I like to go diving in dark places. In dark places like caves, lights are important, and we carry a bunch of them. The rule in cave diving is to carry at least three lights for redundancy: one primary and two or more backups.

I use these lights as backup lights: https://www.divegearexpress.com/dgx-600-twist-handheld-light

They are ruggedly built, simple, and so far quite reliable. They do take rechargeable batteries. There is debate on whether non rechargeable batteries are better suited for use here, centered on discharge characteristics and the human factors involved in remembering to charge a light that you intentionally never use. Nevertheless I do currently use them. I had the pleasure of meeting Bill Main in the parking lot of Little River springs one day; he uses them as well. Consider that a celebrity endorsement. Bill also uses a straight-through manifold without an isolator valve, but that is for another wiki page.

It's important to know how long your main lights and backup lights can burn for. Therefore, I did some burn tests.

My setup is a cheap webcam (the same one used for bird bar) plugged into a cheap macbook. I placed the webcam on a chair pointing at a wall. On the chair I also placed a tupperware full of water. This simulates the diving environment. Then I fully charged the light battery, turned the light on and placed it in the tupperware facing the wall. Finally, I recorded the camera image using OBS. This way I could check the recording to see when the light stopped burning, as well as view the luminance characteristics as the light discharged, without needing to monitor the light.

It turns out the DGX 600 twist light burns for between 90-100 minutes. Over this time period it gets gradually less bright until it finally goes out completely. It is still usefully bright until the moment it turns off.

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The apparent increases in brightness in this video are caused by the automatic exposure control of the camera adjusting for decreasing brightness levels and do not occur in reality.