Why A Canister Light?
If you want Big Light, it's the only way to go. There just isn't any other way to get it. While some very fine one-piece lights have been homebuilt over the years, it remains an inescapable fact that if you want a big enough bulb to produce serious light, and a big enough battery to sustain it, and expect to be able to hold the light and operate a reel or other device, there's no alternative but a cannister light. Canister divelights can look a little strange the first time you see one. Even the best commercial units often have a distinctly home-made look, an impression reinforced by the transparent bodies often used, which put the guts on display. However, these lights have, for a number of good reasons, become the standard for cave and wreck diving and are the first choice of tech and cave divers doing the most demanding dives being done today. Besides sheer output, these light are surprisingly convenient to use, so that they are often preferable to a handheld divelight even when a super-bright light isn't required. The battery canister is tucked away so you are hardly aware it's there, and you've only got to hold a little lighthead instead of a great hunk of batteries and reflector. When you're not using it, the lighthead clips unobtrusively to a D-ring on your BC. Put a Goodman handle on the lighthead, and you can hold and aim it even as you operate a reel, liftbag or scooter with the same hand. When you need both hands for something really demanding, like untangling lines, you can just flip the cable around your neck and let the lighthead hang in front of your chest, where it conveniently illuminates the work area before you. Try that with a UK 1200!

Building Divelights
I had a dive shop owner down in Florida cave country go crazy on me the other day when I tried to buy a reflector from him - he actually told me that the only way heıd sell me one is if I brought in the light to prove I owned it, and showed him the damaged parts I needed to replace. He then went into a rant about how anyone who thought they were going to build themselves a canister light for less than they could buy one (from him, of course) was fooling themselves. He was partially right about that, even though it was really none of his damn business. Anyone who sets out to build a divelight purely to save money will likely find themselves disappointed. However, if you do it because you like to build things, and like the idea of having a dive light you built yourself, with saving money secondary, you should do just fine.

The nice thing about building canister lights is that, since they are modular, nothing you do ever has to be permanent. If you decide you like the light but not the lighthead just make another light head. If you decide you need more or less battery, make a larger or smaller canister. Since the same small parts - latches, stain reliefs and switches - are used on all the models, they can be stripped off the old canister and used on the new (though it will usually be more cost effective to sell it as a working unit). It's a very cost effective way to move up to a canister light, figure out the right combination for your needs, and to upgrade when the time comes. This flexibility comes in handy if you are trying to build a light on a budget. By going with used/surplus batteries and a PVC lighthead you can get a light up and running for well under $100, then upgrade as time and money permit.

The first step to curing leakage is to locate where it's getting in. This is not always as easy as it sounds, since the water will often run to the bottom of the canister leaving no sign of how it got there. Spots that are going to leak at depth will usually leak, albeit much more slowly, in shallow water, so the first step in leak testing is to just leave it in the bathtub over night. With acrylic canisters you can often spot the leak by taking the canister down 30' or so and looking to see where the water is coming in. Otherwise, try stuffing the canister with crumpled newspapers. The newspaper will act as a telltale to mark the input spot (as long as you don't leave it under so long, or the leakage isn't so bad, as to totally saturate it!).

The usual leakage spots are the cord glands, due to settling of the cord, and the screws that hold on the latches. The cord gland is an especially common leak spot on a newly assembled light, due to the cord cover taking a set where the O-ring is tightened against it. Retightening usually cures it. A good test for tightness is to hold the lighthead or canister top in one hand and the cord in the other, gripping it just above where it comes out of the connector, and try to rotate the cord. If it rotates freely the gland is not tight enough. Depending on what kind of watertight connector you are using, what kind of cord, and whether the O-ring is lubed or not the cord may move somewhat, but it should not turn too easily

The latch screws can be harder to fix If you haven't already done it, try using some thread sealant on the screws. Thread sealant is not always completely effective, because the screws tend to move around as the latches are fastened and unfastened and this can be enough to break the seal from the sealant. The cure in this case is usually to stuff the screw holes full of sealant, working it in to get them as full as possible, then install the screws - as they go in they'll force the sealant into whatever gaps might exist.. If the main canister top O-ring is leaking it should be examined carefully for any defects, cleaned if it passes inspection, and relubricated. If that doesn't fix the leakage check the for nicks or roughness on the mating surfaces, and resurface them with fine sandpaper if necessary.

Building HID Lightheads
If you want to build your own, at this point the 10W is the safer bet. The ballast comes already potted and, since it's round, and much more svelte than the more powerful ones, it's that much easier to make a watertight case for it. The ballast compartment can be made an integral part of the lighthead or slug, which makes it an easy job on a lathe. The ballast needs to be able to dissipate heat, so for best results the housing should be made so the ballast is a snug fit in it, so heat will be transferred from the ballast to the housing and thence to the water.

Making a housing for the MR11 version is especially easy since itıs just a tube with a cord fitting on one end and a lens on the other. This can be turned from Delrin or aluminum. The bezel can be either threaded to fit over the tube, or held on by screws like the MR16 lighthead elsewhere in this book. Threading the body has the advantage of allowing the use of smaller diameter stock for the body, and it is often possible to find something ready-made that will do for a bezel. An "instant" version, using 1 1/4" or 1 1/2" schedule 80 PVC (or aluminum if you can find it) pipe is another possibility. If you use 1 1/4" then the inside must be bored out slightly to accept the ballast. If 1 1/2" is used then some appropriately sized brass or aluminum shim should be wrapped around the ballast to snug it in place and make a good thermal connection between the ballast and case. The glass lens can be held on using the` screw-on collar from a PVC pipe union or a similar collar made from a female NPT-to- pipe adaptor.

The ballast is also just the right size for a light press fit into a Maglite D-Cell body, making for some interesting possibilities! Building a test tube head for the 10W is more complicated since the ballast is too big to fit inside a standard reflector. It's a lot easier if the bulb can be moved about 2" from the ballast, since this allows making the slug portion long enough to work with a standard reflector and goodman handle as well as using a standard test tube, however this requires making up a connector, with the possible complications mentioned earlier.

Lithium Ion Battery Packs
Lithium ion, or Li-Ion batteries were considered very exotic only a few years back. Litihium is the lightest of all metals, and very reactive, so by using it in batteries it is possible to build very light cells with very high capacity for their size. This made them ideal for use in high-end laptop computers, where the higher cost could be tolerated, and their widespread adoption for that purpose bought prices down very quickly. They have not been quite as fast to catch on in divelights. Lithium batteries do not like water, and can discharge quickly and possibly explode if allowed to get wet. As a result, several of the major manufacturers of lithium cells warn against their use underwater. Several divelight manufacturers, though, figured, with typical caver logic, that a brighter, longer-lasting light was worth an occasional explosion and pressed ahead and tried them anyway, and the dangers of immersion were found to be considerably less than feared.
Li-Ions are are extremely fussy about charging and easily damaged if it isn't done just right. Usual practice is to equip a pack with a monitoring system that monitors each cell separately to be sure that it isn't being over or undercharged, or discharged too quickly. This required sophisticated circuitry, and has made it difficult for the DIYer to build a Li-Ion pack. However, as Li-Ion batteries have grown more ubiquitous, cheap little off-the-shelf dedicated circuit boards know as PCBs (Protective Circuit Boards) have become readily available, for $5-$15. These are meant to be built right into the battey pack, and make homebuilding lithium pack as simple as building a nicad or NiMH pack.


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