Source code available at https://github.com/ambrogio-galbusera/gr0g, https://github.com/ambrogio-galbusera/gr0g-ble-android and https://github.com/ambrogio-galbusera/gr0g-ble

After some design iterations (aka catastrophic failures), I finally managed to build a working high-pressure system that can spray an atomized air-water mixture to the plant roots

The most difficult component to build is the air reservoir. I tried with different materials, but the best solution I think is the one I am proposing in this post. I reused a leftover corrugated pipe after the installation of lights along the path in the garden.

First, I removed the valve. On YouTube, there are many videos showing and apparently simple procedure to remove the valve, but I never managed to complete this operation. So I opted for a brute-force approach: I drilled the valve with a 2 mm bit, until the air could flow freely. I drilled an hole for the valve in the corrugated pipe and then made the inner tube slide into the corrugated pipe. To contain the expansion of the inner tube when inflated at both the ends of the corrugated pipe, I used a string for rolling shutters kept in place by some screw and bolts (as visible in the final images)

This air reservoir has been tested up to 0.5 MPa (about 5 bars), but the system will be operated at 3.5 bars. At such a pressure, the air-water mixture creates a fine mist even if I can not measure neither the air speed nor the droplets size.

The air reservoir has been connected to the other components of the high-pressure system shown in the diagram below

The compressor is one of those cheap tool designed to be used in case of emergency to inflate flat tires. It works with 12 volts, which is good because I want the Gr0G to be entirely self-contained and to require only a low-voltage power supply in order to be operated
The pressure switch can be adjusted to open the circuit when it reaches a pressure threshold that can be adjusted from 0.1 MPa to 10 MPa. It has 1/4" male adapter
The plastic tee was included in the atomizers kit (more about atomizers in the next post)
The solenoid valve releases the compressed air. This is commanded by the Gertbot opendrain output
The second plastic tee, which sends compressed air to cartridge with nutrients (more details about this in the next topic, thanks to Gene Breniman for inspiring this idea) to create a counter-pressure and improve suction
The ejector, which sucks nutrients from the cartridge and mix them with compressed air

So, the complete shopping for the high-pressure system is

8 fitting from 1/4" male to 6 mm pipe
1 1/4" female tee
2 6mm female plastic tee
1 pressure switch, 1/4" adjustable from 0.1 MPa to 10Mpa
1 solenoid valve 1/4" female on both sides
1 vacuum generator, 1/4" female
2 Italia valve-to-Dunlop valve adapter
1 Schraber-valve-to-Dunlop valve adapter
1 cover female 1/4"
1 10" inner tube with Italia valve

The compressor hose is designed to be connected to a Schraber valve, so I had to build a simple adapter with the components shown in the picture below

The board housing the high-pressure system is shown in picture below

This is the high-pressure system installed in the Gr0G box

On the left, there are the cooler and the dehumidifier and the attachment points for the control board. On the same panel, there is also one of the boxes where the plants will grow