The problem is that there really wasn't much way around admitting that I used schedule 40 pipe and PVC cement to make an incredibly robust pipe bomb.
Dry ice has some very peculiar behavior when it's pressurized while submerged in water. The chamber will steadily pressurize up to about 30psi, and then (I'm guessing) the dry ice ices up enough (with H2O ice) that it stops sublimating, since ice is actually a relatively good insulator. I've seen the some batches sit at 30psi, clearly with more dry ice in the chamber, for more than a minute before it continues to rise. The problem is that the dry ice seems to shatter its ice shell all at once, causing the chamber to go from 30psi to pegged full-scale in a matter of seconds. Handling that manually with a needle valve takes razor focus and a weak sense of self-preservation.
Clearly I needed something better.
Doing some research, I was pleasantly surprised that adjustable automatic relief valves aren't as expensive as I expected. Grainger PN:6D915 is one such relief valve, which comes with four different springs giving you the pressure ranges 25-50, 50-100, 100-150, and 150-200. Select the proper color-coded spring, insert it in the valve, and tighten the bushing to compress the spring until the valve pops at your desired pressure. Once you have it calibrated, tighten the lock-nut to fix it in place, and presto!
Where it gets clever is that, once the piston moves up, the area of the piston over which the pressure is applied greatly increases. This causes a much larger force, which compresses the spring even farther, and gives the relief valve a very distinctive "closed - wide open - closed" action. This is an example of hysteresis, which is desirable in these types of systems, because you usually prefer these "pops" to a slow leak as you approach the critical pressure. Through trial-and-error, I set my carbonation relief valve to open at 80psi and vent until it reached about 55psi. With 50g of dry ice and a typical 32oz fluid charge, I see two or three pops before the dry ice is used up and the chamber is stable.
Of course, once I had this working, this toy was the choice go-to activity whenever any of my out-of-town buddies happened to swing through Davis. Nerds plus dry ice is seeming like less and less of a good idea every time I do it...
Video of my grape juice mishap with Jeff:
Video of Jacob helping to clean up a week later:
For all that is mighty, do not kill yourself trying this.