Tuesday October 05, 2004

Night Diving

Often times when I tell people we've been night diving in Lake Travis they respond with "how can you tell the difference?". Admittedly, visibility in Lake Travis is never on par with blue water which I'm told can be as great as 170 or more feet. Ok, nowhere near that. I think the best visibility I've experienced in Lake Travis has been somewhere around 15 to 20 feet. Still, diving in the daytime murk is somewhat different than diving in the same murk at night.

Vertigo Happens

Roy and I have been diving Windy Point on Wednesdays, nominally with the group from Scuba Land. We've been getting in the water around 7:30pm, which has been right around dusk. Decked out with green and red tank lights and an array of hand-held lights, we attempted dives similar in profile to our daytime dives.

Surprisingly, I didn't feel a great deal of anxiety of being in the water in the dark. I felt pretty confident in my equipment and my buddy, and just tried to enjoy the experience. Of course that all changed once the dive started. I was suprised to learn just how much I depended on visual cues that were either absent or far less prominent in the dark. Our percepetion of our current depth was always skewed too low ( we must be deep, it's dark ). And we also learned that navigation in the dark is somewhat difficult ( learned that a compass is not an optional piece of gear ), and that lights attached to your mask can become a dazzling hazard to your buddy.

So, vertigo. Experienced it for the first time on my first night dive. We were lost at 40 feet in a green haze of dusk, without a compass. Now we can normally navigate pretty well by following the contour of the shore, but we lost sight of the bottom whilst swimming about and got disoriented in the surrounding green. I didn't want to swim out into the lake since we'd heard some props earlier and boats are scary to divers. So our choices were surface or drop to the bottom or swim around lost some more. We went with c) swim around lost some more. I was looking down, hoping to see the bottom and get our bearings again when I started to notice the particulate in the water illuminated by my mask light. But I wasn't sure it was twack in the water, and thought it might be the bottom. And since I wanted it to be the bottom, I swam towards it to see if it was. And then I reached out to try to touch the bottom, and suddenly I'm in the grip of vertigo when my hand passes thru what I expect to be the bottom. As soon as it happened, I realized what happened and I remembered how to fix it. Find your buddy, hang on to him if you have to, and just look at him. Your brain sorts out what your middle ear and your eyes are telling you, and the vertigo subsides. So I find Roy, that was easy, and signal a problem and point to my head and give him hover signal. Sounds good, but I hadn't considered that by looking at Roy I would also blind him with my mask light and that about a third of my signals would get lost in the glare. Roy afterwards said he thought I was signaling a problem with his tank, and he kept reaching around his shoulder. We eventally got on the same page, and by that time the vertigo was under control. It then occurred to me to check my guages, and we had drifted up to about 20 feet from 40. Visual cues absent and the task load of the vertigo episode had kept us from keeping up with our depth. We dumped our BCs to get back to 40 feet and decided to swim off at a 90 degree to our previous imagined heading and eventually hit the shore contour and finished our dive.

Water temperatures are starting to drop, so there's no telling how many more night dives we'll get until spring ( or we shell out for better thermal protection ).

-ejo

(2004-10-05 12:08:19.0) Permalink