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Just adding my two cents, The questions/comments discussed recently about dive light canisters have a lot to do with the materials of construction, although little has been said about the how part. First in understanding the methodology in designing/engineering a battery canister, one needs to understand the conditions of loading in the canister. Combined stresses as a result of an externally applied hydrostatic pressure may cause the canister to fail when they exceed the stress limits (> yield stress) for the material, weld, bolts, or adhesive bond as applicable. A second mechanism of failure for an externally loaded pressure vessel may be referred to as a failure due to instability, or buckling. The chief parameters limiting the potential for buckling failure are the geometry of the canister and the �stiffness� (resistance to deflection as a load is applied) of the material. A simple relationship (but not complete) of collapse pressure, stiffness, and geometry is: Collapse pressure = 2*stiffness/ [(1-posisson�s ratio^2) * (T/D)^3] T- wall thickness, D-Outside diameter So what are the implications? A table of material properties is provided: Material Stiffness factor Impact Factor ~~~~~~ ~~~~~~~~~~~ ~~~~~~~~~~ Std. Delrin 450-500 1.2-2.3 Reinforced Delrin 950 .8-1.0 Polycarbonate (lexan) 350 14-16 Acrylic 450 .3-.4 Polyethylene (Low to Med. Density) 40-150 No break Polyethylene (High Density) 150-175 .4-4.0 Note: The Stiffness factor is in kpsi, but the relative difference is key. Using the simplified formula above, it is clear for a housing with similar wall thickness (T) and diameter (D), the stiffness factor is a significant contributor. Out of the six materials, the reinforced delrin (glass fibers embedded in the polymer matrix) should be able to withstand the greatest pressure/depth, where the polyethylene material housing would withstand the least. This appears to agree with what is observed (i.e. use the polyethylene housing for the shallower depths because it has a lower stiffness). Also, polyethylene would tend to cold-flow or creep over extended periods of applied pressure. Changes in a geometric shape away from a circular cross section would tend to accelerate buckling failure. However reinforced delrin is very very expensive, and its only twice as impact resistant as acrylic. So the next pick may be a standard delrin or acrylic. Two things to note is std. Delrin is also expensive compared to cast acrylic tube. I�m not sure if you can get thickwall delrin tube as a standard product so it would probably be machined from a solid round? However, look at the Impact factor (Izod impact factor for a 1/8� specimen). The impact factor for acrylic is the lowest of the materials meaning it breaks easier under impact loading (e.g. observed while dropping from your tailgate). Delrin would be a much better choice in impact considerations. Continuing on, the polycarbonate is a candidate for a canister (it is used as windshields in military jets because of its high impact resistance) however thick walled standard products are essentially non existent for a reasonable price. A thin wall coupled with a lower stiffness factor equates into a shallower allowable depth range for polycarbonate Lexan. The polyethylenes (Low density, medium density, and high density) have very low stiffness factors and would tend to collapse under lower pressures. However they do have very high impact factors (e.g. beat the hell with a hammer, although it does microscopically damage the material so don�t do it too many times). Therefore if cost is a prime consideration, an acrylic canister for deep depths and a polyethylene one for abusive shallow depths is one way to look at it. If cost is not so important, a standard delrin canister machined from a solid round bar may be the way to go with roughly 4 times the impact resistance as acrylic and a greater buckling resistance than polyethylene. Now if you really want the canister to go deep, lets go with an S-glass over S-glass/graphite fiber filament wound epoxy matrix composite cylinder capped with 6AL-4V titanium closures coupled with a water-blocked cable. $$$$$$$$$ Note, I didn�t speak much about the allowable stress levels of the materials. That�s a subject that has a lot to do with the details of construction. It�s good to see somebody finally stepped the endcap glued to the battery canister (EE). I suggested that to a WKPP diver a few years ago. I don�t know if Barry got that through the grapevine but it�s the second best way to do it. The butt mount like on the AJ is the third. The best way is to float both endcaps to eliminate bending moments (stresses) as a result of the interconnection. This assumes you don�t want to deal with hemispherical endcaps, which would be ridiculous on a dive light. IMHO, Doug Chapman
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