Since Alvin operations are somewhat esoteric, and since they
are carried out in an environment popularly imagined to be extremely
hostile, the degree of risk incurred by the vehicle’s passengers can be
easily exaggerated. This section is intended to allay any fears that
the first time user, his associates, or the members of his family may
Perhaps the most misunderstood hazard is that of the extreme pressure at our maximum operating depth of 4,500 meters (14,764 feet). It should be understood that at no time are the operator and the passengers exposed to other than normal, sea level pressure and temperature. The hull of the submersible is built to withstand an external pressure 25 percent greater than that experienced at maximum operating depth. Thus its integrity is ensured even in the unlikely event that the normal operating depth is exceeded. An exact duplicate of this pressure hull has been tested to nearly twice the operational pressure without failure.
In order to ensure that no water leaks into the cabin, all openings for wires, viewports and the entrance hatch are designed in such a way that they provide a tighter seal the deeper the submersible dives. These devices are thoroughly checked prior to each dive.
While the submersible weighs 17 tons in air, it is so configured that when submerged in salt water it is exactly neutral, requiring only a slight force to move it up or down. It is therefore possible, even if all normal power is lost, to ascend to the surface by dropping a small amount of weight. In order to guarantee that this can always be done, two small emergency batteries are provided within the cabin, either of which is capable of operating all of the weight releases. In primary operation, four 250-pound releasable weights are carried at the start of the dive; two of these are dropped when the submersible reaches bottom to effect neutral buoyancy. Releasing either of these remaining two weights will make the submersible buoyant enough to return to the surface with no further action. If for any reason additional buoyancy is needed, both of the large battery tanks may be released. The batteries, each of which weighs 1,450 pounds in water, are mounted on separate release devices which may be activated by either of the emergency batteries. Both manipulators and the science basket are releasable which would reduce weight, but more importantly, may eliminate the cause of the emergency if resulting from entanglement. In the unlikely event that the submersible still fails to ascend after all these items have been jettisoned, it is possible to release the forebody and pressure hull from the rest of the vehicle. The forebody will then float to the surface where the occupants may be safely retrieved by the surface support ship, which is always in the immediate vicinity and in continuous communication with the submersible’s occupants.
Each year the safety equipment and the other systems receive a thorough evaluation by the technicians and engineers of the Alvin Group. Their work is reviewed and checked by a U.S. Navy certification team and any discrepancies must be corrected before authorization to operate is granted. Prior to each dive, all systems and components are functionally operated by the crew and the pilot. Any safety-related failure or malfunction discovered during these checks must be corrected and the equipment rechecked before a dive can begin.
Over 40 years of operation, encompassing more than 4,000 dives, attest to the efficiency of the rigorous program which has assured, and will continue to assure, the safety of Alvin operations.