Deco + safety stops?

[Dave Whitlow]

Thanks for your replies everyone. I think the original question has been answered. Here's my summary of (what I believe to be) the good answers (some of the points have been made by more than one person, so apologies for lack of attribution):

* People use lower GF high on deco dives than equivalent on NDL dives.
* Part of the reason for the safety stop is to compensate for mistakes like ascending too quickly, that deco divers (or really, more experienced divers) are less prone to.


Are these people trained for back gas deco? I feel like maybe introducing deco earlier in training would help with this kind of situation. Many people would like to be able to spend say 10 minutes at a 35m wreck but don't want to go any further. Do they really need several tech courses and spend thousands of $/£/€ to be able to do that? It's really not that difficult to plan for short back gas deco. Guesstimate emergency SAC rate, decide on a max TTS (say 10 min), calculate min gas, and have a plan for loss of gas. Then NDL+10min is the new limit. 10 min not enough? Just plan for a longer limit instead (assuming sufficient gas).

The only bit that requires significant training is redundant gas, if required. That would depend on individual risk profile. Our club's policy is max 1 min deco without redundant gas.

For deco diving, as previously stated, the safety stop is irrelevant as gradient factors have that covered. However, when all stops are clear it doesn't mean you must surface. I sometimes wait until the next 5 minute interval or just hang around watching jellyfish, because I can!

I use the same GF on all dives and 60/85 works for me as I don't like to stay deep and add extra deco. I rely on my computers to include my ascent behaviour in their calculations and so far it has worked. As a UK sea diver my dives usually end with SMB deployed from depth, and once the SMB begins it's journey I start mine, aiming to be near the first mandatory deco stop when my SMB breaks the surface. It's a fun ride but saves an awful lot of winding and pointless deco.

Diving isn't a cheap pastime and if you really want to spend longer on the wreck, and deep might be interesting, then maybe CCR is worth considering? It saves an awful lot tedious gas planning when all that generally matters is the bailout gas carried and current TTS.

 

[Alekseolsen]

That's interesting. I didn't know it's possible to do DP without intro. That would indeed not be a big burden. For back gas deco you wouldn't even need AN?

Correct.

Here's SDI:

TDI Decompression Procedures Diver -

www.tdisdi.com

If anything I feel the 25 dive prerequisite is on the very low end.

 

[Wibble]

That's interesting. I didn't know it's possible to do DP without intro. That would indeed not be a big burden. For back gas deco you wouldn't even need AN?

Advanced Nitrox and Decompression Procedures -- ANDP -- is a great start to decompression diving down to about 45m/165ft. Other depths and agencies are available, but it's basically around that single high-oxygen decompression gas.

This teaches you the theory behind decompression and all the issues you need to plan for; dive profiles, SAC, MOD, reserves, limits. It covers the major gas switching protocol: NoTox, plus all of the normal underwater issues from freeflow through sharing gas.

It is a very good start from which you can progress to dive on the wrecks which are out of reach with NDLs and single gases.

There's a variant of the ANDP which adds helium to the mix; less narcosis is a good thing compared with being off your gourd on Nitrox.


What ANDP requires is a diver who's skills are sufficiently developed that they can be task loaded and still keep on top of what's going on around them without ascending/descending out of control; especially if you have a poor team mate/buddy who cannot hold their stops.


If you're interested in decompression diving, ANDP (or any equivalent) should be considered as the starting point. You'll be a better diver for it and understand the issues.

ANDP is an excellent course for UK diving where there's a lot of wrecks in the 30m/100ft to 45m/165ft range that would mean extremely short bottom times for NDL.

 

[td275]

Advanced Nitrox and Decompression Procedures -- ANDP -- is a great start to decompression diving down to about 45m/165ft.

I agree with Wibble. Seems if you take an ANDP class you will discover a lot of answers to your questions and more. The class really isn’t that expensive, nor do you have to own any of the equipment (doubles, manifold, dual regs, long hose, deco tank, 40EAN+ reg, etc). That may sound like a lot, but renting from the shop for the class really won’t be that bad. IMHO

The cheaper option would be to see if you can get your hands on the reading materials of the classes. You might be able to learn a bit from that, along with asking questions to a tech instructor.

To be clear, AN is the advanced Nitrox class. Super simple. The materials just teach you all the problems associated with gas mixes higher than 40% oxygen. Problems with equipment, problems with your central nervous system. Hands on portion (I think I am correct) is just one dive with 50EAN then another with 100EAN. Nothing real hard there.

DP is all about the deco procedures. How to plan, adjusting GF, no gas plans, etc. The book and knowledge portion of this class is what I think most of your questions are about. There is a lot of math involved in this section, but rest easy, doing the “work” by playing with the numbers on different plans will help a student really understand what is involved in decomposition. This is where I changed my rec diving habits, because I realized EVERY DIVE is a deco dive. Knowing how my computer works, how to control my dive, buoyancy, assent and understanding how my body feels… helped me adopt a better plan for EVERY dive.

Of course there is a lot more involved in the DP portion, especially the hands on. Such as your first “deco” dive won’t REALLY be a deco (per say). You will set your computer at a much greater conservative GF so that it will go into DECO mode, but yet you are still ACTUALLY on an NDL dive that you will be okay. This is so you can see what your computer will do and not feel like you are getting into trouble by coming too close to your ceiling or something. Plus you clear deco pretty fast with the settings. However I will tell you it is always a cool feeling fo me (and some of my friends) when we used to freak out at NDL getting too close to zero, and now we feel exited to watch it go to zero. Knowing that we can be safe by diving the plan.