Two-stage (Relay) Bell Siphons

I was considering today how to deal with the flow-rate problem for siphon-start, when an idea struck me.  I suspect someone has thought of this before, but a short web search didn't turn up anything fruitful.  There is one fellow I know of that does something similar: he has multiple large grow-beds on a slope, and synchronizes their draining using a 55-gallon drum.  There's a video on YouTube about it.

How about for smaller grow operations?  Let's suppose you have a fairly large grow bed.  First problem: the bed will fill very slowly, so the rate of height-gain of the water may not be sufficient to trip a large-diameter siphon.  Second problem: you want the bed to drain quickly, so you need a large diameter siphon.

The theory of operation is similar to that of a relay: we use a small current to trigger a large current.

The image to the right shows two siphons feeding into the same drain.  The smaller siphon (green) is the control siphon.  The larger (red) is the main siphon. The operation is simple: the control siphon trips quickly, thanks to its small diameter, and starts a rapid flow of water through the drain pipe.  This flow creates a vacuum that pulls water up into the main siphon, thus starting it.  Once the main siphon starts, both siphons will operate until water has been drawn down to the highest intake hole - or down to the bottom of the snorkel tube, if so equipped.

Note that the drawing I have here is not to scale, nor is this configuration of pipes necessarily a good one.  I also did not test the configuration in the drawing.  I did, however, test something very similar:

Two-stage, or relay, bell siphon test

The image shows my test apparatus, with some peculiar plumbing.  The drain for the bell siphon (inside the tank) runs into a 90, which then heads into a tee fitting. The tee sports the control tube (from the top), and a drain out the bottom.  The drain is further fitted with another 90 and some extra pipe, to provide back-pressure.  I was able to start the siphon without the extra 90 and pipe, but it required a high rate of flow through the control.

To test, I filled the tank with water up to the 6" mark.  The main standpipe is 8" tall.  The bell was then placed on top of the standpipe.  I used a hose-to-slip fitting to attach the garden hose to the control pipe.  I was then able to control the rate of flow through the control pipe at the spigot.  With a very low flow rate, the siphon tripped after about 10 or so seconds, which I suspect to be the time it required to pull sufficient vacuum on the main siphon.

The pipes in the image above are a mix of 3/4" and 1", only because I had no 3/4" tees available.  Ideally, the control pipe should be smaller than the main pipe, and of a consistent diameter.  The low rate of flow I was able to use on the control pipe suggests that were the control itself a bell siphon, it would very readily trip the main siphon into action, since a bell siphon would generate a significantly greater rate of flow.  As an added bonus, having two siphons in operation at the same time should produce faster drain rates as well.  And with sufficiently high drain rate, we may avoid the siphon-stop problem.

I would ensure the main standpipe rises 1/2" above the control standpipe, so that there is no chance of the main pipe leaching water and prohibiting the siphon from starting.  It would be good not to go too high, though, since the vacuum generated by the control pipe may not be great enough to pull the water more than 1 or 2 inches up into the main siphon's bell.  I'd like to test this further, using a single bucket, some 1/2" pipe for the control, and upwards of 2" pipe for the main.  2" is probably overkill, but it would be nice to know it works.

One other possibility is to have the control standpipe and the main standpipe together in the same bell (pictured at right).  This would require a large diameter bell, but it would also allow you to use a single snorkel to break the siphon - if so desired.  It would require two bulkhead fittings in close proximity.

Again, the pipes in my drawings and on my test assembly are not ideal; there are probably many configurations possible that would achieve the same effect.

One last note:  it may also be possible - but is, as of yet, unconfirmed - that the control pipe need not necessarily be housed inside a bell.  Once the main siphon starts, the operation of the system reverses: instead of the control pipe pulling vacuum on the main pipe, the main pipe instead pulls vacuum on the control.  So long as the main siphon assembly is operating properly, it ought to be possible to have the control open to the air without losing siphon.  All things being equal, however, I think I'd prefer housing the control be in a bell for the reasons stated above.