Blow off cap for newer style Sherwood CO2 valves. If your cap has blown your tank may have been overfilled or exposed to too much heat. The Blow-off cap is a safety device to keep your tank from blowing up. It must be replaced by a certified compressed gas technician. Do not tamper with or try to modify this device. Note: If purchased by itself a $6.50 shipping charge will be added for Priority mailing. If ordered with other item(s) no shipping will be added. This is to save you on shipping cost on such a small item.