Glass Flotation Spheres
Glass flotation spheres deserve special mention because of their common usage. These implodable volumes cannot be certified by pressure testing because of their susceptibility to failure as a result of minor damage. As a consequence, Alvin will handle instruments utilizing these spheres in only three ways:
- An instrument using glass spheres for flotation will be carried to or from the bottom by Alvin provided the spheres are tethered above the instrument at a distance of at least 100 meters.
- Alvin will move an instrument on the bottom (including limited vertical excursions) provided the flotation spheres are tethered at least 30 meters above the instrument.
- Alvin will approach a glass sphere on the bottom to within photographic range but will not touch it. The only exception here is the case where Alvin approaches to grasp a trigger or release lanyard which is long enough to allow the submersible to back away a suitable distance before activation.
All of Alvin’s electrical systems and through-hull wiring must be UNGROUNDED to limit the chance for corrosion of structural parts in the event of inadvertent grounding of any conductor. Alvin’s electrical systems are frequently checked for grounds during each dive. There are four requirements that each science device must meet:
- No device may permit, or cause, a direct DC path between any source of Alvin power and the submarine’s hull, frame or seawater.
- No device connected to through-hull wires may permit any DC path between any through-hull wire and the submarine’s hull, frame or seawater.
- All devices connected to through-hull wires must provide a DC path from an Alvin power source to all such through-hull wires so that the submarine’s ground detection system can be used to check for inadvertent grounding of through-hull wires. The easiest way to provide this connection is with a resistance of from 0 to 3 Kohms between the instrument’s internal ground and the input power common.
- Isolated or battery-powered equipment may be used, though it must still adhere to these requirements.
NOTE: if grounds are detected on any through-hull conductors, the source of that ground must be identified and that system must be secured for the remainder of the dive. A grounded device will not be allowed back on the submersible until the source of the ground is identified and removed.
The hatch opening is 20 inches in diameter; any equipment which users wish to bring on board must be capable of passing through the hatch with its sealing surface protection ring in place, resulting in a working 19” maximum opening.
All instrument or other water-tight housings taken to depth are considered potential implodable volumes, and as such must be certified for use on Alvin. Implodable volumes are any pressure housing with a non-compensated compressible gas volume at a pressure below the external ambient sea pressure with a potential to collapse.
Certification of implodable volumes to be used with Alvin normally requires a submergence pressure test to 10,200 psi for 10 cycles; held 10 minutes at greatest pressure for cycles 1 through 9, held one hour at greatest pressure for cycle 10. An alternative procedure involves substituting a different maximum test pressure in the same cycling sequence according to the following equation:
Test pressure (psi) = 1.5 x (max pressure (psi) expected during applicable dives)
where maximum pressure = maximum water depth (meters) x 1.487 (psi/meter)
Although pressure test certification for Alvin purposes does not expire, consideration should be given to retesting every five years. The Expedition Leader may refuse to honor a valid certification if he feels the condition of the housing has deteriorated since its last pressure test. Modifications to the housing such as machining, drilling, etc., void a pressure certification and thus the vessel must be tested after any work that could compromise structural integrity.
The test chamber pressure recording chart (or copy) and/or statement of test details signed by the chamber operator shall be delivered to the Alvin office at Woods Hole, and a copy shall be delivered to the Expedition Leader aboard Atlantis prior to ship departure and subsequent equipment installation on Alvin.
Pressure vessels constructed from certain experimental or unpredictable materials such as glass or ceramics cannot be certified for use with Alvin in the manner described above. In some cases, a design review by Alvin’s U.S. Navy engineering oversight body (NAVSEA) is required (a time-consuming process) and in others, certification is impossible. If in doubt about a particular design, contact the Alvin office.
Many user-supplied sampling devices, cameras and sensors are unique, are likely very specialized, and as such may have a higher incidence of problems during a dive series. The Alvin Group strongly recommends that when a science program includes a user-supplied, specialized tool, camera, or sampling device, that sufficient practical spares are available for use in the event of a problem. Examples include spare cabling, o-rings and/or seals, tubing and fittings, valves, power supplies, data interfaces (Moxas) and other critical items that may experience problems or failure during a series.
A general rule of thumb is to expect some form of minor equipment issue, and to arrive prepared to respond to the problem with a reasonable complement of sparing and tools.
The Alvin Group will provide as much expertise and problem evaluation as possible, and where possible and practical, the Group may be able to provide basic equipment support. But for unique sampling devices it is imperative that critical spares are available.
Additionally, the Group recommends that cruise participants bring sufficient ‘dummy’ plugs and caps, capable of protecting sensitive electronic connectors and plugs at depth.
Users should also ensure that they are well equipped with any specialized or unique tools and hardware that are important for the preparation, mounting or support of their equipment.The Alvin Group can provide guidance for spares and tooling as part of the pre-cruise planning process.
The jaws of both manipulators are functionally equivalent and consist of opposing overlapping finger pairs. They are specifically designed to grip instruments which are fitted with a standard “T”-handle. The user should align the “T” with the vertical load. The user is cautioned not to assume compatibility between your tools and Alvin’s manipulators, even if the tools are fitted with T-handles. It is best to seek the advice of the Alvin Group on instruments which have not been previously used with the manipulators, regardless of how dependable they may seem. Many biologically and geologically oriented tools, including a variety of pry bars and other rock breaking tools, soft and hard sediment corers, box corers, and current meters have been adapted for use in conjunction with the manipulator jaws and the associated actuator mechanism. See also Manipulator Section.
Toxicity & Flammability
The U.S. Naval Sea Systems Command (NAVSEA), Alvin’s certifying agency, requires toxicity and flammability testing of all scientific instrumentation brought into the personnel sphere. The testing consists of a 24-hour, power-on bench segment to record maximum component temperatures, followed by 24 hours in an isolated chamber at 100 °F to measure offgassing. The tests are performed at WHOI and a NAVSEA approved laboratory. Items are tagged following testing, and a list of tested material is maintained at WHOI so that repeated tests of unmodified instruments will not be required.
Weight and Buoyancy
The Alvin Group calculates the submersible’s payload as a part of the preparation for every dive. Air and water weights of installed user-supplied equipment and sampling tools are included in these calculations. It is important that accurate air and water (buoyancy) measurements are obtained for all science-supplied items intended for use on Alvin and/or deployed platforms (elevators). The Alvin Expedition Leader will request these weights for all science gear at the beginning of each cruise.
Although use of a model to calculate air weight and net buoyancy of an assembly is possible, it is much more accurate to obtain air and water weights via empirical means. Air weights may be found via standard means (appropriately sized scale). Buoyancy measurements require that the item be submerged in sea water while measuring the resulting buoyancy. Although not typical, users should be aware that some items may be neutrally or positively buoyant in sea water which could have an impact on use during sampling. These items may be prone to float from the basket and thus require additional consideration.
Obtain weight and buoyancy for fully assembled components where possible and fully submerge the items during measurements. To ensure accuracy during measurements, it is important to ensure all equipment air voids and cavities are fully flooded to the extent possible. For larger items, obtaining buoyancy may require the use of a crane and load cell to fully submerge the item in sea water. With prior planning assistance may be available as a part of cruise mobilization (typically the two days prior to departure from port).The Alvin Group can provide additional guidance and assistance as required.