CONVERT - metric teaspoon? (but not metric tablespoon)

["David A. Wheeler" <dwheeler@dwheeler.com>]

I'm going through all the nasty CONVERT units, and found a "does not conform with standards" nasty.  Given all the recent bru-ha-ha about standards conformance and contradiction, I think it'd be very wise to make sure that we don't open ourselves up to a "does not conform with standards" complaint.  My proposed solution: add "tspm", a "metric teaspoon", which is 5mL.  I think we should NOT add a "metric tablespoon", because that is not universal.

Again, more justification below.  I hate unit issues, because they're hairy, but the whole point of CONVERT is to hide the hairiness so that users don't have to deal with them.

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Historically, a teaspoon in the U.S. was 1/6 fluid ounce (about 4.93 mL).  In the UK they had a slightly different value historically (no, that shouldn't surprise you).  Thus, "tsp" has this old U.S. conversion value in all spreadsheet implementations of CONVERT that I know of.

However, things have changed since.  Many countries have officially redefined the teaspoon as being exactly 5mL.  Even the U.S. has 5mL as its official value of teaspoon for purposes of nutritional labels, and this conversion factor is written into U.S. law.

We dare not redefine "tsp", because this would be a silently wrong change to older spreadsheets.  So instead, let's use "tspm" (metric teaspoon) = 5mL.

There's also a "metric tablespoon", which many countries define as 15mL (including the U.S. in law), but at least one country (Australia) has a different value for this (20mL).  Since there seems to be a difference between countries, I think we can reasonably argue that there's some lack of consensus, and it'd be wiser for us to NOT implement that unit yet.  But you can't say that about metric teaspoons, which seem to have a universal definition.

--- David A. Wheeler