I also decided to redesign the boards to be less than 100x50mm in size so that they can be easily manufactured in small batch runs from PCB manufacturers. 100x100mm panel size seems to be a universal cutoff for price variation(at least when it comes to hobby type boards). I utilized this avenue to add M3 mounting holes and standardize the berg stick connectors better.
After using the board for a bit, I realized that most applications call for less than three air channels and I was using the fourth channel almost exclusively for IO. So I put two buttons and a few neopixels on the master board using up the pins from the fourth channel. Now the slave boards can only dock to the bottom or sides of the master boards, and not to the top. The top has easily accessible power connection, USB, buttons and neopixels. I also added LEDs to indicate the state of the pumps and valves. And I put the pumps on analogWrite pins for the atmega328. This will allow me to easily control the pumping speed.
As an additional fun challenge, I decided to only use curved angles in the PCB traces. I think the PCBs came out looking rather sharp. On the practical side, since I manufacture PCBs with a CNC the lack of sharp corners translates to quieter, less jerky operation while manufacturing.
Working with Balloons - Case for flow meter
Programmable Air gets feedback from a pressure sensor. This is because I was following an unwritten intuition that the pressure inside an inflatable is proportional to the volume of air pumped in. Boy! is that intuition wrong! For rubber inflatables like latex balloons or thin walled silicone the pressure initially increases with the air flow but then starts decreasing. The balloon is more of a flow driven device than a pressure driven device. To control something like this a flow meter makes much more sense. The initial case against a flow meter was because of leaks. It makes sense to have a flow sensor to use in conjunction with a pressure sensor as well. I'm on the lookout for a cheap flow sensor.
Result: Flow sensors are too expensive! Like 80$ for a cheap one. Not even gonna think about using it for a while.