Deco + safety stops?

[grassybreakfast]

Hello!

I am a rec diver certified for back gas deco (I am aware that some agencies consider any deco diving to be tech, but not BSAC!) but haven't actually done it except simulated deco during training. I have avoided it because I personally consider significant deco obligation without redundant gas to be outside of my risk profile.

However, now that I am getting into redundant gas (haven't decided between sidemount or twinset and will probably do training for both to decide), I am looking to diving deeper/longer with planned deco.

One thing I find intriguing is how people who do deco think about safety stops. It seems like the most popular school of thought is if you are doing deco, you are planning your own safety margin with GF (or equivalent), and so there is no need for safety stops. That's fine, but then do you do safety stop on a NDL dive? If yes, isn't there a bit of a disconnect in safety margin since completing deco puts you at GF high, which is exactly where you would be if you were just inside NDL? Or do you do safety stop in addition to deco? Or do you also not do a safety stop on NDL dives?

It seems like the most scientifically-based way to manage safety margin is to either set GF high to what you deem acceptable, and aim for a lower SurfGF if possible, or GF high to what you aim for, but be happy to surface earlier if necessary when SurfGF gets to what you deem acceptable.

What strategy do people use?

 

[inquis]

My GFhigh setting for a deco dive is lower than for an NDL dive. Since faster tissues are controlling for NDL, even if I take NDL close to 0, surfGF usually winds up being about the same or lower.

I'm happy to extend the safety stop, because I haven't been in the water terribly long. I typically let the rapid pace divers climb the ladder before me.

 

[grassybreakfast]

My GFhigh setting for a deco dive is lower than for an NDL dive. Since faster tissues are controlling for NDL, even if I take NDL close to 0, surfGF usually winds up being about the same or lower.

I'm happy to extend the safety stop, because I haven't been in the water terribly long. I typically let the rapid pace divers climb the ladder before me.

Thanks that's interesting! If you end up with the same SurfGF either way, why not use the deco settings all the time and skip safety stops? Is it just to dive in places that don't allow deco?

 

[inquis]

I prefer a safety stop because I believe it's very likely that we bubble a little bit. Those few minutes allow enough time for those micro bubbles to make it to the lungs.

 

[steinbil]

However, now that I am getting into redundant gas (haven't decided between sidemount or twinset and will probably do training for both to decide), I am looking to diving deeper/longer with planned deco.

I'm hoping you mean to take some kind of tech class in the configuration of choice (which by default should be a twinset, unless there are good reasons to deviate). Planning to do longer/deeper dives with a deco obligation without using proper deco gases and accelerated decompression is meaningless, in my opinion. You just increase the risk, and gain very little in terms of bottom time/depth.

One thing I find intriguing is how people who do deco think about safety stops

As far as I know safety stops were born because people got bent diving tables to the limits and/or ascending too quickly, or not limiting exposure based on other contributing factors. It's important to note that some of the tables out there are also very aggressive with GF approaching or even exceeding 100 in some cases. Slowing the ascent down with a safety stop and adding a buffer obviously helps increase the safety of such dives, and the great track record of safety stops has carried on into computer NDL diving, with the benefits of decreasing risk of deeper/longer NDL dives, helping newer divers practice buoyancy and making up for other factors like otherwise ascending too quickly, being dehydrated/tired/hungover etc.

It seems like the most popular school of thought is if you are doing deco, you are planning your own safety margin with GF (or equivalent), and so there is no need for safety stops. That's fine, but then do you do safety stop on a NDL dive? If yes, isn't there a bit of a disconnect in safety margin since completing deco puts you at GF high, which is exactly where you would be if you were just inside NDL? Or do you do safety stop in addition to deco? Or do you also not do a safety stop on NDL dives?

There is no need for an arbitrary stop after a planned deco ascent. If you want to adjust the deco plan and add padding to the stops to account for other parameters, that's a different story and something you should be trained for. Also, just adding minutes to the last stops, is simply padding that last stop - no need to call it a safety stop. You can look up other threads on that topic. However after the final deco stop, usually at 6m, I will ascend at 1m per minute - so 6 minutes on the final 6 meter ascent, where the pressure differential is greatest. This is a way of adding conservatism in the most critical part of the ascent, so I guess you could compare it to a safety stop, but just adding x minutes at the last stop and then popping to the surface is less helpful. When doing NDL dives I don't usually do a safety stop, but a minimum deco ascent, this is a GUE protocol that serves the same purpose by slowing the ascent down in 3m intervals, which also is good practice for deco stops. When you're talking about comparing the safety margin/risk profile, you have to keep in mind a couple of things:

- Like @inquis said, on deeper/longer dives the slower tissues are loaded more, and it is well known that increased exposure = increased risk of DCS. So doing a 60m deco dive on GFhi85 is generally more aggressive than doing an NDL dive to 30m with GFhi85.
- Decompression theory is not an exact science, so you can't assume the risk is directly correlated with GFhi.

You can read about ISO-risk deco schedules if you want to understand more.

It seems like the most scientifically-based way to manage safety margin is to either set GF high to what you deem acceptable, and aim for a lower SurfGF if possible, or GF high to what you aim for, but be happy to surface earlier if necessary when SurfGF gets to what you deem acceptable.

That is a reasonable approach. Just remember that GFhi is not an ISO-risk parameter at different depths/exposure, and also that you need to account for a myriad of other parameters that scientists don't even know how will affect your personal decompression and DCS risk.

 

[GF99/99]

Hello!

I am a rec diver certified for back gas deco (I am aware that some agencies consider any deco diving to be tech, but not BSAC!) but haven't actually done it except simulated deco during training. I have avoided it because I personally consider significant deco obligation without redundant gas to be outside of my risk profile.

However, now that I am getting into redundant gas (haven't decided between sidemount or twinset and will probably do training for both to decide), I am looking to diving deeper/longer with planned deco.

One thing I find intriguing is how people who do deco think about safety stops. It seems like the most popular school of thought is if you are doing deco, you are planning your own safety margin with GF (or equivalent), and so there is no need for safety stops. That's fine, but then do you do safety stop on a NDL dive? If yes, isn't there a bit of a disconnect in safety margin since completing deco puts you at GF high, which is exactly where you would be if you were just inside NDL? Or do you do safety stop in addition to deco? Or do you also not do a safety stop on NDL dives?

It seems like the most scientifically-based way to manage safety margin is to either set GF high to what you deem acceptable, and aim for a lower SurfGF if possible, or GF high to what you aim for, but be happy to surface earlier if necessary when SurfGF gets to what you deem acceptable.

What strategy do people use?

I don't believe in "safety stops" for deco dives. I don't know why you would need it. If you think that your planned deco stops were not enough and you now need to do a "safety stop" after you clear deco then you have piss poor planning and you should not have even started the dive.

I say plan your dive and dive your plan.

Pick what you deem an acceptable GF, plan your dive on that GF, when you computer says deco clear then you can safety surface. If you feel to you need to do a "safety stop" once you clear deco then as I mentioned be for you have piss poor planning.

Now I will say there is difference between a "safety stop" and and emergency contingency plan. So yes if you feel some twinging or something odd in a joint or you just don't feel right about the quality of your decompression then yeah listen to your body and spend 30 more min at 6m on O2 (or however much time you need)

 

[inquis]

FWIW, I believe this is related to the time-to-fly. On an NDL dive, we would feel very safe exiting the water with a GF99 of 65, right? At first glance, one might suppose that you just need to wait long enough so your GF99 at cabin pressure was below 65. Unfortunately, a study by Duke Univ. showed you would almost certainly get bent. The fact is we bubble, and it's quite easy to catch some of them with a safety stop.

Plus, it's nice to wait at depth for the ladder to clear rather than in rough surface chop.

 

[Searcaigh]

My typical recreational deep dives are generally 28-30m and my backgas is EAN34

I sling an AL30 with 50% for deco purposes should that occur. Occasionally I run into acceptable deco when photographing subjects on a 30-35 min bottom time.

On ascent I switch to 50% at 21m and before I hit 6m I am clear of deco with a 5 min safety stop showing on my computer, which I follow breathing the 50%, then switch back to my backgas once the safety stop has been cleared.

I run GF 40/85

 

[boulderjohn]

I am one of a growing number of people who use SurfGF as a guide for safety stops on NDL dives. If I see a number I like, I'm ready for the surface. I often have to hang around longer, though, to stay with other divers who are counting down their three minutes. I once did a dive with a group that was so shallow for so long that my SurfGF never got above 19, and everybody still did a safety stop.

 

[grassybreakfast]

@steinbil, thanks for the very informative reply!

I'm hoping you mean to take some kind of tech class in the configuration of choice (which by default should be a twinset, unless there are good reasons to deviate). Planning to do longer/deeper dives with a deco obligation without using proper deco gases and accelerated decompression is meaningless, in my opinion. You just increase the risk, and gain very little in terms of bottom time/depth.

Yeah will definitely be getting formal training in both. My current certification depth limit is 40m, so it's more the redundancy aspect for deeper dives, regardless of bottom time. I don't think I want to get into a lot of deco, and if I do, I'll definitely take the BSAC accelerated deco course.

MultiDeco at GF 60/80 and EAN28 says 20 minutes at 35m is ~9 minutes TTS, and that's approximately the kind of profile I want to be able to do, vs only about 5 minutes of NDL at that depth. This is probably what some people consider "light deco".

As far as I know safety stops were born because people got bent diving tables to the limits and/or ascending too quickly, or not limiting exposure based on other contributing factors. It's important to note that some of the tables out there are also very aggressive with GF approaching or even exceeding 100 in some cases. Slowing the ascent down with a safety stop and adding a buffer obviously helps increase the safety of such dives, and the great track record of safety stops has carried on into computer NDL diving, with the benefits of decreasing risk of deeper/longer NDL dives, helping newer divers practice buoyancy and making up for other factors like otherwise ascending too quickly, being dehydrated/tired/hungover etc.

Yeah that makes sense as an additional buffer for NDL dives. I didn't know the practice started in the tables era (I am way too young for that).

There is no need for an arbitrary stop after a planned deco ascent. If you want to adjust the deco plan and add padding to the stops to account for other parameters, that's a different story and something you should be trained for. Also, just adding minutes to the last stops, is simply padding that last stop - no need to call it a safety stop. You can look up other threads on that topic. However after the final deco stop, usually at 6m, I will ascend at 1m per minute - so 6 minutes on the final 6 meter ascent, where the pressure differential is greatest. This is a way of adding conservatism in the most critical part of the ascent, so I guess you could compare it to a safety stop, but just adding x minutes at the last stop and then popping to the surface is less helpful. When doing NDL dives I don't usually do a safety stop, but a minimum deco ascent, this is a GUE protocol that serves the same purpose by slowing the ascent down in 3m intervals, which also is good practice for deco stops. When you're talking about comparing the safety margin/risk profile, you have to keep in mind a couple of things:

Yeah slow ascent makes sense. I'm not sure if I have good enough control to do 1m/minute!

- Like @inquis said, on deeper/longer dives the slower tissues are loaded more, and it is well known that increased exposure = increased risk of DCS. So doing a 60m deco dive on GFhi85 is generally more aggressive than doing an NDL dive to 30m with GFhi85.
- Decompression theory is not an exact science, so you can't assume the risk is directly correlated with GFhi.

You can read about ISO-risk deco schedules if you want to understand more.

That's a good point. I did assume GF to be iso-risk for a long time, in fact until yesterday when I read another post on here about the Thalmann algorithm. Fascinating stuff. It would then seem like a deeper dive on the same GF would warrant more conservatism (with a safety stop or another method) than a NDL dive to the same GF?