isn't it a 'constantly on' attachment which will want to draw the APT through the pump/outlet pipe and from the reservoir?
Essentially, yes. The
Venturi effect is when the fluid pressure drops as the velocity of the fluid increases as it passes through a constriction. The relationship between fluid pressure and velocity is a fundamental law of fluid dynamics. As the pressure drops in the constriction, the negative pressure can be used in an engineering process to draw another fluid into the system. This is how fertiliser can be pulled into an irrigation system, or air into a protein skimmer for example. While the fluid is flowing through the constriction, there will always be a pressure drop.
How is it connected to the pump outlet?
A peristaltic dosing pump uses
peristalsis to move a fluid from one place to another. When the pump head rotates, it uses rollers to compress and re-expand the tube in which the fluid flows. As it rotates it forms a wave like motion pulling fluid into the tube and pushing it out of the opposite end. When the pump is at rest, the tube is compressed by a roller, essentially acting as a valve. This prevents fluid moving within the tube.
If you suck up a straw of juice then put your finger on the top, you’ll notice that the juice doesn’t come out of the bottom. This is the atmospheric pressure acting on the open end. You can then suck on the end of the straw and you’ll find the juice still doesn’t come out. You’re creating negative pressure with your mouth but because your finger is preventing the pressure from equalising, it doesn’t move. Once you release your finger, the pressure equalises and you get a tasty drink of juice.
Now think of the tube from your dosing pump as the straw, the fertiliser is your juice and the pump head is your finger. The negative pressure created by the Venturi effect is your mouth. While the pump head is at rest, no fertiliser flows into the system. Once the pump head moves however, re-expanding the tube, the pressure in the tube equalises and the Venturi effect pulls fertiliser into the system.
EDIT: Just an addition, the peristaltic head isn’t actually required in a set up like this now and the negative pressure in the Venturi will draw fluid into the system without the ‘pumping’ assistance of the pump. The head is acting as a valve. You could achieve the same thing with a solenoid, timer and check valve if you wanted. The check valve would prevent fluid going the ‘wrong way’ should flow in your main pipe stop and the pressures equalise.