Decompression limits and long safety stops

[tursiops]

This dive had a surfacing GF of 73%. The GF on arrival at the safety stop was 21% and decreased to 9% at the end of the safety stop. It blossomed to 73% over the 1:20 final ascent. It's eye opening to see the increase of the GF during the final ascent. I try to make my final ascents over at least a minute.

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Nice example!

 

[Blackcrusader]

Nice example!

Yeah one of the nice things with uploading logs to the Shearwater cloud is being able to review things like this. As you move your mouse along the GF line you can see the GF. Mine was max 30% at the final surface. Actually the last 20 minutes of this dive was on a reef at around 5m depth so was a really long safety stop. I was actually looking for things to take photos and video of.

 

[Jafo19D]

Humble pov from a new diver.

I think it’s important to know how to use a dive table and more importantly why you’re using it. I don’t think I’d be able to use it today but I understand its purpose.

I did my OW about 4 years ago and my AOW a year later. In OW I don’t recall being taught to use a computer, maybe they leant me one but if they did I certainly didn’t learn to use it. In AOW there was a requirement to use a DC but I wasn’t familiar with it so it was just a “check the box” task.

I’ve been diving a while now with my Mares Nemo Wide and I’m very familiar with it and every once in a while I’ll go over the manual. It’s been a while since I’ve learned something new from reviewing the manual but even today there’s a data point on the first screen (post dive) that I have no idea what it is and the manual isn’t helping.

Anyhow I wish there was quality time devoted to DCs in OW but I guess it’s not practical given the variety of the same.

 

[tursiops]

Anyhow I wish there was quality time devoted to DCs in OW but I guess it’s not practical given the variety of the same.

I'm sorry your OW didn't cover DCs. Many do.

 

[Jafo19D]

I'm sorry your OW didn't cover DCs. Many do.

They were mentioned but they didn’t go over an example of how to use one

 

[tursiops]

They were mentioned but they didn’t go over an example of how to use one

Poor class.

 

[boulderjohn]

In this day and age of basic scuba instruction, people have a choice of taking a class that focuses on tables or a class that focuses on computers. In some cases, it is a choice between agencies. In other cases, it is a choice within agencies.

In that first sentence, I used the vague term people deliberately because it is not at all clear who gets to make that choice. The new OW student usually has no icea the chice exists and does not know enough to make a decision. The new OW student goes into a shop to take a class, not knowing that the decision was made by the shop management. If the shop management chooses to teach the tables and nearly ignore computers, knowing that the students will almost certainly never actually use the tables for diving and will instead use computers, then the student is an unwitting victim.

 

[admikar]

@Alaskan Scuba Dude, post #5 said the SI was 1 hour and 50 minutes.

@Ahmedben, when you say the second dive was "mostly at depth," do you mean it was mostly at a depth of close to 33 meters, or just that most of the dive wasn't spent on safety/deco stops? The first one is what is usually meant by the question, but unless I've screwed up the math, you'd have to have a pretty impressive SAC rate to spend anywhere near 46 minutes at anywhere near 33m and only use 170 bar.

Also, I'm not a tech diver, but for those in the thread who are: wouldn't that kind of profile on air incur a lot more than 11 minutes of deco? My Teric's deco planner says that after 45 minutes at 33m/100 feet on air, I'd need 4 stops totaling 49 minutes, and that's with no residual nitrogen. Obviously coming up even just a little helps a lot in that regard, but even if I drop it down to a max depth of 80 feet (24m) for 45 minutes, I get 3 stops totaling 24 minutes. I'm thinking most of the dive had to be spent at a much shallower depth.

Running 46min at 33m at GF 50/80 gives me deco of 76 minutes. My SAC had to be 8/4 lpm to get numbers he gave us.

OP, just because you had 50 dives in last 2 years doesn't mean a lot. The fact that you didn't even realize that your computer is giving you mandatory deco stop supports this.
Anyway, kudos to you for coming here to get the answers and learn. Learn how to plan your dives and stick to it, or at least pay attention at your DC and go up as you run out of NDL.

 

[Eric Sedletzky]

In this day and age of basic scuba instruction, people have a choice of taking a class that focuses on tables or a class that focuses on computers. In some cases, it is a choice between agencies. In other cases, it is a choice within agencies.

In that first sentence, I used the vague term people deliberately because it is not at all clear who gets to make that choice. The new OW student usually has no icea the chice exists and does not know enough to make a decision. The new OW student goes into a shop to take a class, not knowing that the decision was made by the shop management. If the shop management chooses to teach the tables and nearly ignore computers, knowing that the students will almost certainly never actually use the tables for diving and will instead use computers, then the student is an unwitting victim.

I think they should teach both, at least teach tables in a historical context so they can look at the graph and see times and depths. I’m not saying to teach tables forwards and backwards like they used to, but at least enough so students know what their computers are are based on.
Then they need to teach computers past the point of just saying, “Just follow your computer and do whatever it says”.
I’m not really certain, but somehow I think knowing something about basic decompression might be slightly important in diving.

 

[boulderjohn]

I’m not really certain, but somehow I think knowing something about basic decompression might be slightly important in diving.

Mark Powell wrote an entire book on decompression theory (Deco for Divers), going well beyond the basics, and he never once felt compelled to teach how to use tables. Decompression theory is one thing. Managing decompression is another.

When I taught table-based OW classes, I covered decompression theory on the first day of classes. I went into why nitrogen builds up in the tissues during the dive, and I went into why we need to ascend in a safe way that minimizes the potential for DCS. On the second day of classes, I showed the students how to use tables to manage dives and plan for a safe ascent.

When I taught computer-based OW classes, I covered decompression theory on the first day of classes. I went into why nitrogen builds up in the tissues during the dive, and I went into why we need to ascend in a safe way that minimizes the potential for DCS. On the second day of classes, I showed the students how to use a computer to manage dives and plan for a safe ascent.