Inogen At Home Replacement Column Pair provide a quick and easy way to service the Inogen At Home without having to send in the product for repair. Used as a pair, the columns contain the sieve beds that convert normal air to medical grade oxygen. These sieve beds wear down after prolonged use. Inogen recommends replacing the columns every 18 months to two years. In certain operating conditions, earlier replacement may be required. The Inogen At Home will let the user know when it is time for replacement.