Valves
North American valves have a safety feature called a burst disk. That disk, located behind the outlet is designed to release overpressure inside the tank in case of an fire of the tank being left in intense heat. That safety feature can be a potential problem with failing burst disk releasing all the breathing gas at once, specially if no isolator is used. Some diver plug the burst disk completely or use a higher burst pressure disk in order to avoid that potential problem. Other divers use valves made in Europe, where burst disks are unknown.
Knobs
Valves are the gateway from the high pressure gas inside the tank to the first stage and should be treated as a piece of life support equipment. While most valves come with soft rubber valve knobs some other materials are used such as hard plastic or even metal knobs. The advantage of the softer rubber knobs is that hey do not have the potential of shattering upon impact to a wreck wall or cave ceiling but absorb some of the shock and stay in place. Rubber knobs have a much better grip on almost any surface and with that the potential roll shut of a valve is much higher when diving in overhead environments such as caves or wrecks, and after each contact it is imperative to check the valve and make sure it remains in the fully open position. Metal knobs upon impact can bent and make it impossible to turn the valve and or break off the stem due to the inability to absorb some of the shock. Metal knobs and hard plastic knobs do not have as much grip and somewhat reduce the roll shut scenario but have the other above mentioned potential disadvantages.
K – Valve
The regular K-valve is the simple on – off valve with a single outlet to attach a single first stage to it. While these valves are simple and common around the world they do not offer the option of attaching a second first stage to it, and with it do not have the redundancy of dual outlet valves such as the Y or H-valve. K-valves are no suitable for cold water diving, cavern, cave or wreck diving applications.
H and Y – Valve
The H or Y-valve has two outlets to accommodate two first stages. On either first stage is a second stage attached and both valves are open at all times. If a free flow or complete regulator failure occurs the diver is in the position to self rescue him or herself in order to return safely to the surface. Cold water or ice diving with its potential for regulator freeze up, cavern or cave diving including wreck diving with overhead environments can not be dived without the use of a dual outlet valve for redundancy.
Manifold
The manifold was designed to access two tanks with one regulator two avoid the necessity of changing regulator regularly and the potential of a diver forgetting the change of regulator with the result of the diver having one tank empty and the other full, loosing all his or her redundancy in the breathing gas supply. The first manifold had only one outlet, allowing only one first stage to be attached, not allowing any redundancy in regulators and air supply.
In most double tank dual valve manifold systems, the left valve turns off in the direction of travel (counterclockwise) and in case the valve handle is bumped during the dive against the cave or wreck ceiling it is possible that the forward momentum could shut the gas supply off, leaving the regulator without gas supply. Every contact between any object and the tank valves must be investigated and the fully opened valve position must be assured.
The breathing gas delivery system features a dual outlet valve with two first stages and two second stages. One first stage has to have a 7 foot / 2 meter hose, attached to the right post. One SPG on the left post and one inflator hose on the right post are needed and are divided between the two first stages minimizing a situation both ( SPG and inflator ) are lost at the same time if that regulator had to be shut down. Common sense tells us in event of a emergency requiring a regulator shutdown the dive should be terminated. If the divers are adhering to the rule of thirds, there will be enough gas to return to the surface and an extra pressure gauge is not needed.
200 Bar and 300 Bar threads are in common use the 300 Bar thread is coming from a development in Europe where the trend and tendency is to smaller tanks with a higher pressure while in North America the trend and tendency is to larger, high volume and low pressure tanks which are often over filled. Many diver feel that the longer 300 Bar connection and thread is giving a better seat and connection, which is not necessary until the tanks are filled to 300 Bars.
Isolator or not
The isolator valve, located in the middle of the manifold enables the diver to isolate the two tanks in case of an catastrophic failure such as a burst disk failure or a tank neck / valve o-ring. In case of such an event the isolator valve is to be closed, the dive to be terminated and the diver is to breathe as long as possible form the leaking and depleting air supply, then change over to the isolated remaining tank and if the breathing gas supply is not sufficient ultimately starts sharing breathing gas via the long hose with one of his dive partners. If no isolator manifold is used the complete breathing gas supply will be lost and gas sharing has to be commenced at once, denying the diver self rescue capabilities.
The way an isolator valve can safe our life it can be a potential risk when closed in moment it should be open. During tank filling or breathing gas blending operations the isolator has to be open not to only half fill the double tanks but to avoid the creation of toxic breathing gases when filling oxygen and or helium into only one tank and the air for the blending process into the other. When the pressure is checked a full tank could be the result but breathing could be commenced of the wrong tank with the hypoxic or hyperoxic breathing mixture.
During or before a dive the isolator could be closed involuntarily or just be forgotten to be opened again during the pre-dive valve shut down drill. Such a closed isolator can lead, and had led to deadly accidents. If a diver closed his isolator he will breathe only from his or her right tank. The SPG is not connected to the right post tank but the left post regulator, and will not change its pressure during the dive. This is a very serious situation and the diver should realize that he has not an excellent day on air consumption, but has forgotten to open the isolator and is not using any gas from the left tank. On occasions where divers select to have the SPG connected to the same tank as the breathing regulator the tank pressure may fall to fast, not because the breathing rate is extremely bad this day but because the isolator is closed. Needless to say one must check his SPG often and take the right conclusions and actions from the information collected, and a confirmation of an completely open isolator before entering the water can avoid potential problems.