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Thread: Siphon Problems

  1. #1
    Join Date
    Apr 2016

    Talking Siphon Problems

    Hi All,

    Been hovering around here for around a month, finally made an account. I've been sucked in to aquaponics... first through massive amounts of reading, and then finally deciding to construct a very small setup (would love to build any one of the incredible setups on here, but don't have the money or the time for now).

    Anyway, over the past few weeks I've gotten my fish tank (~5 gallons) set up and cycled, went out and sprang for hydroton for grow media (only needed 10 liters of it for my small growbed, so it didn't burn too large of a hole in my pocket), and have been working on getting an affnan style bell siphon working.

    Unfortunately, finding a wide selection of PVC in my area has been rather difficult, and shipping costs are prohibitive. I finally found a gardening store that had some parts, so I was able to construct my frankensiphon (I'm exaggerating, it isn't really that bad). It's a 50mm bell, 25mm stand pipe (very short in the pictures below), 25-20mm reducer, and 20mm elbows and other pipe parts.

    I was able to order a tiny 6v pump from China which is supposedly rated at 35 GPH (which I thought would be sufficient given my small growbed), but it really does around 22 GPH.

    The problem I'm having, is that the flow from the pump is not enough to get the siphon started (it works when I use the sink faucet to fill it up), and I was wondering if anyone might have some suggestions as to how I could jerry-rig my setup somehow to activate the siphon given the large pipe size and small pump flow.

    Here are some pictures of the growbed & siphon:

    Thanks in advance for all your input, this forum is great!
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  2. #2
    Aquaponics 101 Oliver's Avatar
    Join Date
    Mar 2009
    Show Low, AZ USA
    There are two main issues here.

    One you have already discovered, the pump flow verses the size of pipe in the siphon. You have chosen a 3/4 inch (20 mm) diameter stand pipe, which is really too large for the size of system you are building. I would recommend going with a smaller stand pipe. We have used 3/4 inch stand pipes for grow beds ranging from 40 to 200 gallons. The 200 gallon grow bed would benefit from a larger 1 inch stand pipe but that would require upping the pump requirements.

    The second issue is understanding that a siphon works by having differential pressure between the level of the water in the grow bed (full to near empty) and the lowest point of the siphon drain below the grow bed. If the difference in these two heights is small, then the siphon action will be limited. This often shows up as difficulty in getting the siphon to start or, more often, the siphon starts but as the water drains it slows down and stops before it has emptied the grow bed and continues to dribble out water or oscillates indefinitely. I have found that there needs to be at least 6 inches, preferably 12 inches, between the lowest point of the grow bed water and the bottom of the siphon drain.

    The Affnan improvement to a siphon, be it bell or loop, works best when the ratios between the top and bottom of the funnel diameters are large. It also works best when you have a smooth transition between the two sizes and a continued smooth pipe interior on down to the bottom of the drain pipe. There should be no gaps or other perturbations that cause the water not to flow in a uniform cylinder. The smallest gap in the joints of pipe in the downspout will affect the function of the siphon. Also, siphon stand pipes and downspouts need to be cleaned often, as buildup in there will affect siphon action.

    Another issue in designing a bell siphon is the air space between the top of the stand pipe and the bell cap. It should be as small as possible but not so small as to restrict the water flow. Large amounts of air in this area will affect the starting of the siphon process, as it acts as an air reservoir preventing a vacuum from being created as the siphon attempts to start. If you have a two piece bell shroud, then it needs to be air tight, as the smallest air leak into the cap joint will prevent the siphon from working properly. We have used a simple 4 inch combination shroud and media guard for easy cleaning and a 4 inch wetted rubber cap on all of our systems prior to converting over to loop siphons (which have their own set of problems). Wetting the cap's interior prior to placing it on the guard creates an air tight seal. It will remain wetted until being removed.

    To measure is to know

  3. #3
    Members Apollo's Avatar
    Join Date
    Aug 2013
    Phoenix, AZ
    Sorry, still tiring to learn this new format, check next post.

  4. #4
    Members Apollo's Avatar
    Join Date
    Aug 2013
    Phoenix, AZ
    Good Morning DRH,

    Well...Oliver did a real good job in explaining in detail your problem, I was wondering if it help? I went though all of the same problem and all of the detailed explanation over 3 years ago. I seemed to me to be way too complicated for anyone newbie to follow.

    I finally gave up and made a much easier, cheaper siphon, which has been operating now for over three year. with no cleaning & almost no tweaking.

    Take a look a "Pipe Siphon by Apollo", let me know what you think about would be nice to hear back from you or anyone else having this problem.


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