What size is a typical pony bottle for deep diving?

A typical pony bottle for deep diving is usually in the range of 2-4 liters, or approximately 12-30 cubic feet. This size strikes a balance between being compact enough to carry while providing a sufficient amount of gas for an emergency. During deep dives, having a pony bottle can serve as an additional air supply in the event of a primary tank failure or other emergencies.

The volume of 2-4 liters allows divers to have enough reserve gas to ascend safely while also ensuring that the pony bottle remains manageable and the added weight does not hinder diving mobility. This is especially important in deep diving scenarios, where managing gas supply and maintaining safety are critical.

Other options tend to provide either insufficient volume to be useful during emergencies or may exceed practical limits for size and weight, making them less commonly used in typical deep diving scenarios.