A Vessi owner recently contacted us asking for our take and advice on best carbonation practices with the Vessi Fermentor & Dispenser. Unlike many (most) other fermenters available to homebrewers, the Vessi gives the brewer tremendous flexibility and tools to fine tune the fermentation and conditioning process. So we thought we would share our advice to this user in case it can help others as well.
With the Vessi, you have two options, especially if you transfer the beer after fermentation is done and bottle it or keg it, as opposed to serving it from the vessel itself. We almost always transfer the beer to a keg for serving after fermentation is complete.
OK, option one: leave the Vessi pressure relief valve loose so all CO2 produced during fermentation goes out through the airlock and the beer remains flat during the fermentation process. Then, once all fermentation and clarification is done, you can either transfer flat beer to a keg and force carbonate there, transfer flat beer to bottles with corn sugar and carbonate that way (we do NOT recommend that), or you can use the CO2 bottle in the Vessi and force carbonate for a few days so you have carbonated beer.
Option 2 is what we typically do: We naturally carbonate in the Vessi while fermentation is taking place. After you put the chilled wort into the Vessi on brew day, add the yeast and close the Vessi up, close the pressure relief valve all the way and use the Vessi's CO2 bottle and regulator to pressurize the vessel to about 20 psi. Then, very slowly, start opening the pressure relief valve till you start hearing gas escape from the valve. Finely adjust the valve so that no more gas escapes with about 17 psi. At that point, you know the valve is set to keep pressure up to 17 psi and then release excess gas if more CO2 pressure builds during fermentation. You then pull the valve's ring on top of it to let all the pressure out, down to a zero reading on the pressure gauge and attach the airlock. Once fermentation starts, you will see the pressure gauge start marching upwards and eventually it will hit that 17 psi threshold, at which point you will also start seeing bubbles in the air lock.
This way, the beer continually carbonates while it's fermenting, using the naturally generated CO2 from fermentation and not using up your CO2 bottles external CO2.
One thing you need to get into, if you are not already familiar, is the concept of "dissolved CO2 vs. temp and pressure". Basically, beer absorbs CO2 (i.e. "carbonates") at different levels depending on temperature. Beer at room temperature holds less dissolved CO2, while it can carbonate a lot more at lower temps. So in the scenario above, when you start dropping the temp in the Vessi post-fermentation, you will also start seeing the pressure drop, since head space CO2 will start getting absorbed into the beer as the beer cools. Eventually, it should stabilize at about 6-7 psi at 35-38F temperatures, which should provide you the right level of carbonation for serving.
Of course, all these numbers will depend on how much carbonation you want and what carbonation levels are called for by the different beer styles. For example, British ale styles call for less carbonation (1.5 - 2 volumes of dissolved CO2 per beer volume) than, say, American lagers (3 volumes), and many Belgian styles call for even more (3.5+ volumes).
The best thing you can do is go by a beer carbonation temperature-pressure chart, like this, and shoot for being in the green zone when you reach equilibrium at serving temps. The numbers across the top is pressure at equilibrium in psi. The numbers down the first column are temperatures in degrees Fahrenheit. The numbers inside the table are dissolved CO2 volumes per volume of beer.
Hope this helps! Shoot us a message with any questions or comments you may have, and as always, happy homebrewing!