Very interesting links, thank you both.
Given that it's Rosemont that's still involved, and that the vane is substantially mechanical, I conclude that the vane is the more expensive to produce. ergo, they wouldn't fit them if they weren't the best measuring instrument.
The errors in one of those new tec links left me taken-aback. And more, it left me thinking, 'how would I do it?' Not like that, that's for sure.
It's always been how I think. I was squirting water onto plate glass in my hobby shop, when one of my kids shouted, hey, Dad, look what's on tomorrow's world. Sure enough, a prototype rain detector - which didn't work as well as mine - which wasn't very good.
It is notoriously difficult to measure air molecules. Air after all, contains water, and in differing forms. Looking at those varied droplets I found very, very hard back then. That good old vane would follow a hail of 5mm droplets with ease. And come to that, real hail - all without coming asunder.
It's a bit of kit that's stuck around, not because it's cheap, but because it's a good measuring device.