I'm just building my system now, planning on a timed flood/drain system, and in this am assuming a pump setup like Affnan describes: http://affnan-aquaponics.blogspot.com/2009/06/timed-flood-drain-system.html. I just wanted to make sure all my logic was right...
If I'm understanding his directions and the Rules of Thumb correctly, the generic hourly pump cycle goes something like this:
0 mins: Pump turns on at a rate 4*(aquarium volume) per hour, plant bed begins to flood.
5(ish) mins: Water level reaches the overflow drain, begins flowing back into fish tank through that. The time this occurs at depends on plant bed:aquarium volume ratio and the pore size of your chosen bed media. Assuming 1:1 and Hydroton, 5 mins seemed about right.
5(ish)-15 mins: Inflow continues through pump, outflow continues through overflow drain (primarily).
15 mins: Pump turns off. Only outlet for water is now the smaller hole at the bottom of the standpipe.
15-25(ish) mins: Bed drains through the bottom outlet hole. Finish time depends on the outlet flow rate, which depends on the hole size, though I can't imagine it'd take any less than double the time required to pump the water in (e.g. 10 mins).
25(ish)-60 mins: Bed is dry, system inactive.
60 mins: Repeat.
If that's right, I think I understand one of the reasons why siphons are considered a more advanced option: You can flood the beds more frequently, with the system only draining when the bed is completely filled. This means that all the water entering the bed has more time to diffuse nutrients throughout your plants, instead of running straight from inflow to outflow (as it does in the system above once the overflow drain is reached). Siphons also allow the system to drain faster to offset the more frequent flooding.
So did I get that all right?? I didn't really know about the drain time on the outlet hole... Is there any sort of time/hole size that people have figured out you should be shooting for there, or is it just kinda whatever works?
Thanks for all the help!!!
My timed flood and drain beds take longer to drain actually. The top several inches actually drain fairly quickly but as the water level falls the speed at which the small drain holes drain the bed will slow as the pressure decreases. So the top part of the bed drains quickly while the bottom part of the bed will keep draining or trickling about till the pump floods the bed again.
Truth is both methods work just fine. The timed method just means you need a bigger pump but you run it less time. The 15/45 timing is not absolute just convenient some people will run 15/30 or 15/15 depending on season, drain rate, water quality etc.
With the timed flood and drain you get your extra aeration happening when the water is overflowing down the top of the stand pipe and the bed drains slowly so the fine suspended solids are more likely to stay trapped in the media.
With siphon flood and drain you get your extra aeration as the water siphons from the grow bed and splashes back into the fish tank. With the fast drain you might get more small suspended solids getting sucked out of the grow bed into the fish tank.
Both methods drain and therefor have extra aeration drawn down into the grow bed. If sending the water into the grow bed at just a single point, the constant flow method will see more sliming issues of the media where the water enters the bed.
With siphon systems you have to make sure flow rates stay pretty constant so the siphons stay balanced and since the pump runs constantly it doesn't get the automatic backflush that happens with timed pumping so cleaning the pump and checking the flows should probably be a weekly thing. With timed flood and drain you have the danger point of the timer failing so daily checks to make sure those mechanical timers keep operating is kinda important. If it fails with the pump on it isn't so bad but if the timer fails to turn the pump on then you could be in trouble if you don't check on things regularly.
Hole size. First, make sure you can pull your stand pipes out to adjust the height and hole size later if you need, and also to make cleaning the holes out later if/when roots grow through them. Next Start with less holes so you can add as you see how long it takes to fill the bed to make sure you don't add too many and make it impossible for you pump to flood your beds in the time allotted. I've been using a 5/32" drill bit for the holes for the bottoms of my stand pipes. In the 300 gallon system I have 4 holes in the bottoms of the stand pipes but that only works because I only flood one bed at a time, if my pump had to flood all beds at once, would need a bigger pump to keep up with that drain rate.
Plan on taking a few hours to test the flood and drain to make sure you get the size/amount of holes right so that you can get the bed to flood in the amount of time you want.
Thank you SO much TCLynx. I'm planning on testing the timing tomorrow, so I'll come back here if anything goes wrong!
I still love my bell-siphons.....
With siphons, the optimum flood and drain time is going to be the one that works. That is largely going to be dictated by the siphon size. If the siphon is working, consistently letting the bed flood and then draining the bed then letting it flood again then I would say whatever that timing is happens to be perfect.
Now if you by some chance manage to set up a system where the cycle is really incredibly long and you find your plants are wilting due to lack of water during the cycle then perhaps you need to speed it up.
If you have a tiny little system and the constant flushing every 3 minutes is driving you nuts perhaps you need to slow it down.