[Skulemate]

I don't believe the tubing diameter plays too much of a role in this, though I agree that there is no sense to use anything over 1/8". However, I think that it's best to do these tests with some sort of a reservoir, otherwise, where will the (albeit small) amount of water needed to fill the manometer come from? Furthermore, to do this correctly, you'll need a manometer directly before and after the thing you're measuring... otherwise all you have is pressure at a point.