Instrument Cases and closures2.693 (13.998) Instrumentation LAB: Materials in the Ocean Environment I. Connectors: Required to get signals and power in and out of pressure case. They are often misused and abused and the failure can cause loss of data and instrumentation. a. Mecca single pin. Stainless threaded bulkhead with 0-ring seal. However, a problem exists with the bonding of the rubber to the SS bulkhead fitting. Failure of that bond can lead to failure of the connector (e.g. it leaks and floods pressure case). Seals to pigtail with “molded in” o-ring on bulkhead. Need to “burp” connectors. Also, in-line male-female connectors. b. Impulse XSE rigid plastic. No dissimilar metal problem as plastic. Sealed by o-rings. However, plastic subject to breakage. Also, you can mate the connectors with the key misaligned. They are hard to clean - you need to be sure o-ring is in bottom of bulkhead connector and clean, and o-ring is in male connector and clean. c. Impulse/Seacon XSG and XSK fiberglass bulkhead and RMK and RMG pigtails. The kind I like best and use most often. Plastic bulkhead has no dissimilar metal problem, has o-ring seal to pressure case, conducting pins molded in. Female pigtail makes with bulkhead which has a “molded” in o-ring to help seal. Make sure to burp the connector. Cleaning bulkhead is easy, female pigtail easy unless small particles get inside. Female bulkhead and male pigtail not as reliable. d. Underwater Matable (not really meant to be plugged and unplugged underwater, but really for damp environments – like on deck. Old style is Electro Oceanics which is single or double connector. More recently multiple pins with two connectors per pin have been used – e.g. Martha’s Vineyard Coastal Observatory. The end of the male pin wipes the female clean as it mates. Light greasing only – too much can lead to leaks and corrosion of pins. Problem of sealing bulkhead to pressure case (1) skirt, (2) o-ring on shank, (3) metal base with bonding problem. e. Coax – plastic and D.G. Obrien f. Fiber optic – underwater plugable problem at MVCO because of particulates II. Materials that we use in the ocean a. Aluminum. Most often used for pressure cases. i. 6061-T6 - low pressure, low zinc, lower corrosion ii. 7075-T6 – for higher pressure uses, higher zinc content, higher corrosion. iii. 5400 – lower zinc, not as strong, less subject to corrosion – used in building boats (Navy superstructures) and in buoys. iv. Anodizing with seal and painting are used to protect aluminum v. Best results by using zinc anode vi. Leaving bare allows even corrosion, painting/anodizing concentrates corrosion at scratches. vii. Aluminum is readily available in most shapes, sizes, configurations. b. Titanium. i. Passive in the ocean – don’t have to worry about corrosion.ii. Same strength to weight ratio as aluminum – pressure case with same depth rating is half the thickness but the same weight as aluminum. iii. Harder to get material configured – e.g. no tubes and must get block and machine. Save borings to recycle. iv. Harder to machine if you aren’t experienced and have the right tools. v. More expensive. vi. Available in nuts, bolts, washers, etc. vii. Note that the Sea-Bird Microcat with which you did the CTD profile is made with a Titanium case. c. Stainless Steel – Again used often as fasteners. i. Alloy most used is 316 (must have this stamped on the part or it is probably not 316. This is one of the better stainless steels for use in the ocean. ii. Another type often used is 17-4ph. This is used for load cells (because of the mechanical properties under stress), and for pressure cases (Neal Brown CTD and Benthos acoustic release). The anode for 17-4ph is soft iron. iii. In anoxic environments stainless steel is subject to crevice corrosion. d. Steel: i. Used in ships (KNORR, OCEANUS, ATLANTIS) – TIOGA is aluminum. ii. Anchors, galvanized chain, shackles, etc iii. Coast Guard navigation buoys iv. Older scientific buoys – require regular maintenance, sanding painting,etc. e. Glass is as inert a material as you can get i. Used by Benthos (and others) as flotation spheres. 10” and 17” diameter. ii. 17” diameter in plastic “hard hat” gives about 50 lbs buoyancy. iii. Will go to deepest ocean depths. iv. Sealed by warming air on inside, and mating two halves together (with no grease or sealant on the glass to glass surfaces. If there is something then the glass will tend to spawl, and weaken. Need bare glass to keep the properties of a complete glass sphere. Joint is sealed by tape on the outside to prevent slow air leaks. v. Note that at 2/3 atmosphere pressure inside (or 5 psi difference from atmosphere), a 17” glass ball has about 1,000 lbs holding it together. vi. Another pressure case is made from the glass plumbing you see in scientific laboratories and buildings. You can get a 6’ foot section with round end. Then you seal it on the other end with a flat end cap. This is the pressure case for a RAFOS float (a Lagrangian drifter that tracks ocean currents). f. Syntatic foam i. This is a glass epoxy mix. It is the additional flotation for ALVIN and used in deep ocean flotation spheres (see the bright orange 6 foot diameter spheres on the WHOI dock). ii. It is made from small glass spheres. These are pressurized to the depth rating of the foam, and the spheres that do not crush (but float) are thenmixed with epoxy to hold them together in the desired shape. It can be machined (hard on tools). iii. Heavy for amount of flotation, and expensive, but has its place in the ocean g. Surlyn foam i. A “nearly” closed cell foam that doesn’t absorb water. ii. However as the cells are at atmospheric pressure, the foam collapses when pulled under water – example GLOBE guard buoy. iii. Used by the Coast Guard for navigational buoys and small marker buoys iv. Also used as fender material for boats v. Used by WHOI for buoy flotation – GLOBEC, GoMOOS, Buoy Farm and MVCO buoys on dock, etc. vi. Rolled up into whatever size you want (maximum thickness of one roll is 6 feet). vii. Outside melted and rolled into a hard cover. viii. Ships like to work with buoys with foam flotation as they can hold them tight to the ship without damaging buoy or ship. h. PVC –grey plastic