The removable metal columns provide a quick and easy way to service your Inogen One G3 without having to send in the product for repair. This product is for Inogen One G3 concentrators with flow settings between 1 through 4. Sold and 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 One G3 will let you know when it is time for replacement. Clear step-by-step instructions enable you to easily switch out the column pair without tools - without having to send the Inogen One G3 in for repair like you would with other portable concentrators. (Flow Settings 1-4)