How much wiggle room?

[TSandM]

I completely agree, AJ. On this particular dive, we had agreed ahead of time to dive the RD profile I had come up with. Where we got into issues was when Peter's computer disagreed with the RD profile (something that was unexpected, and due to the setting change) and he decided he did not feel confident enough of the RD profile to naysay the computer. This is one of the issues that's described regarding diving computers for tech, which is that unless the entire team is diving the same device or at least the same brand with the same settings input, the profiles may be different, leading to problems completing the dive as a team.

But Krys wanted an example of how a team could end up deviating from a plan, and I happened to have one . . .

 

[waterpirate]

We all can agree that the point of rigid schedules during training is to " train ". Everything after that training is all part of the big lab project and up to the individual and or the team. AJ is dead on in this issue needs to be discussed prior to entry to avoid a confusing exit.
Eric

 

[kr2y5]

I guess it's natural to think of a plan made beforehand as an "envelope", as someone here put it, that's how I always understood it as well... whether it's max depth and max deco, or the whole ascent schedule that you plan your gas reserves around, and thus can't be "late" for, it's the boundary that you will stay within, and likely won't even get close to. If my understanding of the discussion so far is correct, then at least some people would be comfortable going beyond the "envelope" approach, and leave more to be decided on the fly... I guess with enough experience, this is perfectly feasible, and it all makes sense... This is all just so very different than what I always used to hear... what has been posted in this thread so far is really interesting. Thanks, everyone.

 

[TSandM]

Really, the determining factor is gas. You need enough gas to do the dive, maintain adequate reserves for contingencies, and to do the deco you propose to incur, even if one person's deco gas is completely lost. You cannot expand the "envelope" of the dive beyond what you have the gas to do. You can always make it smaller, or shape it differently, so long as you do not violate the rules on safe reserves.

 

[PfcAJ]

Wiggle room stuff gets bananas if you have a rebreather. Coming from OC tech world and moving into RB tech world, its night and day.

 

[kr2y5]

Really, the determining factor is gas. You need enough gas to do the dive, maintain adequate reserves for contingencies, and to do the deco you propose to incur, even if one person's deco gas is completely lost. You cannot expand the "envelope" of the dive beyond what you have the gas to do. You can always make it smaller, or shape it differently, so long as you do not violate the rules on safe reserves.

Right, the interesting part is what defines that "envelope". Do you express it in terms of a specific "worst case" ascent schedule, for which all the gas has been provisioned (including lost gas contingencies as well as +5 min., +10 feet, etc.), and you simply make sure you don't run "late" with respect to pre-established runtimes (that's the story I was given in the class), or do you define that "envelope" in terms of all the remaining gas reserve itself; the latter leaves more flexibility, but makes things a little more complicated during the dive. Would it be accurate to say that on bigger dives or dives you haven't done before, you do the former, while on the semi-"recreational" dives, you default to the latter?

---------- Post added April 25th, 2015 at 09:51 PM ----------

Wiggle room stuff gets bananas if you have a rebreather. Coming from OC tech world and moving into RB tech world, its night and day.

Interesting. Tell me more, please...

 

[PfcAJ]

Interesting. Tell me more, please...

So above when Litehedded said our planned bottom time range tomorrow will be between 40 and 50mins, with contingency for 60, that means the overall run time could be between 3.5 and 5.5 hours. I've been on a dive where my buddy and I were initially planning on a 4 hour dive and we (intentionally) extended it to 8 hours.

If you've got the deco gas layin' around to cover an OC ascent, you can end up on the hook for some REALLY long decompressions (intentionally or not).

Obviously, these are rather extreme examples, but it shows what you *can* do with a RB. Its a powerful tool, but also makes it easy to bite off more than you can chew if you don't have the right resources and mindset in place.

 

[boulderjohn]

Here is an example of varying a schedule.

My last dive a few days ago was with a student. We had established his SAC rate pretty well, and we planned a dive with what we thought was plenty of reserve gas, both for the bottom part and the deco. Our plan was to follow a pre-planned deco schedule, with computers as a backup. For some reason he went through his bottom gas much more quickly than we planned, although he never got to the reserve. His surprising air hoggishness continued in the deco, and that is where it become a problem. We followed the schedule perfectly as planned, with him leading the deco, but when we still had a lot of time left in the last stop, I saw he was into the red zone on his SPG, whereas I was clearly going to finish with most of my deco gas. (We started with the same amount.) I gave him my deco gas and switched to back gas, noting that on my computer. When we had two minutes left on the scheduled stop, I had him switch back to his deco bottle to finish the stop while I took my deco gas back. My computer now showed that I owed a few extra minutes, and I followed the computer rather than the original plan. When he got really low on his deco gas, he switched to his back gas so he could stay with me, but he had already completed his deco as planned.

I think my history might be helpful in understanding things:

1. When I first started my technical training, we followed the profiles provided by desktop software--no computers allowed, even as backups.

2. Then my instructor crossed over to another agency, and we all had to cross over with him or go without instruction. We were now required to plan the dives using ratio deco and follow those profiles, with no computers allowed, even as backups.

3. Then I switched to a different agency and instructor, and we went back to following profiles from desktop software, but this time computers were allowed as backups, although we never needed them. (Actually, he brought a computer as backup--I didn't have one then.) Despite the fact that we never needed them, the instructor tended to make sure that the computer was clear and happy before we left the water, leading me to wonder which system we were actually using.

4. Then I began to hobnob with instructors and other divers who planned dives and wrote the schedules down, but used them as the backups to the computers that they followed during the dives. I did a pretty fair amount of decompression diving in Florida in February and march this year, and although I did not exactly quiz every diver on the boat, based on my observations of their behaviors both before and during the dives, I would guess this approach was used by the majority of divers. In a recent ScubaBoard thread that I cannot find, someone ran a poll asking people what system they used. When I last checked, this one was in the lead.

5. Although I have never been in this situation, we had a thread on ScubaBoard in which a very well known diver said he and his buddy each carry two computers, all four of the same model with the same settings. They follow the computer, figuring the odds of four computers screwing up are pretty remote.

In my experience on ScubaBoard, the people who follow systems 1 and 2 above are extremely vocal about the vast superiority of their practices, and they will put down any other approach. The people who follow system 3, 4, and 5 keep their mouths shut because they know they will be criticized. This creates the illusion that systems 1 and 2 are the only way tech diving is done.

 

[Trace Malinowski]

New tech divers often feel a lot of time pressure due to the importance normally placed on dive planning and decompression. But, as you gain experience you'll discover that there is a lot of wiggle room involved in most any technical dive conducted for recreation. As was pointed out gas is normally the determining factor in open-circuit diving. But, other factors may also influence a diver's comfort zone. In rebreather diving, most of my friends are concerned with their thermal comfort because, as AJ pointed out, his dives are now longer than half a corporate work day. I recently had an ocular issue due to oxygen toxicity so for me oxygen exposure is my now greatest concern. When following safe diving practices most divers can handle the physiological stresses quite well. As we age or lose fitness ability we need to plan more conservatively. Due to the cost and the experience needed many divers don't start technical diving until they are past their physical peak. We often consider age and fitness into the dive plan.

When dive planning we are able to set up a playing field based upon worst case maximum parameters for combinations of depth, time, gas, decompression, oxygen exposure, thermal comfort, lost deco gases, deco time you are willing to blow off for an emergency, omitted deco and so forth. The dive plan usually sits well with these boundaries allowing you to plenty of room to have fun and avoid stress and pressure due to time, gas, etc.

Today's decompression usually follows bubble models employing initial deeper deco stops, Buhlmann models where stage decompression is done in shallower water, or combinations of both along with other adjustable strategies such as utilizing oxygen windows. A thorough understanding of various models and ascent strategies may help you in the event that you need to change your ascent due to any number of issues. For example, a dive team low on deco gas on a bubble model may decide to switch to Buhlmann where gas would last longer in the shallows.

The question, "What can we do?" sets up the field.

The question, "What should we do?" keeps the players from stepping out of bounds.

 

[MXGratefulDiver]

Here is an example of varying a schedule.

My last dive a few days ago was with a student. We had established his SAC rate pretty well, and we planned a dive with what we thought was plenty of reserve gas, both for the bottom part and the deco. Our plan was to follow a pre-planned deco schedule, with computers as a backup. For some reason he went through his bottom gas much more quickly than we planned, although he never got to the reserve. His surprising air hoggishness continued in the deco, and that is where it become a problem. We followed the schedule perfectly as planned, with him leading the deco, but when we still had a lot of time left in the last stop, I saw he was into the red zone on his SPG, whereas I was clearly going to finish with most of my deco gas. (We started with the same amount.) I gave him my deco gas and switched to back gas, noting that on my computer. When we had two minutes left on the scheduled stop, I had him switch back to his deco bottle to finish the stop while I took my deco gas back. My computer now showed that I owed a few extra minutes, and I followed the computer rather than the original plan. When he got really low on his deco gas, he switched to his back gas so he could stay with me, but he had already completed his deco as planned.

I think my history might be helpful in understanding things:

1. When I first started my technical training, we followed the profiles provided by desktop software--no computers allowed, even as backups.

2. Then my instructor crossed over to another agency, and we all had to cross over with him or go without instruction. We were now required to plan the dives using ratio deco and follow those profiles, with no computers allowed, even as backups.

3. Then I switched to a different agency and instructor, and we went back to following profiles from desktop software, but this time computers were allowed as backups, although we never needed them. (Actually, he brought a computer as backup--I didn't have one then.) Despite the fact that we never needed them, the instructor tended to make sure that the computer was clear and happy before we left the water, leading me to wonder which system we were actually using.

4. Then I began to hobnob with instructors and other divers who planned dives and wrote the schedules down, but used them as the backups to the computers that they followed during the dives. I did a pretty fair amount of decompression diving in Florida in February and march this year, and although I did not exactly quiz every diver on the boat, based on my observations of their behaviors both before and during the dives, I would guess this approach was used by the majority of divers. In a recent ScubaBoard thread that I cannot find, someone ran a poll asking people what system they used. When I last checked, this one was in the lead.

5. Although I have never been in this situation, we had a thread on ScubaBoard in which a very well known diver said he and his buddy each carry two computers, all four of the same model with the same settings. They follow the computer, figuring the odds of four computers screwing up are pretty remote.

In my experience on ScubaBoard, the people who follow systems 1 and 2 above are extremely vocal about the vast superiority of their practices, and they will put down any other approach. The people who follow system 3, 4, and 5 keep their mouths shut because they know they will be criticized. This creates the illusion that systems 1 and 2 are the only way tech diving is done.

... in the real world I find that people mostly want to know that you put some thought into your decisions, and that you have a pretty good understanding of why you make the choices you do ... competence and process aren't the same thing ...

... Bob (Grateful Diver)