The 25mm siphon still relevant but since more parts needed for it and the sheer size make it expensive to make and ship.
Few of my earlier growbed are still having the 25mm siphon installed and since I'm phasing out this size, it will be a good time for me to check exactly its water inflow requirements. This to document for future reference or if anyone interested in making one then they will know what needed to be done.
Water inflow is the amount of water going into the growbed, it is not the pump rating. This inflow will depend on pump rating and how high the water needed to be transported up into the growbed. Water inflow means water "On Tap" that is going into the growbed for this test purpose and not the pump rating.
I uses a Rotameter, which is a very simple float type flow meter to check the water inflow to the growbed. Though it is not that accurate as an electronic meter, but for a simple experiment suffice.
I am testing four siphon and will get the average water inflow to trigger and stop the siphon. Setup for the test is like photo above, I am using an old pump that only capable of supplying inflow of up to 1200 liter/hour. It is sufficient to do the test with this capacity.
Those four siphon tested started at a minimum water inflow of 100, 250, 100 and 200 liter per hour due to their different outlet configuration and their condition, which already in operation for almost two and a half years.
The average for above is (100 + 250 + 100 + 200) / 4 = 162.5 liter per hour
The siphon able to cut-off at a maximum inflow of 650, 700, 800, 1000 liter per hour, again due to their condition and outlet configuration.
The average for above is (600 + 700 + 800 + 1000) / 4 = 775 liter per hour.
To get the sweet spot on what is the inflow for this siphon, it will be the mean of the two average at (162.5 + 775) / 2 = 468.75 liter per hour
I recommend the 25 mm or 1" siphon with a funnel uses inflow of about 450 ~ 500 liter per hour. From above test anything above 250 liter per hour will do, but we will need to give some margin for pump deterioration. 450 to 500 liter per hour inflow at the tap should give good result.
To know how much water flowing into the growbed you have to measure the water head, ie the height water transported up as measures from tank water level. You then need to check the pump rating as given on the pump write up as example below.
Table above, if your growbed inlet is one meter above the water level, then the recommended pump for the 25 mm siphon should be the AS-1000 pump. It is not exactly at the 450 to 500 range but it's more than the minimum inflow as tested above. It is okay to use a bigger pump like the AS-2000 in the diagram above, you will then need to divert excess water away from the growbed.
The above finding serve as a rough guide, and by no means define exactly the size or type of pump needed. Your installation may differ but as a ballpark figure, the above estimation will do.