Diving With Independent Doubles
Independent cylinders were once the standard uniform for any cave diver. They were commonplace in Florida caves and New Jersey wrecks and anywhere the demands of diving required a redundant gas supply. As the years went by innovation and adaptation changed the way divers set up their double tanks. The manifold with isolator became standard equipment on most sets of doubles and could be seen in droves from the caves of the Yucatan to the quarries of Ohio. Many of the divers learning to use doubles have never seen independent cylinders doubled up, as many new tech divers were taught that they were somehow substandard or even dangerous. I had one former solo student tell me as he stood on the Ogden Point breakwater about to plunge, a group of "tech" divers approached him and told him he was going to die diving such a contraption.
So lets look at this system realistically. An independent system allows you to carry two separate gas supplies with independent regulators. No single failure can render you without breathing gas. Unlike the manifold that could possibly suffer a catastrophic loss of gas should one of a number of o-rings fail or mechanical damage to the crossbar isolator prevent any of the o-rings from sealing. The way people use the isolator as a carrying handle I am surprised this hasn't happened more frequently but as such it is an extremely rare failure. Independents don't require any immediate action on the part of the diver should a catastrophic failure occur. Lets say a diver is penetrating a tight space in a cave or a wreck. They encounter a sharp object that causes a tear in an LP hose. A huge cloud of bubbles ensues and the percolation from the ceiling turns the water to a silty mess. To top it off the diver is not able to reach the manifold due to the restricted space they are in. I have personally been in this type of situation. Their only choice is to back out of the hole being careful to locate the guideline and find an area where they have enough freedom of movement to save their remaining gas by conducting a valve drill. The amount of remaining gas will depend on how quickly they were able to perform these steps but there is a good chance they may not be able to save sufficient gas for a safe exit. Also if the failure occurs on the left side of a conventional Hogarthian rig the diver will have no idea how much gas is remaining to get them safely out because they will have had to isolate their only SPG. The independent diver merely locates the guideline and gets themselves to a safer area where they can shut off the offending tank, or if the situation makes it difficult ignore it until the tank runs out. Either way he knows he has enough gas to exit the overhead environment he is in. This is due to simple gas management.
The independent doubles diver uses the rule of thirds, just like the manifold diver but slightly different. Each independent tank has to have its own SPG and the diver should start breathing off the right post and switch to the left post at 2/3rds tank pressure. The diver then makes a switch of regulators and continues the penetration on left regulator. When left tank reaches 2/3rds pressure the diver turns the dive and begins the exit. At this point he continues to breathe off the left post and has exactly the same amount of gas as if he were diving a manifold rig. When the diver reaches 1/3 tank pressure on the left tank he switches back to right tank and continues to the exit. Only two regulator switches are made and the diver has ensured sufficient gas to make a safe exit from any point in the dive using only half of the available supply.
Lets use a hypothetical situation. A diver using independents has a catastrophic gas failure just at the moment he was turning the dive in a cave. The diver at this point has consumed one third of each tank and so whichever tank is not affected has exactly the same amount he used prior to turning the dive. Prudent cave divers will pace themselves when entering the cave so that they will be able to make a speedier exit in case of trouble than what it took to reach the turn point. A diver using a manifold with the same failure will have to perform an isolator shutdown in order to preserve gas. This may cause stress and require focus in an emergency. They may not be concentrating on the most important task at hand, which is maintaining contact with the guideline in the rapidly deteriorating visibility. They may or may not have closed the isolator in time to preserve 2/3rds of the existing tank and as mentioned above, if the left side is affected they won't know for sure until they get out alive.
Prejudice against independents is so strong that one incident puts a smile on my face. A former student of mine was using independents during his tech instructor training. His instructor trainer was making his strong objection known to the potential instructor but the student was making a strong case for his position. So strong in fact that the instructor trainer had to call his buddy to remind him why they hated independents so much. The IT finished with the comment "I would never enter a cave with you in that rig". What is really amusing is that independents in the form of sidemount are gaining in popularity in caves all over the world.
One more point about independents for the traveling diver. You don't need to carry your doubles to remote destinations. Put some wing nuts on your tank bands and carry them with you and rent any tank that is available. You can even just rent two regulators at your destination and remove those extra second stages. Although you may encounter the wrath of a knowledgeable dive shop worker who will tell you how those things are going to kill you.
Independent cylinders were once the standard uniform for any cave diver. They were commonplace in Florida caves and New Jersey wrecks and anywhere the demands of diving required a redundant gas supply. As the years went by innovation and adaptation changed the way divers set up their double tanks. The manifold with isolator became standard equipment on most sets of doubles and could be seen in droves from the caves of the Yucatan to the quarries of Ohio. Many of the divers learning to use doubles have never seen independent cylinders doubled up, as many new tech divers were taught that they were somehow substandard or even dangerous. I had one former solo student tell me as he stood on the Ogden Point breakwater about to plunge, a group of "tech" divers approached him and told him he was going to die diving such a contraption.
So lets look at this system realistically. An independent system allows you to carry two separate gas supplies with independent regulators. No single failure can render you without breathing gas. Unlike the manifold that could possibly suffer a catastrophic loss of gas should one of a number of o-rings fail or mechanical damage to the crossbar isolator prevent any of the o-rings from sealing. The way people use the isolator as a carrying handle I am surprised this hasn't happened more frequently but as such it is an extremely rare failure. Independents don't require any immediate action on the part of the diver should a catastrophic failure occur. Lets say a diver is penetrating a tight space in a cave or a wreck. They encounter a sharp object that causes a tear in an LP hose. A huge cloud of bubbles ensues and the percolation from the ceiling turns the water to a silty mess. To top it off the diver is not able to reach the manifold due to the restricted space they are in. I have personally been in this type of situation. Their only choice is to back out of the hole being careful to locate the guideline and find an area where they have enough freedom of movement to save their remaining gas by conducting a valve drill. The amount of remaining gas will depend on how quickly they were able to perform these steps but there is a good chance they may not be able to save sufficient gas for a safe exit. Also if the failure occurs on the left side of a conventional Hogarthian rig the diver will have no idea how much gas is remaining to get them safely out because they will have had to isolate their only SPG. The independent diver merely locates the guideline and gets themselves to a safer area where they can shut off the offending tank, or if the situation makes it difficult ignore it until the tank runs out. Either way he knows he has enough gas to exit the overhead environment he is in. This is due to simple gas management.
The independent doubles diver uses the rule of thirds, just like the manifold diver but slightly different. Each independent tank has to have its own SPG and the diver should start breathing off the right post and switch to the left post at 2/3rds tank pressure. The diver then makes a switch of regulators and continues the penetration on left regulator. When left tank reaches 2/3rds pressure the diver turns the dive and begins the exit. At this point he continues to breathe off the left post and has exactly the same amount of gas as if he were diving a manifold rig. When the diver reaches 1/3 tank pressure on the left tank he switches back to right tank and continues to the exit. Only two regulator switches are made and the diver has ensured sufficient gas to make a safe exit from any point in the dive using only half of the available supply.
Lets use a hypothetical situation. A diver using independents has a catastrophic gas failure just at the moment he was turning the dive in a cave. The diver at this point has consumed one third of each tank and so whichever tank is not affected has exactly the same amount he used prior to turning the dive. Prudent cave divers will pace themselves when entering the cave so that they will be able to make a speedier exit in case of trouble than what it took to reach the turn point. A diver using a manifold with the same failure will have to perform an isolator shutdown in order to preserve gas. This may cause stress and require focus in an emergency. They may not be concentrating on the most important task at hand, which is maintaining contact with the guideline in the rapidly deteriorating visibility. They may or may not have closed the isolator in time to preserve 2/3rds of the existing tank and as mentioned above, if the left side is affected they won't know for sure until they get out alive.
Prejudice against independents is so strong that one incident puts a smile on my face. A former student of mine was using independents during his tech instructor training. His instructor trainer was making his strong objection known to the potential instructor but the student was making a strong case for his position. So strong in fact that the instructor trainer had to call his buddy to remind him why they hated independents so much. The IT finished with the comment "I would never enter a cave with you in that rig". What is really amusing is that independents in the form of sidemount are gaining in popularity in caves all over the world.
One more point about independents for the traveling diver. You don't need to carry your doubles to remote destinations. Put some wing nuts on your tank bands and carry them with you and rent any tank that is available. You can even just rent two regulators at your destination and remove those extra second stages. Although you may encounter the wrath of a knowledgeable dive shop worker who will tell you how those things are going to kill you.
