For Inogen One G3 System Flow Setting 1-5: These replacement columns are ONLY compatible with Inogen One G3 systems with a FLOW SETTING range between 1-5. If your flow setting can be adjusted to Setting 5, this option is for you. Two columns are included in each order. So if you place one order, you will receive two columns. 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. Sold and used as a pair, the columns contain the sieve beds that convert normal air with lower levels of oxygen concentration to supply a high concentration of 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!