 
> Table 3 are not capitalized except for “degree Celsius".)
>
> and
>
> Decimal multiples and submultiples of SI units: SI prefixes
> Table 5.  SI prefixes
>
> Factor Prefix Symbol
> …
> 10 ^ 9 = (10^3)^3  giga  G
> and
> 10^-6 = (10^3)^-2 micro m(in the table this character is greek letter 
> lower case mu)
>
> I searched for the abbreviation “mu” that we associate with 
> permeability and found
> "Certain quantities, such as refractive index, relative permeability, 
> and mass fraction, are defined
> as the ratio of two mutually comparable quantities and thus are of 
> dimension one”
>
> 7.10.1 Decimal multiples and submultiples of the unit one
> Because SI prefix symbols cannot be attached to the unit one (see Sec. 
> 6.2.6), powers of 10 are
> used to express decimal multiples and submultiples of the unit one.
>
> Example : mr = 1.2 3 1026 but not : mr = 1.2 m (the “1026” is not 
> correctly rendered here_
> Note : mr is the quantity symbol for relative permeability.
>
> (In the document the “mr” above appears as the greek letter lower case 
> mu with the subscript of the greek letter capital tau- I think)
>
> Roy
> Retired NIST employee
>
> Roy Morgan
> k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>
> K1LKY Since 1958
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL

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