Re: SI prefixes with "accepted" units, others

The accepted units include angular quantities such as degrees, minutes, and
seconds. These are not units of time, but I don't think SI prefixes make
any sense to use with them.

That leaves "au" (astronomical unit), "ha" (hectare), "l" (litre), "t"
(tonne), "Da" (dalton), "eV" (electronvolt), "Np" (neper), "B" (bel), "dB"
(decibel).

The later 3 are log ratio quantities; that makes me dubious that they take
SI prefixes. Furthermore, it seems a little strange to allow a SI prefix on
"dB" given that the "d" is already a SI prefix. This wikipedia page
<https://en.wikipedia.org/wiki/Decibel> says:

> The bel is rarely used either without a prefix or with SI unit prefixes
> other than deci; it is customary, for example, to use hundredths of a
> decibel rather than millibels. Thus, five one-thousandths of a bel would
> normally be written 0.05 dB, and not 5 mB


That's a really awkward phrasing, but I believe it is saying "B" doesn't
normally have SI prefixes other than "d" and that you don't use them with
"dB" either. I did some google searches and never found a SI prefix with
"dB". @Joseph: you mentioned "mdB". Do you have a reference?

My conclusion is that only au, ha, l, t, Da, and eV accept SI prefixes in
practice. I'm definitely open to revising that if someone can show that is
not true (of course, people with do all sorts of "wrong" things, but if
enough people do that, then it is acceptable...).

FYI: I hadn't realized this, but "neper" comes from "the neperian (or
natural) logarithm". The bel (named after Alexander Graham Bell) is log_10.
All typically reference some quantity (e.g., 3dbW) because they are just
ratios.

    Neil


On Tue, Nov 12, 2024 at 2:56 AM Joseph Wright <joseph@texdev.net> wrote:

> Hello Neil,
>
> On 12/11/2024 09:12, Neil Soiffer wrote:
> >> The SI prefixes can be used with several of these units, but not, for
> >> example, with the non-SI units of time.
> >
> >
> > That's a little less definitive but does rule out "khr", etc.
>
> Yes. [Also hours are "h", but people persist in using "hr" :)]
>
> > Also, going through the document, I spotted they added four more prefixes
> > in 2022:  10^30, 10^27, 10^-27, 10^-30. I updated the issue to include
> them.
>
> Oh, those, yes - I added them to the LaTeX support back then.
>
> > Unfortunately, as Stephen points out, they also get used when units are
> > combined such as kWh (which is really kW⋅h) and abused as kph (should be
> > km/h).
>
> Indeed: kWh is very odd as people then want to make derivatives, which
> is truly strange.
>
> Joseph
>