Bottom Timer and Tables vs. Computer

[vbcoachchris]

I think the issue is to understand what the computer is actually doing. As mentioned earlier in the thread, we are not even sure the computer is giving you the proper deco (It will however probably give you an adequate deco). Just because the numbers are on a digital display does not make them correct.

Most computers use a Buhlmann algorithm set 30/85 and then add some additional padding. A few like Suunto use RGBM and then add some additional padding. There are even a few VPM models out there.
What is the correct deco? If anyone has a computer that does the correct deco calculation; I will take 2
If you have a computer based on Bulhmann tables and you are comfortable with this algorithm there is no reason not to listen to it. If you like RGBM or VPM and are set on using a computer you should most likely get one that uses these algorithms.

I find the math used for most of the dives we all do (400 ft and shallower and bottom times of 3 hours or less) is very SIMPLE. You do not need a PhD in math to figure the profiles out. If fact you don’t even need a calculator.

So for most people the argument comes down to a few things.

Should I have machine to do a simple task for me or should I perform simple math in my head?

Should I spend $300-$2000 on a calculator to do basic math or should I just spend $150 to get only essential data (depth and time) and spend the remaining money on gear I want, like a canister light or another steel cylinder?

A computer is a tool. I have always felt like you should not use a tool unless you know how to operate it. There are lots of tools that can get you in trouble if you do not know how to use them properly (drills, saws, and even kitchen knives). I once had a buddy bolt to shallow water and leave me alone at 100ft alone, because his computer only said he had 1 min of bottom time remaining. Several things were poorly planned on that dive, but a rapid ascent and buddy separation which can be dangerous was performed because of the data he saw on a digital screen. I have seen people set the computer to Nitrox 32 and forget to change it back to air the next time they dove (which was on air).

At one time I felt like I needed a computer to get me out of the water safely. Then for a time I dove tables with a computer back up. Eventually I realized that I did not need my “binky” and longer, as it wasn’t giving me any additional useful information. I dive in a team, so my team member is my back up. We on agree on a deco before we go down and if the plan changes (as it often does underwater; we agree on the change in decompression before we start the decompression clock. We could ask what if I lost my buddy and my bottom timer, but how many what if’s can you possible plan for. Most divers I know plan for one major issue, then it is time to get out of the water.

I have often wondered why we change our philosophy when we move from recreational diving to technical diving. Very few of us dive computers for tech diving, we all seem to cut tables, because we feel it to be safer for us, yet some still need to have their “binky” confirm they are getting out safely.

I teach dive planning in ALL my classes. I do not think anyone should just go underwater with no planned bottom time and come up when the computer tells them they should. I believe you should have a plan (even if it is a rough plan) and try to execute it on every dive.

I hear the argument all the time about a computer being my back up. I guess I am just confused on what it is backing up. Either you know a deco to get you out of the water safely or you are going to use a computer. If you are going to use a computer and you trust that model of computer it seems logical to me that you should dive an identical model of the computer for a back up and if you trust the tables you cut then bring another copy (and place it in a different location) as your back up. I just don’t see the reasoning behind diving a computer that doesn’t agree with the tables you cut. It seems a lot like two divers who are diving together, but are running different schedules (most of us would not do that).

Unless we ever truly know for sure how all this stuff really works, there will never be a correct answer.

In the end it comes down to what you (and your team) feel comfortable with.

The free Decompression workshop I will conduct in mid September will cover computer diving as well.

Scott

[mattwave]

In the end it comes down to what you (and your team) feel comfortable with.

Scott

:smt038

[Joshua Smith]

[Nailer grabs his "binky", jams his thumb in his mouth, and squints suspiciously at vbcoachchris]

[CaptnJack]

I will offer the following recreational DIR-ness lol.
Computers are fine for recreational dives although you might find DIR types actually doing a little less "no deco" time at intermediate depths (80-50ft). Generally my times at 90-110ft on 32% are very similar to computers, at least for the first dive.

On repetitive dives I find that I can do much shorter SIs than a Vyper just fine. 5 dives (50-70ft) a day in Catalina, 45-75min SIs, no big deal. A Suunto would flip out at that.

I find that I do much more time at 40 up to 20ft than a Suunto wants. Then less from 19ft to the surface. The Suunto likes to just arbitrarily start a 3min timer at 19ft. Not really the way I dive.

Those are the "big" if you could call them big differences between my recreational dives and the Suunto RGBM approach.

NAUI 32% tables are too liberal for me. I don't have them memorized exactly but I think they give 80 or 90mins at 70ft. That's too much for me (and a Suunto for that matter I think). "My body" gives me about 50-60mins at 70ft. After that I would be padding the shallow stops like mad. How much would be based on how much 30-40ft time was in there and how far over my limits I was. So far this is only relevant in MX cause I am not doing hour plus dives in Puget Sound at 60+ft on 32%.

For you, the Suunto is really conservative. Which is probably keeping you from feeling like poo. Which is fine recreationally. I'm not sure if there's an "equivelant" deco computer out there. Most of them use Buhlmann shaped algorithms which are basically using pressure reductions ("only") to offgas and not really using all the tools in your toolbox. If I had to wrap up DIR deco in one sentence it would be to not use just the hammer but to research all the tools in the toolbox to create a "blended" deco schedule using deep stops, gas switches, and pressure reductions. To date, I don't know of a deco algorithm uses all that info, some of it only known from experience.

[vbcoachchris]

[Nailer grabs his "binky", jams his thumb in his mouth, and squints suspiciously at vbcoachchris]

Don’t be too hard on me I still dive my “binky” I just keep it in gauge mode now. Nothing like a $600 Bottom timer.

Scott

[Joshua Smith]

[Nailer grabs his "binky", jams his thumb in his mouth, and squints suspiciously at vbcoachchris]

Don’t be too hard on me I still dive my “binky” I just keep it in gauge mode now. Nothing like a $600 Bottom timer.

Scott

It's cool, it's cool. I strongly believe everyone should dive the way they want to, and if that means you wanna put your binky in "guage binky" mode, so be it. I prefer "full binky."

[dsteding]

Good discussion. Putting my mod hat on, I'm really happy to have non-DIR types posting in here, loanwolf, nailer, others, feel free to engage in the dialogue. My only request is that it remains a bit centered on the "why do DIR/UTD types not use computers"(because of where this thread is located) and not become a "computers versus non-computers" flame war. Most of this information has been covered elsewhere before, but not on this board, and I think it is good to add to the knowledge base here. UTD people like Lynne, Richard and Scott, thanks for participating. CCR gang, thanks too. Pez, thanks for starting this thread.

We've been civil on what can be a hot-button topic. I'd prefer this place to be one for open dialogue on the thinking behind how we dive, and that necessarily involves questions from all types of divers.

Since this is a new forum, thought I'd point that out. I'm off to Montana for the weekend, I'm sure everyone will continue to play nice.

[John Rawlings]

This debate will be with us for a long, long time.....and it's a good one to have.

When diving recreationally I dive with two computers.

When tech diving on Open Circuit I cut tables using V-Planner. I then follow the tables religiously (with 2 "back-up" plans) and set one computer to Gauge mode.

When diving CCR I use an integrated computer along with a non-integrated back-up. With deeper CCR dives I also cut tables (again including back-up plans) and follow the plan.

So, basically I find myself in all 3 "camps" from time to time depending on the type of dive I am doing.

Each "method" is merely one of many tools a diver can successfully use. Personally, I have no problem with any choice that a well-informed diver chooses to make regarding how they will handle their decompression on a given dive, so long as their plan doesn't consist solely of hoping that "God will take care of me no matter I do".

On the other hand, I know divers that have been bent (myself included) using each of the methods mentioned in this thread - computer only, tables/computer back-up, tables only, ratio-deco, etc....you name it, each method occasionally produces a bent diver.

- John

[Penopolypants]

[Nailer grabs his "binky", jams his thumb in his mouth, and squints suspiciously at vbcoachchris]

I have coffee dripping out of my nose. Yay Friday!

[Sounder]

[Nailer grabs his "binky", jams his thumb in his mouth, and squints suspiciously at vbcoachchris]

I have coffee dripping out of my nose. Yay Friday!

That'd make a pretty good quote for a signature line...

[lundysd]

Having experienced 2 independent catastrophic computer failures at depth, I can attest that trusting your life to a piece of electronics in an aquatic environment does have its downsides. The first one scared me the most -- I was a younger diver with a Mares computer, and on a night dive at 80ft it just froze in the on position. Time, depth, deco time remaining -- it all displayed, it just didn't change or update throughout the dive. After 2-3 times of checking my computer I realized that something was wrong and I ended the dive, but it wasn't as obvious as one might think to spot -- I mean how often to we actually sit there and watch our computer rather than glance at it during a dive? Can you imagine if it was air-integrated and I blindly trusted it to tell me I had gas left? I shudder thinking about it

I still use a computer for most of my dives, but I now dive it with the understanding that it can and will fail, and I will be transitioning to a bottom timer (or two) as soon as my comfort level allows it.

[LCF]

I think the kind of diving most of us do here in the Sound is actually the hardest diving to figure out how to manage. Unlike most tech dives, which are done on a specific target and are often square profile, our shore diving tends to be terrain-based and wander around. Even once you begin to read about tools like depth averaging, it's still hard to know how to do it for our profiles. Discussions of depth averaging say that you shouldn't depth average a dive that goes to 100 feet and then up to 40 for a while, but what if the dive just WANDERS from 100 to 40? How do you figure your average depth? Where do you begin offgassing? How much time in the shallows offsets time spent at depth? It's very confusing, and this is where computers excel, is in doing iterative calculations.

After you've done enough dives, you begin to get a very strong gut level sense of what is happening during a dive. This may be based on what your computer has told you, or what you're trying to figure out on your own, or a combination of the two, compared with how you feel afterward. At this point, your confidence in managing without a computer begins to increase, and eventually, you'll get in the water without remembering to set your Suunto for 32%, bend the thing and ignore it, and the next day it will be in gauge mode (at least, that's how Lamont and I did it!)

When I went to Cozumel, and was going to spend a bunch of time diving profiles that were not familiar to me, and were deeper, I thought I'd need my computer or should have it as a double check. Turned out I didn't need it, and I ending up bending it the way I described anyway . . .

As far as using the computer for a backup, I used to think I'd get confused or forget to take the "snapshots" to assess my profile, or something bad might happen and I'd get flustered. If something bad happens, the dive's over and you don' really CARE what your NDLs are any more (note: This is purely in reference to recreational diving, which is what the OP is currently doing). And it turned out that, even if I miss a snapshot or two, I watch my depth frequently enough to have a very good sense of where the dive has been and how long it has been there.

[CaptnJack]

I'm having a hard time thinking of any shore or even boat dives which are wandering from 100 to 40 and back again. Do you have an example? (other than lets go to the I-beams, wait no the Honey Bear, wait no the I-beams, lol)

[Sounder]

I'm having a hard time thinking of any shore or even boat dives which are wandering from 100 to 40 and back again. Do you have an example? (other than lets go to the I-beams, wait no the Honey Bear, wait no the I-beams, lol)

I think she was talking about the time spent coming up-hill, slowing that proper-ascent, with "stops" along the way at the warbonnet, the crab, and the octopus, eventually getting to 40fsw.

But to address your question... it sounds like ascent practice to me!!

[BDub]

I'm having a hard time thinking of any shore or even boat dives which are wandering from 100 to 40 and back again. Do you have an example? (other than lets go to the I-beams, wait no the Honey Bear, wait no the I-beams, lol)

I think she was talking about the time spent coming up-hill, slowing that proper-ascent, with "stops" along the way at the warbonnet, the crab, and the octopus, eventually getting to 40fsw.

But to address your question... it sounds like ascent practice to me!!

In that scenario, you've started your ascent. I'm not factoring that into my average depth.

Ok, so now I'm lost too (it's Friday, so not that that's hard to do)

[loanwolf]

Bottom timers and cut tables are good. But they do not give you the freedom and flexibility a computer can give you. A good technical dive computer will figure out the little things that have been mentioned as well as many other parameters including workload at depth and sac rates to figure total deco. One should alway have backups ie spg's, timers, and cut tables or be ready to do ratio deco also as backups. All they are are tools in the toolbox. I always dive with two computers and cut tables as backup. I have only had 1 technical computer ever fail on in thousands of hours of bottom time, i took it too deep and folded it. Hell I folded a UWATEC BT a few weeks ago in Monterey.

http://www.divecochran.com/cgi-local/ge ... parison.pl

[CaptnJack]

By the time you get to 40ft there's no point in depth averaging anymore. You'll freeze or run OOG long before you'll have a "deco" issue.

For some reason I imagined 100-40-100-40 sawteeth. If that were the case (and I don't know of any local sites doing that) I'd just call it 80ft (depending on time at various depths) and be done with it. Offgassing from a 100ft dive ain't happenin' below ~40ft anyway (gotta get ~2ATA up). Not a friendly profile regardless of what the computer tells you.

[Sounder]

Offgassing from a 100ft dive ain't happenin' below ~40ft anyway (gotta get ~2ATA up). Not a friendly profile regardless of what the computer tells you.

...but this sounds like a "rush to 20fsw and hang there" argument, and I know that's not your preferred plan for dives within NDL.

[camerone]

For some reason I imagined 100-40-100-40 sawteeth. If that were the case (and I don't know of any local sites doing that) I'd just call it 80ft (depending on time at various depths) and be done with it. Offgassing from a 100ft dive ain't happenin' below ~40ft anyway (gotta get ~2ATA up). Not a friendly profile regardless of what the computer tells you.

Was out at Neah Bay last weekend which is pseudo-local, I guess, depending on where you live. We were with Cap'n Mike and the awesome Mark V, and had something very similar to that profile one one of the dives. Not quite as extreme, but a good 40-80-40-80-40 type run. We put in at the last Box Canyon with the cool sea caves, and I took the scooter out on a single dive, and did that cove, Mushroom Rock to the east, and whatever the cove to the west is (unintentionally). I missed the turn off coming back... not making progress against the current with the scooter running in 5 told me I'd gone a little far... In/out/in/out of the coves will do that kind of a profile to you.

It's best to err a bit conservatively here, which lends itself towards picking a deeper "average" depth, but I don't really have a good rule of thumb - 40->100 sawtooth is a lot different than 220-280, in terms of relative pressure changes.

FWIW, I dive two computers - the integrated one in the rebreather, coupled with a VR3 on my other arm as backup. If I'm doing a serious tech dive, I will cut tables as an all-out backup and tape them to my light ballast, although they're last resort, as either computer can do open circuit bailout if needed. However, IMNSHO, the redundancy of the two computers is more than adequate to cover any worries I'd have. It's important from a planning scenario to make sure that I have enough worst-case gas, but my tables are cut so that they're more conservative than what either computer will give me. They give me an idea of what I'm going to expect, and a ground me against the computers' displays during the dive.

[CaptnJack]

Offgassing from a 100ft dive ain't happenin' below ~40ft anyway (gotta get ~2ATA up). Not a friendly profile regardless of what the computer tells you.

...but this sounds like a "rush to 20fsw and hang there" argument, and I know that's not your preferred plan for dives within NDL.

You need to get off the bottom, period. You aren't offgassing at those silly pauses that GUE teaches (recreationally) at the 75% depths and also not at the 50% depth that NAUI is teaching (taught?). E.g. stopping at 70, 60 or even 50ft on a 100ft recreational dive is not doing you any good. You are just lengthening your bottom time at those depths. (although it is a good idea to do a "speed check")

From 100ft you need to get up to 40-45 before you are offgassing, no doubt about it. You can look at this in any software package, buhlmann, v-planner, VPM, decoplanner, whatever. Deco dives are a completely different ball of wax using additional tools and having different 'liabilties'.

Alot of what is taught in "DIR" recreationally are building blocks to more advanced diving. Recreationally, a 30sec stop at 60ft with 30sec ascent to 50ft is trivial so they teach that to build better ascents in general. But if you look at the work DAN and Maroni have done you'll see that these deeper (70-60 ft) stops are not optimal recreationally.

[CaptnJack]

Was out at Neah Bay last weekend which is pseudo-local, I guess, depending on where you live. We were with Cap'n Mike and the awesome Mark V, and had something very similar to that profile one one of the dives. Not quite as extreme, but a good 40-80-40-80-40 type run.

I'd do that profile no problem. Anything greater than a 1.75 ATA swing is where I'd really really hesitate (I'd be doing shortish stops).

[dsteding]

Bottom timers and cut tables are good. But they do not give you the freedom and flexibility a computer can give you. A good technical dive computer will figure out the little things that have been mentioned as well as many other parameters including workload at depth and sac rates to figure total deco. One should alway have backups ie spg's, timers, and cut tables or be ready to do ratio deco also as backups. All they are are tools in the toolbox. I always dive with two computers and cut tables as backup. I have only had 1 technical computer ever fail on in thousands of hours of bottom time, i took it too deep and folded it. Hell I folded a UWATEC BT a few weeks ago in Monterey.

http://www.divecochran.com/cgi-local/ge ... parison.pl

Obviously, given where this thread is, this would be the opposite side of the coin from the approach we take to deco. But, I'm cool with that.

I do run VPM to get an understanding of where I am going in terms of dive profiles, but you can have the same "freedom and flexibility" using some simple rules. If anything, being able to recalculate your deco on the fly will give you the same freedom as a computer, and allow you to shape the profile to what you are comfortable with.

Scott mentioned he'd buy two computers if they calculated the profiles he used to running. I may as well. But, one doesn't exist. Nor do I really want to spend north of a $1000 on one. I recently was offered a good deal on an X1, and declined because it wasn't necessary or needed for me.

As for air consumption,I know my SAC rate cold in various conditions, and have been calculating and predicting my gas consumption on the fly for the past 200 dives or so. I don't need a computer to tell me what that is-and in fact, I'd hazard that this skill is lost on those divers that use an AI computer, because they learn to rely upon it to do it for them.

The reality is there is limited bandwidth in terms of attention underwater. By paying attention to depth, time, and gas consumption for a long time as a recreational diver, I found that the transition to tech diving was an easy one in that respect. I'd already gotten those skills to the point where depth, time, and gas were intuitively monitored and kept track of. This allowed me to layer upon that the additional tasks with relative ease-and where I found myself buddy breathing, keeping track of a maskless diver, and managing an ascent, I found that time and depth were still easily managed.

So, although I'd agree that weening yourself off of computers seems like a daunting practice at first (Pez, going back to your post a while back about "sticking with a computer") by doing so I found I reduced the bandwidth required to keep track of those things. Made my transition to tech diving easier.

I really don't need a backup timer as well, because I'm quite comfortable with the ability of my teammates to keep track of depth and time. The likelihood of two or three failing is miniscule, and even if they did, I'd shoot my bag which has ten foot knots on it. Probably do my deco using my gas consumption as a proxy for time (my deco SAC is 0.4-0.5, know that, easy to track time).

[LCF]

I guess everybody figured out that I wasn't talking about a sawtooth, but our typical shore dive, where you follow the bottom contour down to 100 feet, back up to 60ish and wander around, up to 40ish and wander around . . . What's your average depth, for figuring out how long you can safely be down there? At what point are you "ascending" and no longer accruing bottom time? The answers to these questions are not obvious, and that's where I think studying some decompression software printouts and noting where offgassing begins, and doing some reading, and attending things like Andrew's Ratio Deco seminar (which is far more than Ratio Deco) all help build the intellectual tools to make choices about what data to gather and how to group them.

There are a lot of people out there diving recreationally and using computers. There are fewer technical divers doing so, but there definitely are some. It is possible to dive safely in both realms without a computer doing iterative calculations for you. There are people who believe that it builds a stronger awareness of your dive, if you do these things for yourself, and there may well be some merit to that. There are definitely tools to do this yourself that work.

[lamont]

Did a scooter dive recently that was 20 mins @ 100 fsw at the I-beams, followed up by heading up to 50 fsw and attempting to follow that contour out past cove 1, but winding up hitting 90 feet accidentally. Buddy was on 21%, so we did some decompression shallow.

Generally around here if you're diving 32% to around 100 fsw you want to come off the bottom at around 30 minutes. At that point you need to ascend promptly to around 60 fsw to stay out of decompression. Once you're up at 30-50 fsw, you've got a lot of time before the slower compartments start to hit their limits. If you double the time at 100 fsw and add it to the 30-50 fsw time, that'll give you a fairly good estimate of your loading into the controlling compartment at that depth, and at 50 fsw on 32% you've got so much NDL time that you can spend 30 minutes there even with all the loading from 100 fsw. Then if you spend 10-15 minutes coming up from 30 fsw to the surface that'll fix any brief excursions across the NDL line.

You should also try to keep it so that your bias is towards ascending. Its better to head down to 110 and then wind up at 90 fsw 30 mins into the dive (e.g. olive's den to the I-beams). Similarly, don't ascend from 100 fsw to 40 fsw and spend 30 minutes heading down to 60 fsw. If you do stuff like that, all bets tend to be off (so in the case where we wound up back at 90 fsw, we did a bunch of time at the honey bear -- which cleans up some of the faster compartment stuff -- and then basically took the time we spent going over the NDL and did 1.5x that in backgas deco).

[LCF]

Lamont, that post is a perfect example of what I was trying to say in mine above. You have the tools and the understanding of what's going on, to be able to decide what you need to do, based on where you have been and for how long. Being able to envision the model and how it is going to assess loading in various compartments, gives you a good gestalt for where you need to go and how long you probably ought to stay.

I'll add that, if you're shore diving 32% in Puget Sound, it's highly likely that thermal considerations will keep you out of deco. The 32% keeps you relatively shallow, and having to get back to shore before you're a popsicle means you spend a fair amount of time in that 40 feet and up range. Since I generally aim for no more than about 60 minutes because of cold, I'd really have to be boat diving to get into trouble on 32%.