Question about learning deco procedures

[KenGordon]

Yes, they can, but it gets complicated because the response is really different from different situations.

1. If you are diving using the old system of tables, the tables assumed you were ascending at a certain speed. If you start the ascent near your NDL but ascend slower than the tables call for, you could well have a decompression obligation as a result without knowing it.

2. If you are instead using a computer and ascending very slowly while staying within the NDLs according to the computer algorithm, you are OK.

3. If you are doing deco diving with a software-based dive plan and ascent too slowly to your first scheduled stop, your decompression requirements will have changed as you ascended. As an example, I recently did a technical dive on a wreck, and nearly all the dive teams reported nearly identical dive plans, and every team was using the same computer algorithm. My buddy and I arrived at the ascent line just ahead of the last team to ascend, a team I knew was using the exact same dive plan and the exact same dive computers and settings we were using. When we reached our first decompression stop, I looked back down the line, but they were not in sight. I did not see them emerge from the gloom for several more stops, and they ended up doing 10 more minutes of deco than anyone else did. When asked what happened, they said they did not know--their computer had just kept adding time. I pointed out that they had ascended very slowly from the wreck, and they said, "You're supposed to ascend slowly." Well, the computer wanted them to ascend at 30 FPM, and when they were slower than that, the computer added more decompression time. They were lucky they were using computers. If they were simply following a schedule and then ascending slower than the schedule called for, assuming it did not matter how long it took them to get to the deeper stops, they might have gotten bent.

Some tables are based on arrival time at the stop rather than time leaving the bottom. In that case it doesn’t matter what ascent rate Ian used so long as it is lower than the maximum. Mor3 commonly it does matter.

Divers are generally poor at keeping to the default rates and so computer generated plans are often wrong. You ought to be able to tell though. If the arrival time at a stop is, for example, 60 minutes and you get ther at 63 minutes you are no longer in plan and need to move to a contingency. This should not happen, but it may require altering the default ascent rate to be realistic or going faster.

General paranoia over going up too fast means that actual dives are usually much slower than planned unless the diver has realised and made an appropriate plan.

 

[Edward3c]

Yes, they can, but it gets complicated because the response is really different from different situations.

1. If you are diving using the old system of tables, the tables assumed you were ascending at a certain speed. If you start the ascent near your NDL but ascend slower than the tables call for, you could well have a decompression obligation as a result without knowing it.

2. If you are instead using a computer and ascending very slowly while staying within the NDLs according to the computer algorithm, you are OK.

3. If you are doing deco diving with a software-based dive plan and ascent too slowly to your first scheduled stop, your decompression requirements will have changed as you ascended. As an example, I recently did a technical dive on a wreck, and nearly all the dive teams reported nearly identical dive plans, and every team was using the same computer algorithm. My buddy and I arrived at the ascent line just ahead of the last team to ascend, a team I knew was using the exact same dive plan and the exact same dive computers and settings we were using. When we reached our first decompression stop, I looked back down the line, but they were not in sight. I did not see them emerge from the gloom for several more stops, and they ended up doing 10 more minutes of deco than anyone else did. When asked what happened, they said they did not know--their computer had just kept adding time. I pointed out that they had ascended very slowly from the wreck, and they said, "You're supposed to ascend slowly." Well, the computer wanted them to ascend at 30 FPM, and when they were slower than that, the computer added more decompression time. They were lucky they were using computers. If they were simply following a schedule and then ascending slower than the schedule called for, assuming it did not matter how long it took them to get to the deeper stops, they might have gotten bent.

That’s a really good example of why BSAC use the definition of bottom-time to be:
Time from leaving the surface to time at first deco stop or 6m.

 

[boulderjohn]

That’s a really good example of why BSAC use the definition of bottom-time to be:
Time from leaving the surface to time at first deco stop or 6m.

It is nice that BSAC has that definition, but the definition that counts is the one your decompression algorithm is using, and your actions have to be consistent with that.

 

[ChuckP]

Yes, they can, but it gets complicated because the response is really different from different situations.

1. If you are diving using the old system of tables, the tables assumed you were ascending at a certain speed. If you start the ascent near your NDL but ascend slower than the tables call for, you could well have a decompression obligation as a result without knowing it.

2. If you are instead using a computer and ascending very slowly while staying within the NDLs according to the computer algorithm, you are OK.

3. If you are doing deco diving with a software-based dive plan and ascent too slowly to your first scheduled stop, your decompression requirements will have changed as you ascended. As an example, I recently did a technical dive on a wreck, and nearly all the dive teams reported nearly identical dive plans, and every team was using the same computer algorithm. My buddy and I arrived at the ascent line just ahead of the last team to ascend, a team I knew was using the exact same dive plan and the exact same dive computers and settings we were using. When we reached our first decompression stop, I looked back down the line, but they were not in sight. I did not see them emerge from the gloom for several more stops, and they ended up doing 10 more minutes of deco than anyone else did. When asked what happened, they said they did not know--their computer had just kept adding time. I pointed out that they had ascended very slowly from the wreck, and they said, "You're supposed to ascend slowly." Well, the computer wanted them to ascend at 30 FPM, and when they were slower than that, the computer added more decompression time. They were lucky they were using computers. If they were simply following a schedule and then ascending slower than the schedule called for, assuming it did not matter how long it took them to get to the deeper stops, they might have gotten bent.

I just started AN/DP and haven't got thru the intro yet but is this a good example of why your dive computer should be running the same math as your planning software, ie, Scubapro verses Shearwater?

While running a computer in gauge mode, you'd still have to make adjustments to plan if arriving late at a stop? I have a lot to learn.....

 

[Edward3c]

It is nice that BSAC has that definition, but the definition that counts is the one your decompression algorithm is using, and your actions have to be consistent with that.

The BSAC 88 Tables are based on the definition. That said most just jump in and rely on their computer to tell them what to do.

 

[boulderjohn]

While running a computer in gauge mode, you'd still have to make adjustments to plan if arriving late at a stop? I have a lot to learn.....

I believe that this happens more often than people recognize, and it is a potentially serious problem.

In my first years of technical diving, I used a bottom timer and followed a (usually) predetermined dive plan. Everybody I knew did, and many people still do it that way today. It was years before I saw someone using a computer to guide a dive.

In this system, people usually have a written plan and two contingency plans written on slates or wet notes. Frequently the plans tell them when to leave the bottom and at what depth they should do the first stop and how long to stay at each of the stops. The plans assume that the diver will ascend to that first stop at a specified ascent rate, usually 30 FPM. There is also something called "run time," which is how long the total dive should last, including what time you should arrive at each stop. In theory, if you arrive at the first stop behind your planned run time, you will adjust your stops.

In reality, I know darn well that many people, including me at first, focus on the depth of the first stop and the schedule of the succeeding stops without proper regard to the run time. These divers arrive at the depth of the first stop and begin the predetermined plan for depths and times for the stops, and in some cases they do not realize that they had arrived at that first stop long after the planned run time because of their very slow ascent. I know one specific case in which two divers got bent doing that, as was revealed when they ran the dive profile from the computer they had in gauge mode. (It also revealed that they were deeper than they thought for much of the time, and it showed they miscounted their time on the last stop--all errors that a computer would have caught.)

Divers who do this kind of diving without the ability to check the profile of a computer later may finish many a dive believing they executed the dive perfectly without ever knowing how much they screwed up. "Well, maybe you can make mistakes, but I don't," they can safely say, knowing there is no way to tell if they did or not.

 

[boulderjohn]

Here is another way that people can make an error using pre-planned schedules. (I am thinking of a couple individuals I know right now.) They plan a dive for a certain bottom time and depth, and they follow that written plan to the letter. Almost.

The bottom time they calculate starts at the surface, so it includes the descent time. There are different ways to descend. Some people take a leisurely descent, while others like to drop like rocks, getting to the bottom as soon as possible, many going over 100 FPM as they drop. Dive software programs have default descent speeds, often about 50 FPM, and if you do;t plan for that, it can make a significant difference. Let's see how that difference affects a dive by comparing two divers on a dive to 300 feet.

Diver A follows the software default speed of 50 FPM. It takes him 6 minutes to reach the bottom, so for the first 6 minutes of his dive, he has averaged 150 feet of depth. Diver B drops at 100 FPM, reaching 300 feet in 3 minutes. He averaged 150 feet for the first 3 minutes and 300 feet for the next 3 minutes, so after 6 minutes he has averaged 225 feet.

Check out the software and see how much difference in dive plans arises from spending 6 minutes at an average of 150 feet compared to spending 6 minutes at an average of 225 feet.

 

[ChuckP]

I haven't bought V Planner or planning software yet but will and am curious as how much that effects a plan.

How does your GF effect plans? How do you choose your GF? Does the GF you choose change if your diving to 150' verses 300'? I dive a Scubapro now but am switching to Shearwater for purely planning purposes.

Some of these questions are why I was so torn about who to go to for the training. I choose to stay local for classroom and some dives but will complete most dives elsewhere. Local is old school thought thou, this whole science of diving is constantly evolving and some new things turn out to be not so right and some to be way better. Deco diving appears to be one huge gray area and I for one really do not want some doctor telling me I can't dive for 30 days cause I just got hurt.

Maybe I'm wrong but the physical part of deco diving is easy - it's the brain part that poses the challenge.

 

[scubadada]

Hi @ChuckP

V-Planner runs VPM only, probably not what you are looking for. MultiDeco runs Buhlmann and VPM. There are multiple planning applications running Buhlmann. Others can advise you as to what they use

Sounds like you have quite a bit to learn based on your questions

 

[boulderjohn]

How does your GF effect plans? How do you choose your GF? Does the GF you choose change if your diving to 150' verses 300'? I dive a Scubapro now but am switching to Shearwater for purely planning purposes.

Some of these questions are why I was so torn about who to go to for the training. I choose to stay local for classroom and some dives but will complete most dives elsewhere. Local is old school thought thou, this whole science of diving is constantly evolving and some new things turn out to be not so right and some to be way better.

The most important part of what you wrote is the part I bolded above. It is indeed constantly evolving. My technical diving processes today are very different from that I did only a few years ago and far different from what it was like when I started. It is critical that a technical diver keep in touch with the overall diving community in order to stay in touch with the latest research and thinking. I had some recent discussions with people I had not seen in years and was quite surprised to learn that they are completely unaware of what has been happening.

I wrote a blog about that a few months ago in which I pointed out how very hard it is to keep up with things because so many of the people who profess to be experts keep repeating outdated information for others to read and become misinformed.