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Filling pressure housings with a fluid can be an effective method of equalizing pressure across the housing, however some points which have been touched on should be considered. Two of the more common fill fluids for submerged electrical systems are silicone oil and caster oil (not castrol motor oil). They are dielectric (do not conduct electricity) and do not generate significant carbon during arcing (relays), relative to other fluids. However silicone oil can be an environmental concern. Another oil which I have seen used is mineral oil, purchased from a livestock supply store. It is not as good as the silicone and caster, but it should work well for dive lights. It does however contain lots of moisture. As with any fill fluid it should be degassed in a vacuum environment (6 to 12 hrs) to remove the dissolved gases (air) in the fluid; the vacuum also will tend to dry the moisture (boiling off at the vacuum pressure). I have a customer who uses peanut oil in some of their systems because they once had a silicone oil leak which turned into an environmental nightmare. Seems to work ok. If you are going to fill the cannister with oil, or some other liquid, why not sandwich a rubber diaphragm across one end of the cannister (supported by an end cap drilled with several holes). This allows ambient pressure to be transmitted via the diaphragm to the internal fill fluid. The pressure inside will now be almost equal to the ambient pressure. Your only problem now is the components inside the battery cannister. If you use the "sealed" gel-cell lead acid batteries, you will have to open the tops of each cell and remove the rubber bellows so that the dielectric oil can flood inside the cells. I have done this by peeling back the top on a PowerSonic 7 amp-hr battery. If the gel electrolyte is relatively incompressible (I haven't tested gel-cells yet) then the equal pressure around the plates may not cause distortion and plate failure. Or maybe it may (remains to be seen). If you use a silicone fill fluid, do not use any silicone parts inside your cannister. The silicone oil will cause swelling of the silicone parts. Strange isn't it? Most all of the SO, SOW type cables with a neoprene jacket and insulation are compatible with the fluids mentioned. Switches will flood with oil and should be no problem. Since I mentioned cables: The typical cable configuration uses a compression gland/o-ring or sleeve to seal the cable. This does not seal the inside of the cable. If the outer jacket of the cable is nicked, water will migrate through the inside of the jacket around the insulated wires and into the light cannister. If the cannister is fluid filled and pressure equalized this water migration will be reduced because of the low to nil pressure gradient. However if it isn't, like 99% of the cannister designs, this is one way to get water into your cannister. Ultimate solution: use a bulkhead penetrator which totally isolates the cable from the inside of the cannister (can be costly). Lower cost solution is to use a SO or SOW cable which has a thicker outer jacket than an SJ cable and inspect and replace as needed. Many a day has been ruined by charging batteries in a closed compartment. "Sealed" lead-acid batteries generate a little gas during charging. This must be vented off or you end up with a bomb. This is one reason why many rechargeable systems require you to remove a cover to plug in the charger so the gases can be vented. You need to provide a valve to vent off the gas while charging a fluid filled cannister. A little tricky but certainly can be done. Just my two cents. Doug Chapman
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