Gases are obtainable packed in cylinders or empty cylinders from a vessel can be refilled for use. The most commonly required gases for marine use are listed below. When ordering, please specify the type of gas required. Charges for cylinder rental or gas refills are applied separately and the guidelines regarding what type of valves are used on the cylinders, how frequently the cylinders must undergo pressure tests, and if cylinders can be exchanged will vary by country and manufacturer.