Hydraulics & Pressure
Residual Pressure
Residual pressure is the pressure that remains in a supply while water is actually flowing. The gap between static and residual is what tells you how much more the source can give.
When you open a line, the supply pressure drops from its static value to its residual value. A small drop means the source has plenty left; a large drop means you are already near its limit. The residual reading is the operator's live gauge of remaining capacity.
On a weak hydrant the residual is everything: pump the line to its target, read the residual, and the margin above the floor is all the room you have. Reach for more and the residual craters — and a starved line can lose its water entirely, not just sag.
Where this shows up
Calculator
Hydrant Available WaterLesson
Static & Residual Pressure