On: A comment on Hacker News led to 4½ new Unicode cha...

[Slartie]

Filled circle means "vent is blocked, so no air can pass". Empty circle is an unblocked vent through which air can pass.

[gmac → Slartie]

I think the problem is those meanings can plausibly be reversed, so it's hard to remember which way round it goes.

A better way to do it that I've seen is to represent air flow with wavy lines or similar on the 'open' symbol.

[alecmg → Slartie]

Pretty sure in the last car it was the opposite. Filled circle meant air was coming.

So you can see why this is confusing