Why a fan curve only tells half the story — and which of the three numbers actually belongs in a fan selection.
Every point in a duct system has three pressure numbers attached to it, not one. Two of them — static and velocity — can rise and fall in opposite directions at the very same spot in the duct without anything being wrong. The third, total pressure, is just their sum. Knowing which of the three to hand to a fan curve, and which two are mostly there to help you get to that number, is the difference between a system that hits its design airflow and one that quietly falls short after startup.