From: drake@Autodesk.COM (Dan Drake)
Newsgroups: rec.aquaria
Subject: Re: Wet/Dry Problems ?
Date: 27 Jul 92 21:28:47 GMT
Organization: Autodesk Inc., Sausalito CA, USA

Speaking of preventing breakage of the siphon by accumulaed bubbles,

harold@wam.umd.edu (James B. Harold) writes:
+ In any case, not wanting a new pump (I'm using an 802), I solved
+ my problem with the venturi of a powerhead.  I have a Hagen
+ 402 in the tank to provide water circulations (gives the 
+ rainbows something to play in).  I raised it a little, hooked
+ the venturi into a connector I glued into the syphon, and
+ *presto* no more bubbles.  Every now and then I entertain
+ myself by watching a bubble get sucked into the venturi tube
+ and into the tank (clearly I'm easily entertained :-)  ).
+ Anyway, another alternative solution.  You may have to fiddle
+ a little to get enough suction off of your venturi for this
+ to work. Oh...one caveat.  Plug the powerhead into the same
+ power supply as the sump pump.  If the sump pump shuts off
+ and the powerhead keeps going, the venturi will suck water
+ out of overflow box until the syphon is broken. 

Interesting that this problem -- the venturi breaking the siphon that it
was protecting, if the powerhead stays on when the pump stops -- doesn't
seem to have been mentioned before.  Having just done my first wet/dry
installation on the weekend, I have a bit to add to this.

When the sump pump stops, the water drains back until _both_ (1) the
main siphon stops draining because the water level is down to the bottom
of the teeth in the skimmer box and (2) the return stops back-siphoning
because the water is down to its outlet (or to the siphon-breaker hole
that you drilled just below normal water level).

If (2) is higher than (1), then the venturi won't break the siphon:
as the level gets down to the bottom of the skimmer box teeth, water
trickles into the skimmer box, is sucked up into the venturi, is blown
back into the tank, and cycles.  As a practical matter, the main drain
will shut down and leave this cycle going indefinitely -- probably.

If (1) is higher than (2), then the water level will drop below the
skimmer box teeth, and that cycle won't take place.  You may still get
away with it, though: If the end of your siphon tube in the skimmer box
is _higher_ than the end in the prefilter box, then that end of the
siphon will break while there's still enough water in the prefilter to
cover the end of the tube.  When the sump pump goes on again, the water
will again flow into the skimmer box, and the siphon will restart
nicely.  Of course, in the interim, your powerhead will blow masses of
bubbles into the tank.  

But if you let the siphon break on the prefilter end, it won't start
without manual priming, and your pump may burn itself up pumping the
sump dry when power comes on again.

For belt-and-suspenders operation, then:

 (a) Plug both pumps into the same outlet.

 (b) Drill a siphon-breaker hole in your return tube, as previously
     recommended by experts in this forum; and put it above the level of
     the bottom of the skimmer box teeth, if practical.  Don't forget to
     drill it so that it angles down just a tiny bit; when water comes
     spewing out of there as the pump restarts, you'll prefer to have it
     spew into the water.

  (c) In the end of the siphon tube that goes into the skimmer, drill a
      little hole above the level of the end that goes in the prefilter.

Your mileage may differ, but my tank actually behaves as this theory
says it should.
     
Why must all this sound so diabolically complicated?  After months of
cogitation and trepidation, I found that it was really quite easy and
safe to rig the thing when I finally got around to it.

--
Dan Drake
drake@Autodesk.com

All theory is against the freedom of the will; all experience for it.
                                        --Samuel Johnson