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Gas Supply

It is the configuration of gas supply equipment, perhaps more than any other factor, that differentiates recreational diving equipment from cave diving equipment.

Gas Supply

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.

  • Single cylinders provide insufficient range and reserve gas for cave diving.
  • A single tank valve and regulator first stage provide insufficient redundancy in case of valve or regulator failure.
  • Even a “long” 36- to 40-inch octopus hose is nowhere near long enough when cave divers must share gas while passing single file through restricted passageways.

The standard cave diving gas supply consists of items designed to overcome these limitations, and includes:

  • Double cylinders, ranging in size from double aluminum 80s (the norm for divers exploring caves in Mexico’s Yucatan Peninsula) to double steel 95s and 104s (commonly used among cave divers in north-central Florida, where caves are deeper, and gas consumption rates higher).
  • A dual-orifice, isolation manifold. These permit the use of separate first stages for each second stage. Should a free flow occur, the gas flow to the affected regulator can be shut off without affecting the flow to the remaining regulator. Additionally, should manifold damage occur, the isolation valve allows the system to be divided into what are, essentially, two separate cylinders. The use of DIN fittings helps ensure a stronger connection and avoids the damage that could be caused by yoke screws impacting the cave ceiling.
  • A primary regulator, consisting of a first and second stage connected by a seven-foot low-pressure hose. This facilitates single file gas sharing through narrow passageways. The primary first stage will also generally have a short, low-pressure inflator hose that connects to the diver’s BC air cell.
  • A secondary regulator, consisting of a first and second stage connected by a 22- to 28-inch low-pressure hose. Additionally, this second stage will have a neck strap made from lightweight bungee cord. It is designed to keep this back-up second stage close to the diver’s mouth, so that he or she can switch to it quickly, after having donated his or her primary second stage, with its long hose, to an out-of-gas diver. The secondary regulator first stage will also typically have a low-pressure inflator hose for the diver’s dry suit, and a submersible pressure gauge on a short, 22-inch high-pressure hose (this length allows the SPG to be clipped off to the D-ring on the diver’s waist strap without there being a lot of excess hose).
Long Hose Routing

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.

Instrumentation »




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