Common definition of not really SI-units

[HansOlsson]

Sort of related to unit-checking, but can be added independently.

There are some common units used with the SI-standard, that are not SI-units. I believe it would be good if we have a common understanding of them, and currently the specification only mentions that base and derived are allowed - but not these ones. In particular it might be good to state that it is only recommended to use them as display units.

First we have the non SI-units that are acceptable to use with the SI-standard (according to the standard!), and I would in particular want us to consider:

• min (1 minute=60 s)
• h (1 hour=3600 s)
• d (1 day=86400 s)
• l (1 litre=1 dm3)
• eV (1 electronvolt=1.602176634e-19 J)
• (superscript o) spelled "deg" (degree=pi/180 rad)

Additionally the list contains - we might include some or all of them:

• au (astronomical unit=149 597 870 700 m)
• ′ (minute=1/60 deg)
• '' (second = 1/60 minute); in astronomy spelled "as" as in "mas", "μas", "pas"
• ha (hectare = 1e4 m2); I have no idea why it was kept but "are" was dropped.
• t (tonne =1e3 kg)
• Da (dalton = 1.660 539 066 60(50)e-27 kg)
• Np (neper)
• B (bel)
• dB (decibel)

Notes:

• I find the spelling of ' for minute and '' for second as problematic, as it can also mean feet and inches.
• The decibel is problematic as the SI-standard clearly define that dB is for 10*log10(A1/A0) - whereas MSL has 20*log10(...) (We had that discussion at some point).
• I'm not 100% clear on the exact status of degree Celsius (for non-differences), but clearly it is in use in practice.

And then there are the ones that aren't really acceptable according to the standard, but still used and have a clear definition:

• debye (1 qC.m=0.299792458 debye, IIRC)
• English ones like: degree Rankine (=5/9 K) and Fahrenheit (same with offset)
• there are also other ones such as "Ångström" =1e-10 m, but since we want English letters it doesn't fit.

[HansOlsson]

Language meeting:

• Should we have both of l and L, or only one? (Simplest with both?) Both: Dag, Elena, Hans. Only L: Henrik:
• Add first list (keep open)? Poll: Favor: Henrik, Dag, Quentin, Elena, Against:
• Add debye (1 qC.m=0.299792458 debye) ? Poll: Favor: Henrik, Dag, Quentin, Elena, Against:
• Recommend to stick to SI-units for unit and only use others for displayUnit, with motivation .... Favor: Henrik, Dag, Quentin, Elena, Against:

Abstain: Markus