Foam filters and lift tubes

Discussion in 'DIY' started by rbk, Sep 12, 2015.

  1. rbk

    rbk Member

    What is the relationship between the length of a lift tube and the strength of the flow through the filter?

    What is the relationship between the diameter of the lift tube and the strength of the flow in a foam filter?
  2. Aquaticus

    Aquaticus Administrator

    Great questions! I don't know the answers. I use 1/2" and 3/4" pvc lift tubes, mostly 1/2" in smaller tanks.
  3. Marine590622

    Marine590622 Advisory Board Staff Member

    The longer the lift tube the more flow, assuming the air bubble is released at the bottom of the tube, The longer the bubble rises the faster it rises. A larger diameter tube is only usefull if you can fill the tube with air bubbles.
  4. tjudy

    tjudy Advisory Board Staff Member

    Smaller bubbles have a greater surface area (as a mass of bubbles) and move more water than large bubbles do. However, an airstone in the bottom of the tube block some flow of water. So if you are going to use air stones you will want to go with a narrow stone and wider tube. Not as easy to find as you might think. One way to do it is to drill a 1" diameter hole all the way to the bottom of the sponge (assuming slate or tile on the bottom. Use a 1" diameter lift tube (pvc) with a 1" connector on the bottom. The connector is wider than 1", so it can be held very snuggly in the 1" diameter hole... right at the top, no need to push it to the bottom. Cut the lift tube to the length you want. I like it to rise to about 1" below the surface of the water. Use a piece of rigid airline to hold the airstone all the way at the bottom of the sponge filter (running through the lift tube).

    If you want to get really creating, you can slice a small hole through the side of the sponge near the bottom into the cavity, and run airline through it into the open space. Pull the airline up the hole, attach the stone and pull the line back down the hole so the stone in in the bottom of the sponge.

    The bubbles will be funneled into the lift tube by the pvc connector, and there is very little of the inside surface area of the hole in the sponge blocked by lift tube.

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