I think the whole smplicty is quite deceptive here. It is not simple at all. The whole flow conrol uses two different mechanism there to my understanding. One is purely Newtonian (inertia) and second is derived from the Poiseuille's Law and pressure drop gradients.
Very clever due to elegant engineering employed at minimal cost!
As product it is just perfect!
BTW Nice hacking jobbie there pH! Did you enjoy doing it, huh? C'mon, we know you did!
