This is a great idea, CA99. As you note, the implementation could be done more cleanly. Back in 2009 I started making plans for a water gun with the same overarching goal, so I'll share what I found. (This was never built, in part due to a lack of time, but also because I recall the valves needed were too expensive for me at the time.)
I don't remember all of the details, but here's the pneumatic circuit and my attempt to decode how it works 5 years after drawing it:
This uses pneumatic circuit symbols
, and some of them appear to be non-standard. The rectangles are complex valves. The flow paths through the valves are represented by the arrows. When the valve on the left is pressed (the button on top means it's actuated by hand), you can replace the bottom square with the top square. The flow paths will change. The valve on the right is similar, except that it's actuated by compressed gas from the left side of the piston. There's a needle valve on the feed to the actuator to add some delay for that.
The top is an pneumatic cylinder, which is something like this:
Basically, when you pressurize the left side, the piston and rod moves to the right, etc. The rod will be attached to a piston pushing water with the standard check valve setup. The water chamber is not in the drawing I have, unfortunately. Essentially, the circuit here shows a way to automate a piston pumper with a separate reservoir.
If that didn't confuse you enough, I'll try to explain the circuit's operation.
The circuit actuates a piston that pushes water out of the nozzle or pulls it in to the PC. In mode 1 (shown in the drawing), the compressed gas source (the triangle; by "compressed gas source" I mean some sort of regulated gas system) leads to a plug (the x). The piston will be positioned to the right, such that water has been pulled into the PC. When the button/trigger is pushed, mode 2 begins. The left side of the pneumatic cylinder is exhausted to the atmosphere. The compressed gas source leads directly to the right side of the piston, pushing the piston to the left and thus firing water from the PC. When the trigger is released, mode 1 starts again. The left side of the pneumatic cylinder starts to fill with gas from the right side. At a certain point, the right valve is tripped by the pressure actuated switch (the needle valve controls the delay) and the right side is exhausted to the atmosphere (the dot means leads to the open air). This allows the left side to move the piston fully to the right.
As for specific parts, I have a spreadsheet that lists the following valves: Clippard's MNV-1, Clippard's MMA-41NAS, and Clippard's FV-5P. I'm not sure if these valves have high enough flow rates, but I imagine they do.
One problem with this design, that you are aware of, is the required check valve in the nozzle. This'll add a lot of turbulence to the flow. I think using something like a lightweight ball kept in place with a small elastic band as a check valve would be one way to reduce the turbulence.
Another problem is tap shots. I'm not entirely sure if they'll waste a ton of air or what. I figure getting tap shots to work well might require getting the needle valve timing just right and/or modifying the design a bit. I did try to make this design avoid wasting air if possible, which is why the air leftover after pushing water out the nozzle is used to refill the chamber.
I hope all of this isn't too confusing. I remember being very confused when I drew up this design myself, but I think I understand it now.
CA99 wrote:There is also another issue I'm missing: I'm not sure how quickly a regulator can work. This design assumes near-instant air-filling of the chamber, which may also be a problem.
Most regulators list a flow rate that you can use to get a rough estimate of the fill time.
To do this with good accuracy would require some more complicated math. For those interested in this route, I can suggest a few books. The Analysis and Design of Pneumatic Systems
is a pretty good book on the subject, though I'd use the ISO valve equations from a book like Pneumatic Drives
(I made this post in a rush, so forgive any egregious writing errors.)