Originally Posted by noteworthy
Also notice in the design picture you posted that the venturi is before the valve. It must be after the valve or you'll shoot water out the air line. A major flaw.
sorry, but this is not entirely correct. what you are saying will only occur if you have to close the valve enough to lower the flow extensively, and this is only true if the pressure created by the closed valve is greater than the pressure created by the pump. if a decent pump is used than this can be fairly difficult to achieve if the skimmer is designed properly. if you were to put the valve before the venturi than the venturi will not be working efficiently. it needs the full pressure from the pump in order to draw the air in fast enough to form tiny bubbles. the lower the pressure gradient in the venturi valve the larger the bubbles in the reaction chamber.
BTW-that valve is used to shut off the flow of water if i needed to remove the pump for any reason. it can also be used to fine tune the flow, but should not be necessary if a large enough skimmer drain is used.
a good rule of thumb: a proper venturi valve will add 7' of head pressure to a pump. use this point on a pumps curve to figure out the proper exit pipe diameter for the skimmer.