The Replacement Columns for the Inogen At Home unit are removable for quick and easy service. They are sold and used as a pair. Each of the columns contains the sieve beds. The sieve beds are responsible for turning normal, ambient air into medical-grade oxygen. Depending on how often you use your concentrator and your climate, your Replacement Columns will need to be replaced every 18 months to 2 years. The Inogen At Home unit will let you know when it is time for a replacement.