When considering pump Lift (or head), it is only applicable to the distance above the water's surface that it is pumping the water. It makes negligible difference if the pump is on the bottom or at the surface. This is due to the buoyancy of the water in the pipe compared to the buoyancy of water surrounding the pipe...they are the same. Once the pipe leaves the water...you have to compare the water buoyancy to the density of air which is greatly different, gravity starts to apply. Think of it as when you pick up a big rock that is under water, when it comes out of the water...it gets alot heavier and harder to lift...Picking up an item with the same density/specific gravity of water (a water balloon) while it is in the water takes no effort, once that balloon gets out of the water, you start to feel it's weight. That's when the pump starts to "work".
You can get some resistance (or back pressure) due to the length of the pipe going considerable lengths, but for pond and lake depths, even that is barely a factor.
The main specifications to consider are...Flow (GPM for water or CFM for air), Pressure at that flow (PSI, inches of H2O, mainly caused by the nozzle restriction), and Watts (power usage at the systems operating conditions). Given those variables, we can evaluate the pump's efficiency.
Last edited by Quarter Acre; 06/28/19 08:37 AM. Reason: Added "GPM for water"
