Dive Computers Deco Planning & Bailout

Dive Computers, Deco Planning, and Bailout: A Beginner's Guide
By Rob Infante



The title of this article can be taken at face value: that the information is most useful for people just starting out on closed circuit. I also mean it as a self-reference: with a little over a year, and 165 hours, I am not an authority. I offer my observations because I've been asked a number of questions about these three topics, and hope others can use what I've learned to get a boost up on that ole' learning curve (as well as make a more educated decision before plunking down their hard-earned.) At this point I have experience with the three stand-alone closed circuit computers (as opposed to Vision, Hammerhead, Shearwater Meg or Optima electronics), as well as with using a multigas open circuit trimix computer.


Constant PO2 Dive Computers

I have three different computers, all with their pros, all with their cons. I find them to be the tetchiest pieces of gear I own, and a constant reminder to plan my backup tables carefully.

My first choice is the Hydrospace Explorer, simply because I like the RGBM algorithm. It is not so easy to switch gasses for bailout, and the controls are about as intuitive as DOS, so you better practice. There are 10 conservatism settings. If you put it on C4 it matches the stock vr3 algorithm at C0 fairly closely. You need to play with the conservatism settings a bit and see what makes you comfortable. I put it on C0 for a warm water, relatively easy 160' dive in Florida and came out feeling kind of crappy - sore and not happy. I now usually do C1 in warm water, C2 in colder, without issue. Reliability is its Achilles heel: mine died on its first dive, then its 25th. Be prepared to wait several weeks to get it back when it inevitably dies. The computer interface didn't work for me at all, as it doesn't like Dells and it doesn't like Windows XP. It did succeed at killing my rather pricey and scarce battery though, as I left it hooked up to the patch cord overnight. When you do change batteries make sure to crank down the battery compartment cap hard, or the connection will briefly break and leave you with no deco information. Sounds like all good reasons to avoid it, but when it works it works great.

In contrast the best thing about the VR3, at least in my experience, has been its reliability. I've taken it for something like 50 hours of diving without so much as a hiccup. It comes with either a color or monochromatic screen. I chose the latter because I've heard the color screen can be hard to see in strong sunlight (it also gave me a good excuse to not pay another $300.) The interface is very nice, with minimal help from the manual you can set it up and work the functions. Having said all that, I kind of hate it. The stock algorithm has no sense of time, and over and over I've found myself screamingly aggravated by its inability to give me a straight answer on remaining deco time, even when following all the stops accurately. While the VR3 nominally is a deep stop computer, it is also famous for jumping stops, for example 180' to 130' to 70'. Huh? What about all those stops in between? I wind up adding some time to all the 10' depths in between, and then it goes berserk, pumping 60 minutes of deco up by another half hour or more. It is imperative that you watch which gasses are turned on, or your deco will be stretched even longer. One option is to get the VPM-B/E algorithm added to the VR3, but at another $300 it's not a cheap decision to make. On the plus side though, I've been told it does then give very accurate information, such that your Time-To-Surface and your watch seem to be on the same planet. For another $75 you can also download books to it, which is a pretty cool function at an outrageous price. I think first I'm going to try laminating some sudoku puzzles. One thing that makes it a very good backup computer is its low self-esteem: Violate it all you want, it will never lock you out. There is a well known NE diver who uses it to back up RGBM tables. He said he hopes he never has to send it back for service, since he has scarcely done a single dive where he didn't blow off remaining deco!

The other computer I use is the new Shearwater GF, sometimes hooked into my Sport Kiss, sometimes not (Prism.) I got one from the first batch of ten, and frankly was expecting a lot more problems than it has given me. It's a big-ass brick, but fits pretty nicely on my arm, and is even simpler to use than the VR3. Its biggest drawback to me is the lack of an RGBM algorithm, but the selectable gradient factors (I like 10/85) do a nice job. Like the VR3, it sometimes thinks minutes are 80, 90, 120 seconds long (this past weekend it took four minutes to clear the last minute of my 20' stop), which has also given me fits. Other times, like when I did 35 minutes at 150', it tracked with my Explorer almost to the minute. I'm still figuring it eccentricities, but so far I'm pleased.


Multigas Open Circuit Computers

By the time they get to diving CCR, most everybody has some sort of multi-gas computer. These are still very useful for recreational ccr diving, and even for some limited technical diving. The more gas blends the better, since each gas blend lets you more closely approximate the PO2 within your loop at a given depth.

When I first began diving closed circuit my computer was a DiveRite Nitek HE. This served me very well for the first 45 hours, when most of my diving was done to 130' max, and my deco rarely exceeded 20 minutes. What I would do is work out whatever the FO2s would be to match a PO2 of 1.2 at the various depths I anticipated being at. I would then dive at 1.3 for conservatism. (I have a table in excel with all the various depths and PO2s, p-mail me if you would like a copy.) With the Nitek you get 7 mixes, which makes it pretty useful for getting the right depth/mix. If you don't have a slot for a precise mix just use the next deeper one (ie lower PO2), the difference between the two only adds conservatism. If for some reason your bailout is not represented in those numbers then reserve a slot for it as well. I would sometimes up the FO2/PO2 for the 20' stop to something higher, 1.4 typically, if I wasn't concerned about O2 exposure and wanted to get out of the water a little faster. You can either set the mixes in ascending or descending order. If you do the deepest mix first it will be a little more conservative, but you'll have to do more scrolling when you change mixes. The other way it will give a PO2 alarm the whole way down, which is a pain, but then is marginally easier to switch. It takes a little more attention to dive this way, since every time you change depths you need to fiddle with the computer. It's not too onerous though, and cultivating an increased time/depth awareness is a worthwhile pursuit.


Backup Tables

Computers die, some of them often, some of them rarely, but dive long enough and you'll find yourself looking at either a blank screen or bad information (Woohoo! 45 minutes at 180' and I have no deco!) Cut a bunch of tables on Vplanner or Gap and stick them in your wetnotes, or else cut them for each dive and leave them in there. Pretty soon you'll have a pretty decent library with all sorts of options to get you home. I also like to keep a PDA in my dive bag with deco software, that way I can take time on my surface interval to cut very accurate tables for dive 2. You should be looking at your computer often enough that if it does suddenly die on you, you'll still have a pretty good idea of your deco obligation. The Buhlmann algorithm (like the Niteks use) is not my first choice for deep diving, but it is helpful in that the deco time its going to show you will largely be at shallower depths. I always try and keep my Time-To-Surface in mind, because computers do die, and I have finished off my deco stops becasue I remembered what my obligation was. After a while you can work a viable deco plan for gradient models as well, as you become more comfortable with how the total deco time breaks down (and then pad it generously, of course.)

You can cut your backup tables as precisely as you care to, but I wouldn't sweat every possible depth, so long as your tables are at least cut for deeper than you are. So your computer dies at 45 minutes at 120', but you've only got tables for 130'? Who cares, just do the extra deco. On Vplanner the difference is 20 minutes vs. 26, would it really help you to be able to shave those 6 minutes? Not likely, you'd probably add them right back in anyways, I know I would. BTW, when cutting backup tables I prefer to cut them with less conservatism. I want to know what I NEED to do, I'll do the padding myself.

So how can you plan for failure of both the loop and the computer? Easy. In V-planner, add a one-minute OC line at the worst possible time/depth for your loop to go down. For Gap, tick off for OC deco and select the appropriate bailout from your gas list. You don't need to figure it out at every minute of the dive, just find out what your worst-case deco obligation is, and how much OC gas you need to do it with. Presto! Now you have tables to get you home.

For planning OTUs and CNS limits, V-planner and Gap will show you that. Neither should be a factor at this point though. Good to know, but ultimately you will be limited either by temperature, or how much bailout you feel like bringing with you.


Bailout

Bring lots. Please? Rebreathers have gotten a bad name with some Captains in the Northeast US in part because folks are not taking their bailout needs seriously (for example, diving the Doria with an al40, or even an al20!) Either software will show you how much you need. Then bring more. Think of your rebreather as an abused Doberman you adopted. You can love it, go all sorts of places with it, have lots of great times together, but never forget that it can turn on you in an instant. I like to begin every dive by putting my hand on my bailout reg: this confirms that it is free and builds muscle memory so I can grab it faster without fumbling. I also like to use a sideport reg, so when the excrement hits the impeller I can plug it right in without worrying about where the exhaust T is. Personally I bring an Al80 with me on most every dive but the shallowest, since underwater a 40 and an 80 feel pretty much alike. If I'm going below 150' or so I add an al40 of trimix, and then either hang an al40 of O2, or else factor in the boats hanging O2 reg in my planning. I prefer this method to the more common AL40-bottom mix/Al40-O2 or 50/50, because it gives me more breathable gas at depth. I also plan my bailout so that if I can't get back to my O2, I can still finish my worst-case-scenario deco with what I have on me.

Team bailout is an option some folks like to do, whereby the team carries enough gas to get 1.5 team members to safety. Personally, it's not for me, and not just because I spend so much time solo. I may like you, I may like diving with you, but I'm not willing to cede you the power to make my kids orphans. That's waaaaayyyy beyond my comfort level. If team bailout works for you, just don't ever forget that without your buddies nearby, you ain't gonna make it.


Conclusion

There are lots of ways to skin a cat in terms of inert gas planning, and I hope some of this information proves useful, or at least provides food for thought. A final cautionary note: Extended constant PO2 exposure, along with the deep stop algorithms many of us dive, have nowhere near the body of data that more traditional Haldanean models do. Decompression diving of all flavors has a large element of no-guarantee, hydrate-and-hope statistical whimsy to it, and you can be right, so right, and still be bent like a pretzel. So do your homework, don't push the limits, and never, ever skimp on bailout.


Discuss this article HERE