A Complete Scuba Diving O-Ring sizing guide and reference chart. Designed to provide a quick reference and identification of the 12 most common o-ring sizes. This handy little card is perfect for keeping on you while diving, or as a reference when you're at home. It has the 12 most common scuba diving O-ring sizes, as well as their uses, outlined on the back. So whether you're out on a dive and need to check your equipment, or just want to be prepared for the next time you need to replace an o-ring, this card has you covered.