It is the configuration of gas supply equipment, perhaps more than any other factor, that differentiates recreational diving equipment from cave diving equipment.
Recreational divers typically use a small single cylinder, connected to a regulator consisting of a single first stage with a yoke connector, a primary second stage on a 30- to 33-inch hose, an alternate air source second stage on a slightly longer hose and a high-pressure hose connected to some sort of pressure gauge and instrument console. In some instances, the alternate air source second stage will be integrated with the BC low pressure inflator, reducing the total number of hoses from four to three. Ideally, items such as alternate-air-source (“octopus”) second stages and consoles should be clipped of to the wearer’s BC. Unfortunately, they are all too often allowed to dangly freely, where they can be damaged by — and cause damage to — the environment.
This configuration has proven perfectly adequate for recreational diving and has been the standard for over 20 years. However, in terms of the needs of cave divers, it fall short in several areas.
The standard cave diving gas supply consists of items designed to overcome these limitations, and includes:
The greatest vulnerability of this configuration is that the manifold is subject to something known as "left-hand roll off." This is where contact with the cave ceiling can result in the left-hand manifold valve being turned off accidentally.
It is for this reason that the primary regulator, with its seven-foot hose, is mounted on the right. In a gas-sharing situation, should the donor experience a left-hand roll off, he or she will know instantly (as that is the regulator he or she is breathing from), and will be able to rectify the situation simply by reaching back and turning the gas back on. In contrast, were the recipient to be breathing from a regulator attached to the left-hand post and a roll off take place, he or she might drown before being able to communicate to the donor that a problem had occurred.
The most common way in which cave divers manage their seven-foot, primary second stage hoses is to route the hose down and underneath their dive light’s battery canister, then up across the chest, around the back of the neck, and into their mouths. This not only facilitates the rapid deployment of this hose in an emergency, it also provides a way in which the wearer can store his or her long hose without needing the assistance of another diver.