How does that work in a cave?
Backup Computer for Rebreather Divers
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How does that work in a cave?
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I leave an analog depth gauge in the "first" deco bottle that I come to which is in a dive rite thigh pocket. It also has my ipod shuffle in it which is convenient. Time can be done roughly based on gas consumption, or in my case roughly by number of songs that I go through on my mp3 player. Sh!t really hit the fan if I end up at that point though...
The analog gauge is more in case the pressure sensor goes wonky, but I do dive with a normal watch on most of my dives
MegDiver792
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Not being a caver, I'd like to assume you know the cave depths well enough already to have a reasonable idea. I certainly do on wreck penetrations.
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knowing depths on a wreck penetration is a cakewalk compared to a cave whre you have multilevel profiles and no necessary reference to where you are
you generally have good reference points to where you are, but if the cave is 40ft tall, you don't necessary know exactly what point you're at. Certainly nothing like having a knotted line from the surface....
With cave sites you can learn the depths pretty easily and don't really need a backup depth gauge because the stop depths are fairly static. You might have minor variations due to water level changes, but it'll be close enough to get you out of the water.
Devil's eye: 50' is at the start of the gold line, 40' is by the log at the back of the cavern / top of the restriction, 30' is the floor of the cavern, 20' is the overhang right inside the cavern, 10' is at the little shelf.
Peacock: 20' is right at the mouth of the cavern. Watch out for people doing a giant stride to get in.
Eagles Nest: 120' is the top of the debris cone, 80' is the ceiling of the ballroom, 50' is the top of the shaft, 30' is where the sign is on the tree.
etc. etc. etc.
Devil's eye: 50' is at the start of the gold line, 40' is by the log at the back of the cavern / top of the restriction, 30' is the floor of the cavern, 20' is the overhang right inside the cavern, 10' is at the little shelf.
Peacock: 20' is right at the mouth of the cavern. Watch out for people doing a giant stride to get in.
Eagles Nest: 120' is the top of the debris cone, 80' is the ceiling of the ballroom, 50' is the top of the shaft, 30' is where the sign is on the tree.
etc. etc. etc.
I'd be more concerned about trying to count knots/use an SMB in a strong current on an ocean drift dive. What you may think is 50' may really be 30' due to current pulling a line.
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I use two computers. One connected to the CCR - one standalone in constant PO2 mode.
That should be fairly standard practice.
That should be fairly standard practice.
joshk
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I have two computers reading the cells. The standalone Petrel was running in CC mode for a while, but I recently switched it to gauge mode because accessing the stopwatch/runtime/average depth was easier on cave dives.
I'd keep the standalone in CC mode if I only had one live computer. It's better than nothing and frankly it was always close enough unless I run the PPO2 much differently than what it's expecting.
I'd keep the standalone in CC mode if I only had one live computer. It's better than nothing and frankly it was always close enough unless I run the PPO2 much differently than what it's expecting.
If you you do some basic trigonometry you'll find you need about a 55 degree angle from the vertical to have 50' of line out from a 30 ft depth. That would require a pretty strong difference between the current at depth and the wind and current at the surface on a drifting deco.
I've seen some exceptions where we had a strong out flowing tidal current at depth (about 1.5 kts), at depth and a strong long shore current (about 4 kts) closer to the surface, where the line has done some very strange things. But, it usually sorts itself out before you get to the first stop depths, even if they are in the 100-130 ft range.
Those kinds of angles are more likely to happen if you've shot an up line from the wreck in order to stay on the numbers. I never dove with an up line with a length less than 2x the maximum depth. On a 200' dive that meant a 400' up line. That would accommodate a 60 degree angle from the vertical on the up line. If I needed more than that I needed a bigger SMB, and it was a lot more practical to clip another one on the line with large locking gate carabiner and send it up the line.
I won't bore you with the hypotenuse (line length) and opposite side (depth) of a right triangle, and the resulting angle from the vertical. But I can provide some useful rules of thumb.
15 degree angle from the vertical, add 5%
25 degree angle from the vertical, add 10%
35 degree angle from the vertical, add 20%
45 degree angle from the vertical, add 40%
55 degree angle from the vertical, add 75%
60 degree angle from the vertical, add 100%
Obviously you want to keep the angle on the line less than 45 degrees, because the errors beyond 45 degrees start to get huge, so just ignore the last two and be sure to bring a large enough SMB.
You can simplify it by thinking of it as 5% for 15 degrees and then double it for each additional 10 degrees you can't screw it up. 5% doubles to 10% at 25 degrees, which doubles to 20% at 35 degrees which doubles to 40% at 45 degrees.
So..if your up line is angled at about 35 degrees, that's about a 20% error in depth versus line length. If your first stop is at a 130 feet, you can either:
- multiply 130 x 1.2 to get 156, or
- break it down to 100 x 1.2, and 30 x 1.2 to get 120 + 36= 156
If you can't do that last one in your head, or at least on a slate, you might want to consider a different sport.
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My point was not the depth of the deco stops themselves, but rather the differences between having your only computer crap out in open water versus a cave. In open water you can immediately shoot a bag or SMB and start your ascent, or tie in and create an up line and then start your ascent if you have to stay on the number.
The options are different in a cave where you may be a few thousand feet back with several substantial changes in depth on the way out. The major changes through and along horizontal changes are no that hard (Cow for example) as you more or less know the depths if you've been paying attention. However, when the passages are much more irregular and where you may have pretty significant depth changes, it becomes a bit more difficult to guestimate the resulting deco, unless it's a dive you've done in the past.
The options are different in a cave where you may be a few thousand feet back with several substantial changes in depth on the way out. The major changes through and along horizontal changes are no that hard (Cow for example) as you more or less know the depths if you've been paying attention. However, when the passages are much more irregular and where you may have pretty significant depth changes, it becomes a bit more difficult to guestimate the resulting deco, unless it's a dive you've done in the past.
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