An airlock is used to allow the co2 to escape without allowing oxygen, flies or any other unwanteds in.
A hydrometer reads the specific gravity of a liquid, which is the only sure fire way (other then other SG instruments) to know the alcohol percentage as well as when fermentation is complete.
Do you have it sealed airtight? I am really confused what your setup is. You must let the co2 escape in fermentation. If you are wanting carbonation, you still let the co2 out and fermentation to complete. When fermentation is complete, you add a bit more sugar to it, bottle away in a pressure proof container, such as a beer bottle or champagne bottles.
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