I'm a craft beer lover and started gaining weight. In order to be able to enjoy my passion I had to change my diet and I'm now all Paleo... all the time. In order to enjoy an entire growler of fresh ale I must pace myself. Thus I have to keep a growler's contents alive as long as possible. Thus I made growler cap, similar to the ones I use on PET bottles with a gas/CO2 fitting is a necessity. Please note: With no pressure relief valve, you must NOT overfill a glass growler. I always set my gas below 10psi and I would recommend you wrap the growler in a thick towel when adding pressure.