For refills, use distilled water without Baking Soda. That's because the water is being consumed by electrolysis, while the soda stays in the device. Adding a mixture of water+soda will create an imbalance and may result in device overheating and/or fuse blowout.
PRECAUTIONS: There are things you should know here that are not obvious to everybody:
--First of all for safety’s sake, never do it with the electricity on. Turn the ignition switch fully off AND pull the fuse out.
--Second, never fill out new water into an empty HOT JAR. Let it cool first.