"Twenty Watt lights :- nice if you own one, but not really necessary for cave diving." These were my sentiments exactly, until, that is, Joint Hole started going, and my buddy started to use one regularly. His description of the new sumps we had been through were very different from my own. He had seen walls, roof pockets, beddings and landmarks which were completely out of reach to my meagre pit lamp and aquaflashes. His overall impression of the sumps then, were far more detailed than my own, a definite advantage. Yet he had only been following exactly the same lines as me. "Wait 'till the bad visibilty returns," I remember thinking, "the twenty watt light will be just excess baggage. Nothing can cut through our oxtail soup type vis."
Wrong again ! Although the beam is reflected back as with any other system, the intense light given off illuminates a far greater area around the diver's field of vision, making line following and junction manoeuvres far easier, not to mention gauge and compass reading. I remember counting the pound notes I would need to buy one. Seventy seemed to be about the going rate. Was it worth it for a bit more light ? A few more dives with my buddy convinced me it was.
I looked around for the few that were available; all were secondhand, and all seemed to have design faults that could cause problems for the diver. I felt very reluctant to part with my seventy big ones. Then the thought struck me. I knew exactly what I wanted out of a twenty watt light:- long duration, totally waterproof, very rugged, maintenance free and easy to charge - so why not build my own ? The drawing is for a lamp which fulfils all the points mentioned and can be built for around half the cost quoted earlier. My own light is built to this design, has been in constant use now for about eighteen months and has never once failed or let me down. The duration of the batteries is unchanged from new. The bulb I used is a 6 volt, 20 watt halogen and gives around one hour twenty minutes of useful light using 4 Amp-hour rechargable batteries. Each cell provides 1.2 volts, so five cells are wired in series to give 6 volts. My own light did not include a fusible link, although this should be a consideration. Charging time is dependant on which manufacturer's cells are used, but is usually around 12 to 14 hours for a full charge. Charge rates vary between 350 and 500 milliamp. The headset can be any make that is available provided that all the holes are sealed to stop the ingress of water, although no damage will result should the headset flood. The reflector must be made of metal to withstand the heat generated by the bulb. The battery container was just a length of stainless steel tube with a cap welded on one end and a cable gland fitted for the entry of the headset cable. The whole unit fixes snugly onto one cylinder with snoopies, and is a very useful item of equipment.