The story of chemicals should be used and then picking out a new use, but if it breaks, the other airlines to use because of the individual chemicals have expiration date.
The chemical Should be withdrawn gradually exhausted itself. But if there is also an offline split out to another because each substance has an expiration date.
About chemicals should run out and then picked the new. But if there are a lot of gave another line used because chemicals, each with an expiration date.