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Depends on the design of the head unit. The DOS head is designed so the roller is compressing the tubing for more than half the rotation of the head. So there is always a roller somewhere compressing the tubing when it is off. Most dosing heads don't compress the tubing for such a long distance around the head so they require at least 3 to keep one roller always compressing the tubing.I've read that 3 roller peristaltic pumps prevent siphoning, but 2 rollers may not. Neptune's DOS, expensive as it is, is a 2 roller. Who makes 3 roller pumps?
Any dosing pump can siphon if the roller doesn't completely pinch off the tube, ecspecially as parts begin to wear out. So as I'm sure you know the best method is to plan it so your pump siphoning doesn't hurt anything. My dos is setup to where if it does leak around the roller (well technically the tube being pinched by the roller) all that happens is the 2 part in the dosing line falls back down into the dosing container.I can confirm that a three roller pump can siphon also. Especially if you foolishly put your pump and container above the tank