This hack works a lot like the Vacuum Beer Bleeder below with one modification for managing the extra pressure.
Again you are going to need a few fittings:
- Hose barb - McMaster Carr 5454K62 $0.62
- 10-32 Female Hex Coupling - McMaster Carr 5454K71 $0.36
- Check valve - McMaster Carr 7757K41 $3.23
- Instant hose fittings - McMaster Carr 5779K102 $1.78
- 1/8" ID x 1/4" OD vinyl tubing
I know that adds up to more then $4, you will only one of the instant hose fittings. If you haven't see these before the are kinda like ad external hose barb. There is an o-ring inside to seal around the tubing and some little barbs to grab the outside of the tubing. The really cool part is you press on the front of the fitting and it releases the tubing. Kind of like a simple quick disconnect, but you don't need a fitting on the tubing. As for the check valve you only need this if you want to carbonate something and store/transport it. If you only want to quickly carbonate something like a beer sample or water you won't needed the check valve. So you could do this for just over $1 a cap.
So to get things started. You will need to drill a 3/16" hole in the center of the cap. Next thread in one of the hose barbs making sure to include the little gasket they give you. Now to make sure the hose barb stays in place thread a coupling onto the end of the hose bard now sticking thru the inside of the cap. Tighten this being careful not to strip it. Now just add some hose and the check valve if you desire. Use the instant fitting to make the connection to your CO2 regulator.
I find that 30psi and about 1min of violent shaking will carbonate any cold (40F or lower) liquid. Let it sit for another minute still under pressure before you remove the cap. I mostly use this for making soda water and carbonating beer samples.Note: You could use a regular #10-32 nut in place of the coupler just make sure it is brass or stainless.