2016-01-21

Relay Siphon - Theory and Practice

I have done some additional experiments with the relay siphon, and am pleased with the results.  Using a 1/2" diameter control siphon, I was able to start the main siphon in a very short period of time.

Here are some numbers:

Relay Siphon Test
1" drain, 3/4" main standpipe, 1/2" control standpipe
Fill Rate0.01449275362in/sec
time to 1/2" spillover195sec
height at 1/2" spillover5.326086957inches
volume at spillover13257.96562cm3
main siphon start time14seconds
main drain time17seconds
main drain rate779.8803306cm3 / sec


Most of these numbers are approximates, since the timer is human-operated and the measurements are not exact.  What we can be certain of is that the relay does actually work.  The 1/2" spillover height is the water level where the 1/2" control standpipe begins experiencing water flow.  The siphon start time is the time between when the control siphon starts and the main siphon starts - that is, the control siphon operated for 14 seconds before the main siphon began to pull water down.  From the time the main siphon operated to the time it finished was 17 seconds.  Note that during this time, the control siphon had lowered the water level as well, but by only a very small amount.

Here's a picture of one of the other relay tests:

The presence of the trap does not necessarily help nor hurt the control siphon.  It does cause air to be trapped in the control bell, so the water level does rise quite a bit higher than without the trap.  The above picture shows the 1/2" control (left) feeding into the 3/4" tee from the main (right), into a 90 and down a 3/4" drain pipe.  The drain rate on this setup seemed slower than the 1" drain pipe, but unfortunately I didn't have time to pull any timing numbers.  But again, this test was primarily about the function of the relay.

To ensure the water level was slow enough not to trip the 3/4" standpipe, I capped the control pipe and let the water rise.  As expected, there was insufficient flow and insufficient back-pressure to trigger the 3/4" siphon.  Uncapping the 1/2" standpipe caused a flood of water into the drain, which then triggered the main siphon, thus demonstrating that sufficient flow from the control was all that was necessary to induce the main siphon.

Replacing the bell on the control siphon allowed it to function as designed: by starting its siphon at sufficient water height, and providing sufficient flow via siphon-action alone to trigger the main siphon.  I was able to watch the main siphon start using my snorkel bell (described in an earlier post).  It would start several seconds after the control started full siphoning.

While a single large main siphon will certainly draw water out of a grow-bed quickly, I could see this mechanism also finding use in long grow-beds where the risk of poor circulation may be significant.  You should, in theory, be able to tee-in a number of main siphons and have them trigger off of one control.  Once one of the main siphons starts, the flow should be great enough to start any remaining main siphons that did not start from the control flow alone.  I'd like to experiment with this in the future.

All this fast draining potential has me asking another question, however - one that I don't readily see much information on in the literature I've read to-date: how long should plant roots be exposed to water before the grow-bed is drained (or air after it has drained)?  If it takes an half hour to fill the bed, and 60 seconds to drain it, the roots may be exposed to air for at least 15 minutes before the water level is high enough to touch their tips.  I'm hoping there is other literature out there, especially among the hydroponics community, that addresses this.  The closest thing I think I've seen to a number is with the timer-based flood-and-drain systems: 15 minutes of fill time, 45 minutes of drain-and-empty time.

More work to do!

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