PAPER Under High Pressure: Spherical Glass Flotation and Instrument Housings in Deep Ocean Research AUTHORS ABSTRACT Steffen Pausch All stationary and autonomous instrumentation for observational activities in Nautilus Marine Service GmbH ocean research have two things in common, they need pressure-resistant housings Detlef Below and buoyancy to bring instruments safely back to the surface. The use of glass DURAN Group GmbH spheres is attractive in many ways. Glass qualities such as the immense strength– weight ratio, corrosion resistance, and low cost make glass spheres ideal for both Kevin Hardy flotation and instrument housings. On the other hand, glass is brittle and hence DeepSea Power & Light subject to damage from impact. The production of glass spheres therefore requires high-quality raw material, advanced manufacturing technology and expertise in Introduction processing. VITROVEX® spheres made of DURAN® borosilicate glass 3.3 are the hen Jacques Piccard and Don only commercially available 17-inch glass spheres with operational ratings to full Walsh reached the Marianas Trench ocean trench depth. They provide a low-cost option for specialized flotation and W instrument housings. in1960andreportedshrimpand flounder-like fish, it was proven that Keywords: Buoyancy, Flotation, Instrument housings, Pressure, Spheres, Trench, there is life even in the very deepest VITROVEX® parts of the ocean. What started as a simple search for life has become over (1) they need to have pressure- the years a search for answers to basic Advantages and resistant housings to accommodate questions such as the number of spe- Disadvantages sensitive electronics, and (2) they need cies, their distribution ranges, and the of Glass Spheres either positive buoyancy to bring the composition of the fauna. The discov- Because of the compressive force of instrument or sampler back to the sur- ery of swarming snailfish at 7,700 m by water pressure, the pressure case design face for recovery or to establish neutral University of Aberdeen’s (UK) Ocean- and the material selection for buoyancy buoyancy for manned or remotely op- lab so far presents the culmination of elements or pressure housings are vital. erated vehicles to dock and lift the these researches. Although the oceans Like the crew compartment of the package (Figure 1). have been investigated for a long Trieste 50 years ago, the ideal shape is time, the deeper depths present a chal- FIGURE 1 a sphere. Because of its geometry with lenge to exploration due to the ex- no corners, a sphere distributes the ex- ® fl treme environmental conditions that Benthic lander with VITROVEX otation ternal forces of the water evenly over its spheres. exist there. It is totally dark, constantly structure, making it the strongest possi- cold, and the pressure is immense. At ble shape. The material chosen may be a depth of 1,000 m, the weight on steel or other metals, molded plastic, every square centimeter is 100 kg but ceramics, or glass. increases to 1,100 kg at 11,000 m. The use of glass is attractive in many Still, researchers today have a suite of ways. Glass has an immense strength-to- stationary and autonomous instrumen- weight ratio and it is inherently cheap. It tation available for hadal observation. is corrosion resistant and nonpolluting. All these instruments have two funda- Additionally, glass spheres are trans- mental requirements in common: parent, nonmagnetic, and electrically Winter 2009 Volume 43, Number 5 105 nonconductive. Command and con- Handling spheres at sea can be borosilicate glass 3.3 with standardized trol of instruments, including upload- nerve wracking when opening and clos- physical, chemical, electrical, and op- ing mission profilesordownloading ing the fragile glass in bumpy seas. The tical properties, also well known as data, may be done through the glass hemispheres take some practice to feel DURAN®.Thiskindofglasswasfirst with hall effect or reed switches, infra- comfortable moving and lifting them. developed by the German glassmaker red, or blue tooth. Radio and flash- A rubber bumper over the exposed Otto Schott in the late 19th century. ing light recovery beacons have been glass faces brings some measure of pro- Borosilicate glass is created by adding shown to work effectively housed in- tection, but a system design that pre- boron to the traditional glassmaker’s ternally.GPS,ARGOS,orIridium cludes the need to open the sphere at frit of silicate sand, soda, and ground transceivers as well as VHF radio links sea, as described above, is preferred. lime. Borosilicate glass has a very high penetrate the glass without problem. The glass sphere housing requires physical strength and very low thermal Status lights and LCD displays are vis- some skill to seal. The sealing surfaces expansion coefficient, about one third ible to deck crews before deployment. must be very well cleaned and free of that of ordinary glass. This reduces ma- The high-quality glass may even have any grease, oil, lint, or other foreign terial stresses caused by pressure and been polished to create a viewport sec- material. The low-pressure seal around temperature gradients, thus making it tion for high-resolution digital cameras the equator is made with butyl rubber more resistant to breaking. Borosilicate or sensors utilizing light. The use of ex- and wide black tape. A vacuum port glass is commonly used in ovenware, ternal pressure-compensated lithium is quite useful. A pocket altimeter whereitisknownbyitscommercial polymer batteries, as described else- mounted to the interior is easily viewed name of “Pyrex®” (Figure 2). where in this issue, means the spheres providing confidence the sphere is need never be opened at sea, and the sealed and not slowly leaking. The FIGURE 2 use of small vessels of opportunity sphere is protected in a thick wall becomes more attractive. Indeed, a re- LDPE hardhat, which also simplifies Production of VITROVEX® hemispheres re- search team from Scripps Institution mounting. quires high-quality raw material, precision of Oceanography/UCSD deployed molds, advanced manufacturing technology, two free vehicles, described below, and processing expertise to meet the challenge of ocean trenches. using 17-inch VITROVEX® spheres to History of VITROVEX® 8,400 m (27,500 feet) from a 53-feet Glass Flotation and boat in November 2006. Instrument Housings As appealing as it is, glass has, how- All pressure housings depend on ever, some drawbacks in that it is diffi- geometry, outside diameter, wall thick- cult to machine accurately, it is brittle, ness, and material to reach their de- and hence is subject to damage from sired design depth. VITROVEX® glass impact and spalling. The glass may spheres of 10- or 13-inch diameter spall when cycled. The bearing stresses can withstand pressure at 9,000 or under standard connectors can cause 7,000 m, respectively. Larger 17-inch spalling at the corner of the spotface spheres are made to reach 6,700 m, and the diamond drilled hole. A large and now to 9,000 m and 11,000 m. As a result, VITROVEX® flota- 4-mm-thick “flat washer” with a larger The flotation spheres and instrument tion spheres and instrument housings diameter o-ring can be used to spread housings are composed of two mated show very little deviation in shape even out the load over a larger area. The glass hemispheres that are evacuated under the high pressure found in ocean connector o-ring rests on the top of and locked into position by a sealant trenches. Since two hemispheres have the “flat washer,” which has a smooth and protective tape. Once the spheres to be put together to form one sphere, finished o-ring surface. Some under- are sealed, the two hemispheres are matching the geometries is critical. In water connector manufacturers, includ- kept together by the atmospheric addition to precision molding of the ing SubConn and Teledyne Impulse, pressure on land and the pressure right type of glass, skilled craftsmanship have created connector bodies that of the water column when deployed. directs the pressing of each hemisphere are specially adapted to glass housings. VITROVEX® spheres are made of to exactly the same dimensions, outer 106 Marine Technology Society Journal diameter, inner diameter and wall FIGURE 3 thickness, as all the others of that size. Examples of VITROVEX® glass products. The mating surfaces require a triple grinding process: milling with diamond tools, manual smoothing, and manual polishing to ensure the parting plane sealing faces are honed to a precise flat- ness and finish. The congruous surfaces are so closely matched that when two hemispheres are set together, with no butyl and tape at the parting line, and 7,000 and 6,700 m pressure rating in Trench, and even modified to func- a hard vacuum is pulled, it takes a day the early 1990s. So it was in the nature tion as a towed camera in the Sea of for the vacuum to bleed down molecule of things to face the challenge of going Cortez to confirm the presence of by molecule. Place the butyl rubber and deeper. bacterial mats (Figures 4 and 5) tape on the seam, and it might take This development was driven by (Hardy et al., 2002). forever. As a result of precision form- various scientific institutions such as ing, VITROVEX hemispheres of the Scripps Institution of Oceanography/ ® FIGURE 4 same outside diameter and wall thick- UCSD (Kevin Hardy, 2000, in col- ness are completely interchangeable laboration with Emory Kristof, Na- Deep Ocean vehicle DOV Mary Carol, built from and can be replaced individually. Dur- tional Geographic Society), and later, a single VITROVEX® 17-inch sphere, is recov- ing assembly, they are aligned along the Oceanlab from the University of ered on the stern of Scripps Institution of ’ the outer circumference only; there is Aberdeen. Oceanography/UCSD s R/V Sproul by Scripps engineer Kevin Hardy in August 2003.