The linear pumps I am using have an 11' maximum recomended depth, but in my opinion that would be really pushing it. My max pond depth is 10' at full pool (bout 10.5 right now!) but also the diffuser base is sitting up probably close to a foot from the bottom. So in reality even though the depth is 10' the pump is looking at about 9'. I get good air flow at that depth and according to the performance curve for a similar pump of another brand I should be getting in excess of 2 cfm.

To a vane or rocking piston pump, talking the difference of 8 or 10 or 12 feet depth would make very little difference in flow rates. These type pumps are positive displacement and the drop in performance is due to any slippage of air around the sealing surfaces as well as the displacement vs the amount of compression (much like the compression ratio of a car engine).

But a linear compressor is quite different. They use a diaphragm with a solenoid style electric motor that pulls the diaphragm in one direction then coils on the other end pull the diaphragm in the other direction. The maximum pressure that the compressor is able to develop is related to the power in the coils in each end of the linear motor. The coils can only pull so hard and the travel of the diaphragm limits the volume of air. This happens at a frequency equal to the AC frequency of 60 times per second in the US. As back pressure is exerted against the diaphragm, this back and forth pulse does not pull the diaphragm as far in either direction. At some pressure (around 7 psi in the performance chart posted above) the motor can not pull the diaphragm in either direction and simply hums at the 60 cycle frequency with full pressure within the diaphragm chambers (diaphragm at each end of the shaft each with an inlet and outlet valve arrangement) and the shaft simply stays still in the middle. The pump "strokes out". Anyone familiar with a closed center hydraulic system it operates in a similar manner with the pump stroking out at the designed pressure. At this point the motor will be drawing the same electricity (I think, I'm no electrical engineer) but only maintaining maximum pressure with no flow.

The fact that there are no bearings and that the armature of the linear motor floats on air with the only friction involved being the flexing of the diaphragm and compression of air, they are very efficient at pumping air AT LOW PRESSURES. A conventional pump can't touch the efficiency. But as pressure increases where a conventional pump and motor maintains flow much better, the linear pump drops off drastically.

That is why, when used within their design capabilities, the linear compressor will pump a lot of air on little electricity. But efficiency rapidly drops off with pressure (depth).

John

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