Accelerated Deco (ERD) is it worth it?

[Step]

I would very much like to know:
(I have no access to planning tools other than BSAC88/Nitrox table and my Air only Aladin Pro so can’t look this up for myself.)

1.
A. How much bottom time in the 30-50m range can I get using 1-2 deco mixes and 30%-21% as back gas while keeping deco to 15 min or less.
Or what is the max. depth I can still get 30 min of bottom time when doing 15 min deco or less.
B. Same question but with air as back gas and 100% for deco only.

2. How effective is accelerated deco. i.e. does it make sense to use it to cut
10 min deco down to say 5 or is it only adequate to cut 40min to 20min?
Or in other words by what percentage can I cut my deco time.
(I plucked those numbers out of the blue and am not sure how realistic they are but you get the drift)

3. How realistic is to everything on air as a back gas I stage with100% O2 for deco?
Then I would only need to O2 clean one reg. an no cylinders as the one would always get 100%. I also won’t need an analyser as everything I will ever use will be 100% or 21%. (Temporarily at least until I move back to where I can get up to 40 pre-mixed and then would buy an analyser)


I guess the above depends on how conservative I want to be. The answer is not very.
I’m interested in what you’d be happy with, for now, and will decide what level of risk I’m happy with after I have done the course.
(as I’m not sure how conservative what I’m doing now is maybe that would be a good reason to do the course already?)


Why am I interested
I’m thinking of doing the BSAC extended range course. Manly to learn about accelerated Deco.
(If there is another reason why I should want to do this course I would be interested to hear that as well)
I’m sure the course will be very interesting but if I’m unlikely to use what I learned I rather spend the money/time on a weekend away diving.

I did the BSAC advance nitrox course and although it was very interesting so far it has just been an expensive waste of money.
I have not used nitrox in the 2 years since because my diving does not justify paying 8quid for a fill, never mind the cost of a, personal filter, nitrox analyser, and annual O2 cleaning of cylinders and regs. I would definitely use it on the odd occasion if only I could get pre-mixed up to 40% round here.

So far I have dived to see sea creatures so I could just chase the no-deco limit up the slope and don’t have to do deep as most of the time, most live is shallow, and the most interesting is below the kelp but above the mud...
Now I want to have a look at some wrecks, but to be of interest as a wreck rather than because they support lots of live, they need to be very intact! This usually means deep and ascending through the water column with nothing interesting to look at.
If I don’t get 30 min bottom time all the preparation and expense for the boat etc. is not worth it in my opinion so with my sinus issues I tend to descend slow => so this means a dive time of >40min. if the dive goes below 25m.
Although I have no trouble in holding a stop I get bored on the 3 min safety stop. Now for a superb dive I guess I’d be happy to hang for 5min but I doubt I have the patients for longer than15.
I want to dive what is local and not have to travel all over the country to find intact wrecks in my depth range. I.e. adjust the diving to the wreck I want to dive not fit the dive site to the dive I want to do (within reason oc course).

[Tony Jay]

Most my diving has been air + O2 deco, the reason for this were a small stage (we have low breating rates so 3 litre is massive along with twin 10's of air) so I can see where you are coming from.

If you have easy access to nitrox (and know your dives in advance) then nitrox + O2 is very compact and a nice compromise

As for what the time and depth is ..... Depends on the planning tools and if you choose to do deep stop etc etc.

I was generally doing 20 mins mandatory stops using a vytec 50% RBGM (pre DS) and 20/85 GF Buelman.

For the 30 to 50m depth ranges it really does not seem to make any difference in total run time if you use 50% or 100% (and I could get 100% more easily than 50 !)

A good rule of thumb is that O2 deco will halve to 6m (and shallow shops) .Using O2 you can do the 5,4,3,2,1 metre stops at 6m with out lenghtening the stops !

This type of deco setup is not very "normal" but works wonderfully for some.
If you are near rebreather friendly dive shops then you can get o2 top offs, which are actually cheaper per dive than 50% fills (as you only pay for what you use)

I think in the BSAC I'm not supposed to be using o2 at 6m, but I did a TDI cert which says its ok !

Tony

[Olle]

As for 1 & 2, I can’t answer them now as I don’t have access to planning software at work (nor does it look anything like work were my boss to glance over my shoulder). I would download HLplanner. It’s free, easy to use and it’s what we used on my ERD course to plan the dives.

3. 100% O2 is often worse than say, 80% as you can switch to 80% earlier (deeper) and breathe it for longer. It’s also cheaper.
(Edit: Just re-read and that doesn't at all answer your question. Air + stage of happy gas works nicely!)

I found the ERD course very useful for oodles of reasons. There’s a couple of hours of kit-phaff in there just looking at your kit and making it more optimised for more gases (adding D-rings, moving regs, where to hang stages, etc) In water skills such as DSMB deployment, stage handling, reg switching was also mighty useful.

[Rob Dobson]

HLPlanner is free and so is FreePlanner (www.freeplanner.org). If you can post here you must have access to a computer and an internet connection....

30-50m range on a BG that has an END of 30-50m... Do yourself a favour and do an adv nitrox & deco course and then do a mix course. Agency doesn't matter but find a decent insructor (which may or may not be your mates at the local BSAC club). This stuff is important, your life depends on it so please don't cut corners and try to do it on the cheap.

[liamm]

V planner is an excellent tool if you
have access to a computer or PDA

[witchieblackcat]

Originally Posted by Step

I would very much like to know:
(I have no access to planning tools other than BSAC88/Nitrox table and my Air only Aladin Pro so can’t look this up for myself.)

1.
A. How much bottom time in the 30-50m range can I get using 1-2 deco mixes and 30%-21% as back gas while keeping deco to 15 min or less.
Or what is the max. depth I can still get 30 min of bottom time when doing 15 min deco or less.
B. Same question but with air as back gas and 100% for deco only.

2. How effective is accelerated deco. i.e. does it make sense to use it to cut
10 min deco down to say 5 or is it only adequate to cut 40min to 20min?
Or in other words by what percentage can I cut my deco time.
(I plucked those numbers out of the blue and am not sure how realistic they are but you get the drift)

3. How realistic is to everything on air as a back gas I stage with100% O2 for deco?
Then I would only need to O2 clean one reg. an no cylinders as the one would always get 100%. I also won’t need an analyser as everything I will ever use will be 100% or 21%. (Temporarily at least until I move back to where I can get up to 40 pre-mixed and then would buy an analyser)

1a. More than on air. vPlanner will tell you how much.
1b. More than on air but possibly not as much as your nitrox mixes.
2. Quite. vPlanner would show you.
3. I wouldn't. Air on the back and a rich mix is better because you can swap onto it deeper.

Originally Posted by Step

I guess the above depends on how conservative I want to be. The answer is not very.
I’m interested in what you’d be happy with, for now, and will decide what level of risk I’m happy with after I have done the course.
(as I’m not sure how conservative what I’m doing now is maybe that would be a good reason to do the course already?)

Sounds like you're looking for more risk than some of use would consider necessary. Do the course and you'll at least understand the risks rather than taking the rather high risk of relying on Internet advice.

Hope that helps

[Step]

Thanks for all replies esp. to Olle and Tony Jay,

I know that there is free soft ware available.
But for now I can't install programs on the computers i have access to.
Last time I looked on Hplanner, freeplaner etc. pages they all seemed to require me to install the thing on my computer.
(I may well be too stupid to find the relevant page on the site in which case I’d be grateful for the link)
Unless there is a web based program where I can punch in the time, depth, mixes etc. and it tells me the stops without me needing to install it on my computer it is at present no use to me.

I can understand that the information I was asking for may take some time to generate. I.e. a few examples comparing air only, Nitrox back gas only, Air /nitrox plus rich mix...
So if nobody is prepared to spend the time for a stranger. Fair enough!

What does puzzle me though is that part of the answerers to this kind of questions (even though they may perhaps mot meant to be) always seem somewhat condescending/patronising.

Originally Posted by Rob Dobson

Do yourself a favour and do an adv nitrox & deco course and then do a mix course….
This stuff is important, your life depends on it so please don't cut corners and try to do it on the cheap.

Originally Posted by witchieblackcat

Do the course and you'll at least understand the risks rather than taking the rather high risk of relying on Internet advice.

(I acknowledged in the OP I don’t quite understand risk levels. How does this disqualify me from being given the information I ask?)


I included my background and motives hoping to avoid the belitteling don't get ahead of yourself, donb't fuck about get the proper training. etc replies.



Why can people not assume I want the advice /favour for good reasons?
Why can I not be treated like an adult, given the information, and be trusted to use it appropriately/responsibly and to make my own choices?

I wanted the information to get a better picture of what accelerated deco can and can't do for me.

I wanted the information so I can judge if it is worth my while and money to do the course or if I rather forget all about 30min. bottom times >30m as it will require more hanging than I'm prepared to do regardless of accelerated deco.

I think my questions were reasonable things to ask.
(unless it takes hours to generate, in which case why not just say so, instead of treating me like an idiot saying: "do the course, then you'll know")


Don't worry. I won’t dive the profiles you give me.
Not before I have done the course and double checked/optimized them anyway.


As for not being very conservative.
What I meant is:
I dive an Aladin Pro, not a Suunto set to max possible conservative mode.

[Paul Oliver]

ERD is well worth it, however you will have to do longer than 15 min of Deco on the course.

20 min of deco will generally cut to about 15 min after a gas switch, however ERD is more about redundancy and planning than just running a deco stop.

If you only want to do that long you may as well stick to the Adv Nitrox qualification. I personally do up to about 15 min on backgas on my 15L

Why do you need to buy a personal filter and analyser? Thats all done at the shop and no need for your own.

[ardhill]

Originally Posted by Step

(unless it takes hours to generate, in which case why not just say so, instead of treating me like an idiot saying: "do the course, then you'll know")

Step, the way you worded your question about comparisons and what dive times can you get in these parameters, to be honest would take hours and many tables. If you could shorten your depth variation a bit, it would help. 30-50m 1-2deco gases 21-30% back gas is just too many variable to ask someone to generate tables for you. Never mind what level of conservatism you want.

Here is a basic idea using air as back gas and 100% deco on a VPM-B algorithm +3 conservatism:

Dive to 35m - 15min Bottom Time

Using Air & 50% - 5.5mins deco plus ascent time
Using Air & 100% - 5.5mins deco plus ascent time
Using Air only - 18.5mins plus ascent time

[MattS]

Originally Posted by Step

What does puzzle me though is that part of the answerers to this kind of questions (even though they may perhaps mot meant to be) always seem somewhat condescending/patronising.

It is perhaps only condescending/patronising if you take the attitude that you know better. The answers you were given were good ones - whether or not they are what you want to hear.

Why can I not be treated like an adult, given the information, and be trusted to use it appropriately/responsibly and to make my own choices?

Perhaps some of those people that are responsible are fed up seeing idiots attempting to learn technical diving off the Internet and subsequently filling up recompression chambers? Some of those idiots came to technical diving because they were not comfortable doing long deco hangs...it is sort of the antithesis of the technical attitude where you use the techniques to get what you want out of a risky dive with the minimum of risk. I decide on my bottom time and then do the deco it requires.

I was doing the air times for a few years before venturing outside BSAC for Nitrox and technical training 12 years ago. I have been a bit of a Nitrox evangelist ever since.

I wanted the information to get a better picture of what accelerated deco can and can't do for me.

Well let's see what I can do then. Very roughly speaking 30mins at 30m to 50m on air + 80% or higher - deco times will approximate bottom times. Obviously it depends on the exact depth and time the longer and/or deeper the more deco and at 50m for an hour I might spend 90mins (150% of bottom time) decompressing . You can usually make it out the water faster on BSAC 88s if the profile is not off the end of the table.

Nitrox tables (other than 88s) and accelerated deco tables are based on EAD extrapolation. It is more theory than experiment so most people use a conservative (compared to 88s) table based, to one extent or other, on Buhlmann's. Even so very few people stick exactly to the table they have chosen and will alter their profiles according to their gut feelings. It is very difficult to say what is or is not correct because the statistical verification models just do not exist. If this worries you, you probably should not be technical diving.

Anything over 75% will keep your PPO above 1 for the bulk of decompression. As shallow stop times increase the greater the difference this makes. For short stops there will be virtualy no significant difference over backgas or 50%. It takes time for accelerated deco to do it's stuff. Personally I don't bother accelerating any less than 30 minutes of air stops, about 15 mins on 80%, but I do the air stops up to 15 mins on 80%.

There are a number of cons when it comes to using 100%, CNS exposure rises significantly and heavy sea states can make a 6m MOD very uncomfortable. With 80% the gas switch is out the way before ascending into the surf zone in any remotely divable conditions. 100% offers little theoretical time advantage over 80%. It has the advantages of being devoid of Nitrogen and the treatment gas for DCI.

I wanted the information so I can judge if it is worth my while and money to do the course or if I rather forget all about 30min. bottom times >30m as it will require more hanging than I'm prepared to do regardless of accelerated deco.

A basic rule of accelerating decompression is being prepared to do the air stops in case you lose access to the deco gas. Personlly I do not think it is enough to assume you can do the longer stop times, you should try them and be comfortable with them.

Ideally you would be using a Nitrox backgas to extend the bottom time at 30m to 35m but yes it can get expensive. Using 50% to decompress from your 30min 30m air dives, according to the air tables, will cost similar to accelerating deeper/longer dives. What is likely to change is how you feel after the dive. Only you can say if it is worth it, but it stands a chance that if you do not think so you are not going to find much use for accelerated deco.

As for not being very conservative. What I meant is: I dive an Aladin Pro, not a Suunto set to max possible conservative mode.

Once you start extending times well beyond no stop times a lot of additional risks come into play. DCI is quite a small one. DCI is however almost inevitable if you run into the consequence of those other risks. The worse possible scenario is having to surface with the equivalent of a massive backgas decompression obligation because you fell prey to one of the other risks. Hence technical diving tends to favour those who are adventurous in their objectives and conservative in their plans.