Add the units: parts per hundred thousand (pcm), million (ppm), billion (ppb), trillion (ppt), and quadrillion (ppq)

[eliascarv]

Reference: Parts-per notation - Wikipedia

closes #684

[juliohm]

Appreciate if someone can review and merge this. It is a very useful unit for chemical analysis.

[cstjean]

We define our own ppm/ppb, but it's its own dimension, which feels better than having 3ppm == 0.000003.

[eliascarv]

Sorry @cstjean, I didn't understand your comment, could you explain?

[cstjean]

We have

@dimension PPer     "PartsPer"          PartsPer
@refunit ppb        "ppb"               PartsPerBillion     PPer         false
@unit ppm           "ppm"               PPM                 1000ppb      false

(perhaps that's not super clean, but eh)

I'm not sure what's more appropriate for Unitful, but for us, in terms of "units exist to handle conversion code and prevent errors", making it its own dimension is the right call, even if it means having to add a /ppm division here and there.

Are there circumstances where you'd want 1ppm to be equal to 0.000001? For us, it's no, but I can definitely see that it could be true in some calculations, and in that case, this PR makes more sense than our code. Dividing by 1000000ppm sounds a bit awkward for that conversion.

What do you think? For us it doesn't matter (I think?) as we can override the units. But obviously since changing it afterwards is difficult, so I thought I'd raise the issue at least.

[sostock]

Percent, Permille, and Permyriad are dimensionless, so adding a dimension only for ppm, ppb, ppb, and ppq would be highly inconsistent. And changing Percent, Permille, and Permyriad to use that new dimension would be breaking.

SI also defines 1% == 0.01, and it doesn’t seem controversial to me.

[sostock]

Reference: Parts-per notation - Wikipedia

The Wikipedia page also mentions pcm (per cent mille). Does it make sense to add that as well? Then we would have all units listed on the page.

[eliascarv]

@sostock, added.

[juliohm]

Thank you all for the input. We are looking forward to seeing it merged and released. We already have some downstream applications that will benefit from these new units.

[eliascarv]

@sostock, the PR is ready. Can you check if it's ready to merge?

[eliascarv]

@sostock, could we have a patch release with this PR?

[sostock]

No, this is a feature, so it cannot go into a patch release. I will release it as v1.19.0 shortly.