Theoretically, in ideal conditions, it makes no difference where you put your filter in a gravity water filter system. However, placing the filter at least a few feet below your water source has practical advantages.
The difference in pressure between the filter inlet and outlet is what forces water through it.
In a ‘bottom filter’ setup, each foot of elevation between the water source and filter inlet creates .434psi of hydrostatic head at the filter. Ignoring atmospheric pressure, the filter inlet pressure is .434psi for each foot of head and the filter outlet pressure is 0.
In a ‘top filter’ setup, for each foot of elevation between the filter outlet and the lower end of the tube, there is also .434psi of hydrostatic head. The head creates a ‘vacuum’ effect at the top of the tube equal to .434psi. Ignoring atmospheric pressure, the filter inlet pressure is 0 and the filter outlet pressure is negative 0.434psi for each foot of head.
In a top filter system, head between the water surface of the source vessel and the inlet of the filter may be sufficient pressure alone to force water through the filter.
However, for the ‘vacuum’ effect of the top filter system to work effectively, the tube must be primed and handled carefully so that prime is not lost. This is where the bottom filter setup has an advantage.
In the following pictures, I try to demonstrate the theoretical pressure differences.
In this picture, a tube is filled with water, both ends leveled and a pressure gauge attached. The gauge reads zero.
In this picture, the gauge is at the bottom with approximately 34” of head. (34” of water is approximately 63mm of mercury). Gauge shows about 60mm hg.
In this picture, the gauge is at the top with approximately 38” of head below it. (38” of water is approximately 71mm of mercury) The gauge moved from 0/320 down to 250; a decrease of 70mm hg.
These two pictures show the overall setups.
My observations and calculations confirm that the pressure differences across a filter should be the same in theoretical top filter and bottom filter setups.
My personal experience with Sawyer, Seychelle and Frontier Pro filters is that several feet of head between water source and filter work best because of the priming issues mentioned above.
Conversion factors used:
1 inch = 25.4 mm
Specific gravity of mercury = 13.6