[sdiy] What does it mean?

[Magnus Danielson]

From: "John L Marshall" <john.l.marshall at gte.net>
Subject: Re: [sdiy] What does it mean?
Date: Sun, 16 Sep 2001 18:34:46 -0700

Hi!

> The international unit is used in place of the decimal point. for example:
> 1k3=1.3k, 2M2=2.2M, 2u2=2.2uF, 4n7=4700pF=0.0047uF.

Hey!

4n7 = 4.7 nF, you forgot that one.

> I'm not sure why those of us in the USA don't use the international unit
> nano for capacitors. Just habit I guess.

Indeed. It is bad habbit. OK. This is the story... if you go very long
back in time, these units where not quite as developed as they used
to. The need where there however.

In 1960, the 11th CGPM conference adoped the first prefixes tera,
giga, mega, kilo, hecto, deka, deci, centi, milli, micro, nano and
pico. Some of these (mega through micro) had been in extensive use
before that.

> Years ago we didn't use pico either. We used micro-micro (uu).

Indeed. Before 1960 this was the way to survive the lack of nano and
pico.

mu = mili-micro = nano
uu = micro-micro = pico

Such clobbing together of units is expressively forbidden nowdays.

Also, the number before shall be above or equal 1 and below 1000.
That is, one shall allways use the right prefix at all times. For
instance, you may never write 0.47 uF, it should at all times be
470 nF. Sadly, few people realize this.

However, bad habbits is hard to kill. 

I must admitt, all to often I also write things like 3k3 etc. It's a
handy short-form.

> But then frequency was measured in cycles per second rather than
> Hertz.

Indeed. The Hertz was voted through by the CGPM in 1948.

> Conductance was measured in Mhos (ohms
> backwards) rather than Siemens.

Siemens was a very late unit, voted through in 1971.

> Watt-Second rather than Joules.

Voted in 1946.

> Take care reading large international numbers in the USA the comma is a
> separator and the period is a decimal point but elsewhere the period may be
> the separator and a comma the decimal point.

Indeed. Also, miljard means diffrent things inside and outside of USA.
This is why one should use SI prefixes.

I strongly recommend people to find NIST's Special Publication 330. It
has the full SI system, is free for download and is in a nifty PDF
format. Go and get it! Better... go and actually USE IT!!!

Cheers,
Magnus