What are the differences between manifolds and independent cylinders and which is the best system to use?
The question of manifold usage is a very heavily discussed subject, with many different and quite polarized opinions on the subject. Some people are adamant that manifolds seriously reduce the safety of twin cylinders and without suitable support will place the diver at great risk. Others would vehemently deny that using a manifold significantly increase the diver's risk but that the additional safety factors and comfort easily make up for any slim possibility of mechanical failure. Even with manifold users there are differences in how to use the manifold.
It is not realistic to attempt to settle these differences in this column, for in many ways the principle of personal preference takes priority. However it is important to understand that ensuring adequate safety and redundancy with any diving configuration should be the most important factors. Before we can look at the pros and cons of a manifold system we first need to look at the history of manifolds.
The original manifolds were developed in response to the early (pre 1960's) diver's need for an increased gas supply while using their twin hose regulator. As larger cylinders were not common, and fill pressures were low, a single yoke fitting was manufactured which was connected to two cylinders. This system is still in use today in specialised mine & fire rescue sets.
With the advent of the single hose regulator the first true diver's manifold was developed which connected two independent cylinders and allowed for two regulators to be attached. This 'crossover manifold' equalized the pressure between the two cylinders and the two cylinders in effect became one breathing system. The manifold was a yoke connection and as this could be bumped against an overhead projection and damaged there could be the loss of all the air in both cylinders. This system can still be seen in use even today, but these are potentially dangerous systems not designed for redundancy or emergency air situations.
Both of the above systems should be treated as a single air source for all dive planning.
In the late 1960's Dr George Benjamin developed the first isolation manifold. This 'Benjamin Manifold' had a connecting bar between the two cylinder valves, allowing for the regulator of either cylinder to have access to the air from both cylinders. What differentiated this system from the previous type of manifolds was that should one regulator free flow or O-ring fail then that valve could be turned off effectively isolating it from the air cylinder. The second regulator then would continue to have access to both cylinder contents. There was however no way of isolating the cylinders form one another once this bar was connected.
The current isolation manifold design was produced in the late 1970's by Sherwood. Now called the DIN Double Genesis Manifold this system is similar to the Benjamin manifold but includes an isolation valve in the connecting bar, therefore allowing each cylinder to be independent of the other. Now available from most valve suppliers (OMS, Dive Rite, Scuba Pro, Sterling, etc) this system has become the accepted manifold system for many cave and technical divers. It is this system that we shall consider with regard to the manifold versus the twin independent system discussion.
The only difference between independent doubles and the isolation manifold is the connecting valve and bar between the two cylinders. Both systems can be run as separate cylinders by diving the manifold in the closed off or isolated position. The argument against is that no matter how tested & secure the connecting valve and bar is there is still a chance that an internal O-ring or valve component may be damaged (say from being knocked). Those in favour of this system point to a fail free record for properly connected and banded cylinders and the increased safety. Should one regulator fail then that regulator alone can be isolated by closing the relevant cylinder valve and then, by opening the centre isolation valve, the working regulator can then have access to both cylinders.
Another difference in opinion is even in the use of these manifolds. Some of the cave and technical training agencies in the USA have advocated diving with the manifold open and then closing the centre valve in an air loss emergency. Others advocate running with the manifold closed as described above, but instead of swapping regulators to access the separate cylinders the diver should open the manifold valve to allow the cylinder pressures to equalize, then close the valve off to maintain cylinder independence.
In effect the use of a manifold is a personal decision.
The one requirement is that the equipment configuration must allow for an adequate redundancy and air emergency plan. The question of the connecting isolation valve being the fatal weak link will come down to the diver's own acceptance of the chance of failure. Remember that one person will not hold all the answers. But before trying to use a manifold (or any new gear configuration) we should always practice in a safe and protected environment and if necessary seek training in the systems we wish to use.