A gravity-feed siphon topoff system

by Chris Paris for Reef Aquarium Information Depot

The following is a log of a description of this that I gave on FishRoom.

You say: Jasonv wants to know about my automatic topoff system.
You say: I have a 5 gallon water jug with a narrow mouth, and a #7 2 hole 
   stopper that fits it.
You say: The jug is suspended above the sump.
You say: There is rigid airline tubing going from near the bottom of the jug, 
   through one of the holes in the stopper, after which it's connected to 
   flexible airline tubing, and that flexible airline tubing goes down into 
   the sump. A siphon is started between the jug and the sump, thorough this 
   airline tubing, and a knot is tied in the tubing to get the right drip rate 
   (that actually works).
You say: As water leaves the jug through the siphon, air has to come in 
   through the other hole in the stopper.
You say: The other hole also has rigid airline tubing through it, but rather 
   than going down to the bottom of the jug, it stops just inside the jug. 
   Just outside of the jug, the rigid airline tubing connects (through a 
   homemade connector) to 1/2" flexible tubing.
You say: The 1/2" flexible tubing also goes into the sump, and its end is held 
   in a pair of clamps (actually 3/4" CPVC tubing clamps) that lets you 
   position the end of the tubing precisely relative to the sump's water level.
You say: In fact, the bottom of the 1/2" tubing defines the water level. Quiz: 
   why?
Jasonv says: Because when it hits water it stops the other syphon?
Jasonv says: Or water just circulates.
Jasonv says: Or niether. :)
You say: Right the first time, because air has to be able to enter the jug in 
   order for water to leave through the siphon. So water flows exactly when 
   the sump level is below the bottom of the 1/2" flex tubing.
You say: The only slightly tricky thing is that the jug should itself be 
   rigid. Mine is glass. I first tried this with a plastic "gasoline can," and the 
   vacuum generated by the siphon would contract the jug, which would allow 
   water to flow long after it was supposed to stop.
Jasonv says: Ah, good point.
Jasonv says: Why a knot in the airline tubing and not a valve of some sort?
Jasonv says: Being such a perfectionist I am shocked you depend upona  knot in 
   the tube for flow control.
You say: You can use a valve too, but I found that the green Penn-Plax airline 
   control valves (a screw that closes an opening) would leak a little air, 
   which would poison the siphon. Also, I've heard that those valves can clog 
   with calcium.
Jasonv nods his head in understanding.
You say: I figured a knot would loosen over time, but apparently it doesn't, 
   at least not between fillings of the jug. I find myself loosening the knot 
   intentionally in order to start the siphon after filling the jug, so I have 
   to adjust it then anyway.
You say: Oh, and you don't like getting kalkwasser in your mouth from starting 
   the siphon? Then start it by blowing on the air inlet tubing.
Jasonv says: Is your tank covered?
You say: The tank is covered but the sump is open. [For several months
now, the tank has been completely uncovered. --cap 5/21/96]
Jasonv says: I'm wondering if just replacing evap. water with KW will work for 
   me.  I dont see how I will get significant evap, except through the skimmer 
   anf possibly the bio-wheel.
You say: Yeah, that's a good point.

What I didn't explain above is why the airline is 1/2" tubing instead of something smaller like conventional airline tubing. The reason has to do with the fact that when the air supply is cut off, water gets drawn up into the 1/2" tubing, and often rises up to the level of water in the jug. When the sump level evaporates enough to expose the end of the tubing, the water that earlier was drawn into the tubing has to be able to fall out in order for air to get into it. This works reliably with fat tubing but not with thin airline tubing.

You can dispense with the water siphon by drilling a hole in the bottom of the jug (with a diamond abrasive bit) and taking water directly from the bottom. Both ways have disadvantages. For now I'm sticking with the siphon.

Actually the water flow doesn't stop dead in its tracks as soon as the air line gets covered. There's a period where water continues to flow out of the jar, and sump water gets sucked up the air line. This stops, theoretically, when the water in the air line is at the level of the water in the jug.

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