NOTE: These replacement columns are ONLY compatible with Inogen One G3 systems with a Flow setting range between 1-5. 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 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. 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 as you would with other portable concentrators! Sieve beds filter out the nitrogen from the air coming into the concentrator Air gets concentrated into oxygen after passing through the second filter-Malfunctioning sieve beds will not give the oxygen dosage needed to treat COPD or other diseases that affect the lungs Clear step-by-step instructions enable easy switching out of the column pair without tools-Cost efficient Dimensions: 2″ W x 2″ L x 5″ H