You should first understand the difference between "quantity decreases until it indicates zero" and "quantity decreases to zero".
Then you should start to lock at the pictures as well. (e.g. standpipe)
Sorry, but did you read the headline of this "Question" forum? I can quote it for you:
If you are a professional pilot or your work involves professional aviation please use this forum for questions. Enthusiasts, please use the 'Spectators Balcony' forum.