Check Valve limits the flow of gas to one direction only. Use a check valve between the bubbler and generator to prevent unwanted pressure build up in the HHO generator, and a safety against a low level bubbler. A check valve, such as used on torches, is recommended. Many homemade ones are being designed for hobbyists to build.
Many are available at welding supply houses and aquarium shops (they are used in fish tanks).
There should always be a check valve install on your HHO generator.