Carbonator Cap
 

I've recently picked one of these up but find it hard to use?
Here's what I've been doing.

Leaving 3" of headroom
Gassing at 30 psi and shaking like a mad man
Keeping my beer super cold

I cracked it open and found it was flat and that was after 2 days of adding after the bottle softened up.

What am I doing wrong here? I practiced on some Gatorade and failed miserably.

I have a DIY cap and CO2 cartridge cornie charger but these are my steps:

leave headroom equal to the original soda
squeeze the bottle to force the air out and cap
chill beer
gas, shake, gas, shake, gas, shake, gas, shake, gas
drink within 24 hours

Are you forcing the air out before adding gas?
Are you shaking and adding gas several times as the bottle softens?

http://www.truetex.com/carbonation.htm

Yes, what he said. You have to push the air out first. Cold liquid absorbs more gas. 30 psi is enough for soda, way too much for beer.

i think the point is that you probably need to keep adding co2. if you fill the headspace to 30 psi that goes into solution pretty quickly and the new equilibrium pressure is much lower, so you need to add a few more bursts, like the post above says: gas, shake, repeat several times, you might have better luck. but if using multiple high pressure bursts you are guessing at how many additions it will take to get the final equilibrium pressure within a normal range for beer

"squeeze the bottle to force the air out and cap" is key

I'm betting on not enough gas ups.