The first came through a deliberate approach with a tested methodology, the second came because I recognised the first as already existing. (It made me think of another problem, leading me to find another solution, and that looked a bit like a deer scarer)
It's a deer scarer.
Actually the first solution is also a solution for some other problem, so I think I'll save it for solving that.
A deer scarer works by having a tube pivoting around a point roughly in the centre like a set of scales. One end is blocked and made slightly heavier. The other end is made slightly longer. Everything is set up in such a way as to make the thing sit with the open end up.
If you add a trickle of water to the open end, it eventually fills the tube. when it does it reaches a point where the water in the longer end overcomes the extra weight in the heavy end, so it tips.
|Thanks for the use of the pic Cassiopeia_sweet|
Once the water is dumped, the heavy end thumps down onto a stone making a pleasant (yet apparently scary to deer) sound, that eases you into the now.
My version would be quiet.
My version would also be less graceful. If you made it out of 200mm PVC tubing you could dump a bucket full of water each time in a short compact unit. You could also place it somewhere else and pipe the sudden dump of water to your grow bed.
To make a grow bed flood for most of an hour, drain suddenly, then flood again, this might work.
Create a bell siphon that has a standpipe too large for your pump's flow. This will mean the siphon wont trigger, but the standpipe will allow water to circulate back to the fish tank or sump just by normal overflow through the standpipe. In other words, it will act as a constant flood grow bed.
Next mount a deer scarer above the siphon and divert a trickle of water from your pump to it, so that it tips once an hour (add a tap on the outlet for adjustment).
The sudden rush of water into the siphon area will be enough to decisively trigger your oversized siphon, draining all the water in mere moments to everyone's delight. So now we have a "mostly flood, and some drain" system. We also have an interesting, steam punk, bit of low tech kit moving around, rather than the sedentary vista provided by a constant flood grow bed.
This could be configured so that your grow bed drained every 12 hours or any desired time, making sure you had no dead areas of rancid mush hiding in the corners of your (almost) constant-flood grow bed.
Thanks for the heads up Nom. ]