[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Ivo's Anti-Scum Surface Skimmer (long)


> [How does your setup deal with the surface agetation CO2 loss problem?
> >From your post, it was'nt clear whether or not you run CO2.]

Surface agitation is almost nill. Water flows into the device very gently
and the tank CO2 level is quite good: ph is 6.6 to 6.8 (7.4 with no CO2),
kH is around 5.

> [Do you run your setup only at night or 24-7?]
It runs continuously. 

> [Please tell us more.]	

Here is the theory:

The trick in surface skimming is to suck up the surface film and keep it
inside the skimming mechanism. Surface film can easily move outward even
when the main water flow goes inward. I figure there are two ways of
achieveing the desired result. The standard way used by overflow boxes 
actually spills the water to mechanically break the surface film and keep
the scum inside the box. This is of course not desirable if we need to 
keep CO2 from leaving solution. It also only works with gravity-fed sump
systems, not with canister filters in an airless syphon loop. The other 
way is to use a gentle water flow from a higher level in the tank to a 
lower level into the skimming box but with a level difference that is 
sufficient to prevent the film from migrating backwards into the tank. Under
the particular parameters of my setup this level difference is about one 
inch. One of my former versions of the skimmer had a difference in level 
of about 1/2" and I could see water being drawn into the skimmer at a rather
high flow rate but the surface film itself swirling in and out at the skimmer
input. At the same time we must prevent the water level difference in 
getting too large since them air may enter the filter's intake syphon and 
break it. This can happen if the skimmer input gets clogged, or if the 
water level in the tank drops too much. This control can be achieved by
an underwater weight-regulated valve, similar to the valve in a pressure
cooker. This is by the way the mechanism I figuerd is used by the Eheim
surface extractor.

Here is the implementation:

The device was built from a large gravel vac mounted upside down and
secured to the back glass wall by two suction cups. The canister filter 
intake with strainer and all goes into the vac's mouth. At the bottom end
of the vac I removed the hose, enlarged and reshaped the hole with drill
tools and glued a nylon nut of proper size and shape to act as the valve
rest, a small piece of plastic tube to protect the valve and a strainer at
the bottom end. The valve was built from a beveled faucet stopper with a 
stainless steel screw hanging under it and with stainless steel nuts screwed
on it until it gets the proper weigth. By choosing the correct weight the 
valve sets the minimum internal water level to the desired safe height. The 
device must be carefully adjusted in height inside the tank until water flows
from the tank level to a lower level in the vac's tube. Too little weight in
the valve and it will be always open, not letting the water level inside the 
skimmer to drop. Too much weight and the water level will drop too much, 
allowing air to enter the filter intake. 

As you can see the device needs a few adjustments to the particular setup 
where it will be used. It also depends rather critically on the tank water
level. My tank is well covered and keeps its level almost constant for
several days, so the skimmer needs almost no adjustment. If I notice that
it stopped skimming and all water is flowing in thru the valve, I just top
off the tank until the skimming action restarts and the valve patially 
shuts off.

I attempted to draw an "ASCII picture" of the device; hope it helps in
understanding its principles.


               |  |
               |  |  <---  fluval intake with strainer
               |  |
  ........     |  |      ..........      <--- water level at tank
         |     |  |      |
         |     |  |      |
         |     |  |      |
         \.....|  |....../  <---  water level inside skimmer
          \    |  |     /
           |   |  |     |
           |   |  |     |
           |   |  |     |    <---  gravel vac body
           |  \    /    |
           |   \  /     |
           |    --      |
           |            |
           |   _______  |
           |   \_____/  |   <---  valve (in floating, open position)
           |      T     |
           ----\  |  /---   <--- gravel vac end cap with valve rest 
               |  | |
               |    |       <--- tube (cut from an old UGF riser)
               |    |
               \    /
                \  /     <--- strainer               
                 --

The main drawback of this design is that all surface debris go directly 
into the filter's intake strainer and quickly clog it. To clean it I must
reach into the tank and slide the skimmer underwater until the filter intake
strainer gets exposed. I am working now on a modified design that
hopefully will fix this problem. I will replace the gravel vac's mouth
by a plastic box with two compartments separated by a fine-mesh screen. 
One compartment will hold the filter intake, the other will have an 
adjustable surface intake (loosely inspired on the Eheim extractor) and
act as the skimming box. In this way the larger debris will be caught at
the screen and would be easily removed thru a small opening in the back
of the tank cover. Again, I will be using only cheap plastic parts, mostly 
leftovers from UGFs I had in the past. The only new part I bought for this 
version is a Lee's Economy Corner Filter that will be the main skimming box
and costs around $1.75. 

-Ivo Busko
 Baltimore, MD