You may find with your Big Mouth Bubbler that the lid doesn't seem like it fit's, if so please read the below!
An important first point to take from the below if that if there is enough co2 being produced to push the lid up, then the beer is protected from oxygenation by this blanket of co2 and the brew is not suffering for the lid being up or off.
The universal lids do a great job of providing an air tight seal on the Big Mouth Bubblers. In fact due to the larger service area, they almost do too good of a job which is why the lid rises. Some brewers have found that letting the lid dry after sanitising can prevent the lid from rising. Other brewers experience such a vigorous fermentation, that they add some weight to the top of the lid. If at least one ring of the outer gasket is in contact with the Big Mouth Bubbler, your fermenter is sealed. we understand that can cause concern, but this is a similar issue that many brewers experienced with a traditional carboy/bung. If there is enough fermentation to push the lid up, there is enough co2 being created to protect your brew and keep it safe!
Tips to stop the Bubbler lid from popping off during fermentation:
Let the sanitiser air dry to form a better seal between the BMB gasket and the wall of the fermenter (This is especially true for Star San. In fact the instructions on the Star San labels state that all applications should be allowed to air dry. Most brewers overlook that)
Flip the silicone gasket over to put the larger flange on the bottom. The additional surface area helps hold the lid in place
Some brewers do experience such vigorous fermentation that they resort to adding some weight to the lid but the co2 that is being produced at that time would mean that the beer is fully protected
Northern Brewer do not have plans to make any changes to the lids and will continue to educate customers on how this new design behaves.