Mine fill in about 8 or 9 and drain in about 30. I don't know that there is a right or wrong way or whether any real study has been done on the subject, although no doubt there are many opinions.
440 gal tank, which is generally not full. Six 100 gal media beds.
A 14 minute cycle is just about perfect
I agree with Jonathan, 14 minutes for F&D is right on. My understanding is that the flood brings in the nutrients and moisture, then the drain draws in the oxygen. 15 minutes may be ideal, but I would think a wider range is acceptable.
Why, if you don't mind? My thinking was that I wanted several beds draining at the same time, all of them aerating the water. Too, it was a way of increasing total water volume and therefore stability, according to conventional wisdom. Slow drain pulls in just as much oxygen to the media as fast drain. I can easily drain faster by using a standpipe with a bigger hole so I'm curious as to why 14 minutes is better than some other length of time. Almost all of my beds are either draining or filling at any given time and the system aerates during both the fill and the drain. Actually, my beds begin to drain as soon as they begin to fill but they fill faster than they drain so eventually the bed is full. If a shorter cycle is better I'm open to it.
Jonathan Kadish said:
A 14 minute cycle is just about perfect
I really don't think anyone has done research to show 15 minutes provides the best health for your plants, it is just conventional wisdom. The length of time is not about the fish, it is more about keeping the environment in the root zone in good shape so the bacteria, worms (if you have em) and roots are all well oxygenated. If you flood for much longer periods you would start to see anaerobic zones where you can have bacteria that decompose the fish waste and create gasses that are not good for your system.
I'm gonna go out on a limb here...but i'm thinking that the whole 15 minute thing has much more to do with the fact that most timers are built with 15 minute increments on their dials than any scientific research. This 15 minute number (like how grow beds need be 12" not more not less) was probably picked up on and folks ran with it, and it is now carved in AP stone. Be it timers or siphons. So don't mess with it ;)
I could be wrong though.
@Jonathan, don't people run constant flood, constant flow systems without those problems you mentioned? I guess adequate oxygenation would be key...
Atmospheric pressure drives the drain. I have to laugh when I hear people say the the water sucks down air into the growbed when it drains. That's nonsense. The weight of the atmosphere pushes down to the degree you let it.
What we are trying to do with FD is maximum root exposure to air without drying theroot system or the BB colony. Some hydro growers flood and drain six times a day. Are they right or wrong? It doesnt really matter. I flood and drain twice per hour without timers. Why? Because of the evaporation and drying caused by Texas heat on lava rocks. My personal opinion of someone that says 15 minutes as a rule tells me that they really dont know. Use your judgement. The less you water the better the root system will be for absorbing moisture. Check out some aeroponics on youtube to see how well developed a root system can actually become when it's not constantly saturated.
I've had the roots of different plants (including ones that supposedly don't like being in water) totally submerged 100% of the time, and the roots were just massive and the plants yielded very well in terms of fruit. But then I had decent (if not a bit over sized) air pump and a number of good quality medium pore air diffusers churning the water a good deal. I believe a well oxygenated environment for the roots cannot be overstated, whether flood and drain, constant flood, DWC, or whatever form your hydroponic sub-system may take.
What seems nice with FD is that you get away with using less air pumps/stones. It would seem that as long as you take your particular environment into consideration (flood/drain cycles in an Alaskan summer might not need be the same as in Phoenix in late July...or how people have used coir as their media in the hot tropics, where that same material would probably suck in a temperate zone climate) and give the plant roots as much time 'for oxygen' without drying them out, that may be 'best'. Those times can obviously differ according to each individuals set of circumstances. (Climate, temps, choice of media, relative air humidity, system size ratios etc..)
The flood/drain cycles in AP are probably more frequent than in in hydro due to needing to move around the entire volume of your fish tank at least once an hour.
Who said AP water needs to be turned over once per hour?