Ok, so I programmed the arduino to fire the injector 20 times a second and hold it open 80 ms each pulse. Wired up the pump, and connected the hydraulics with surgical tubing (and zip ties, after the hoses popped off the first time) =) Using regular water for now.

Latex tubing expands in such a way that it provides approximately constant pressure, apparently. (people use variations of it to power homemade cps supersoakers).

#1 Wow, the clicking from the relay I bought is annoying... it's a lot louder than the injector, which is quiet.

#2 It basically works.... fired it up and the injector pulses water at what appears to be the programmed rate. Dispensing volume and pressure unknown, but it's pretty quick... maybe approaching the rating of the injector, which is 300cc per minute.

#3 Need to solve pressure regulation before it makes sense to do any measuring... the pump I bought is adding pressure to the system faster than it leaves the injector, and that's with an undersized power supply. I might have gotten a pump that's too beefy (500 ml/ minute and 40psi). Clearly need to get some kind of pressure sensor, and shut off the pump when pressure is above target level, and probably control the current too. The latex tubing can only handle so much volume before it expands (and that's destructive, it can only handle that a few times).

#4 I didn't have the right arduino pin cables, so I jury rigged with bits of wire and alligator clips. It works, but if water leaks it's going to short out and potentially electrocute me. Need to get the right bits and bobs and re-arrange the parts so as to minimize mixing water and electricity.

#5 The jets from the injector come out with a good bit of force. Looks like some of the nozzles on the used injector I got are clogged... will need to get some brake cleaner and try to salvage the clogged ones.

But yeah, it basically works, and it's pretty clear that it will be able to adjust dispensing rate to match a given container. (above some minimum drop size, which depends on the pressure, and physical characteristics of the injector).

Yay, fun!