It’s a good question! The reason for those specific dimensions is that the facility that we designed the system with uses Montel mobile racking and the dimensions of each bay is 98"x42". Larger trays - like a standard 4’x8’ or 4’x’4’ - would over hang the racks and collide with the trays either side when the racking is moved together.
We’re working with several other facilities on their build out and they too are using the 42" wide Montel racking - which is a fantastic system, by the way. You should definitely check it out… http://www.montel.com/en/markets/agriculture-mobile-shelving-storage
You effectively double your square footage when you use the vertical space.
The AQUAvalve is a mechanical “float” that sits in the base of the tray (either a flat tray or one of our pre-moulded modules) and is connected to a reservoir using tubing. It will fill the tray with water or nutrient solution to a depth of 3/4" then stop the supply and will not reopen until the plants have used all of the water in the tray. It basically uses a vacuum chamber to lock an internal float in place to prevent the liquid from entering the tray until it has been completely emptied.
Here is an animation that explains how it operates (it’s a little old and clunky, but it explains it pretty well) https://youtu.be/K1Dn-1V8Za4
Hope this helps.