Staged Deco

[beejw]

Hi all,

Im just curious as to what staged deco is exactly, and what the most used gas for staged deco is, whether is be 100% O2, or an EAN mix. And does this mean that you stage the bottles at a specific point on a line?

Also, is staging bottles a generally accepted practise, or do mose divers prefer to carry the stage bottles on a sling?

The reason i ask is that ive noticed in my readings, many areas where staged deco is often used, such as diving on deep wrecks, where little or no current is present (such as the bikini attol) that many well trusted computers are not acceptable to be used in these conditions. I did a little research, and all i could come up with was that the acceptable computers all had gas switching capabilties, whereas the others didnt.

I understand that any information given here in no way constitues training and i would never attempt diving in conditions which exceeded my current training. I have plenty of time to go out and explore technical diving later, as im only a young 18, but i am just curious.

cheers,

Beej

 

[miketsp]

The major issue here is guaranteed accessibility to your stage bottles.
If you are cave diving you normally (?) come out the way you went in by following your line and current is not normally an issue. So leaving stages along the route is not unacceptable.
Wreck diving is another issue.

Can you guarantee you will not get blown off the wreck and have to come up off the line? Are conditions calm and will they stay like this?

So the general consensus is that if you are relying on a stage gas for wreck diving you should carry it with you and not leave it hanging at your deco depth.
Now I've dived some sites where the deco gases are left hanging but the DMs leading the group carry stage bottles sufficient to handle a minimum deco obligation if the group has to come up off the line for any reason.

 

[beejw]

Hmm, i see what you meen.

But what usually constitues your stage gas? or does it depend on personal preference for spending less time hanging at your deco depth haha.

 

[SDAnderson]

Im just curious as to what staged deco is exactly, and what the most used gas for staged deco is, whether is be 100% O2, or an EAN mix. And does this mean that you stage the bottles at a specific point on a line?

Staging bottles means leaving them in a secure location for recovery (and use) at a later point in the dive. Staging bottles doesn't automatically equate to decompression diving but that is often the case.

Deco means exceeding the non-decompression limits, when it is unsafe to ascend directly to the surface, thus creating a virtual overhead environment. Elimination of the virtual overhead is accomplished through the use of time and, typically, special gas mixes to facilitate off-gassing of saturated tissues. The additional gases used to decompress with may be staged or carried (slung) with you.

The gases used in decompression vary depending upon the situation. Most decompression divers use a number of standard mixes, including a number of high O2 nitrox blends (all the way up to pure O2) or trimix blends. Other divers may custom blend their gases to meet the needs of a specific dive profile. In any event, the blends are calculated to minimize the amount of decompression time needed while balancing a bunch of physiological, logistical and mechanical requirements.​

Also, is staging bottles a generally accepted practise, or do mose divers prefer to carry the stage bottles on a sling?

Most open water divers are reluctant to stage bottles, since conditions or events may make it difficult to recover them. Cave divers tend to be more willing to stage bottles, since their route is more predictable. Technically, if you keep the bottle and don't drop it you should call it a sling bottle and not a stage bottle.​

The reason i ask is that ive noticed in my readings, many areas where staged deco is often used, such as diving on deep wrecks, where little or no current is present (such as the bikini attol) that many well trusted computers are not acceptable to be used in these conditions. I did a little research, and all i could come up with was that the acceptable computers all had gas switching capabilties, whereas the others didnt.

It sounds like you're tossing a number of different issues into the same hat. Wreck divers and other open water divers usually prefer to carry their bottles with them and won't drop them. If they stage the bottles, it's usually either at the entrance to the wreck where they can be recovered when exiting or at a pre-determined depth where they plan to switch breathing mixtures.

None of this relates to the reluctance on the part of many divers to utilize computers when doing decompression dives. Many divers don't like to rely on computers to calculate their decompression obligations and prefer either to pre-plan their dives, including decompression stops and gas switches, or they learn how to make the decompression calculations in their heads. In either instance, the role of the computer is minimized in order to avoid relying upon a mechanical device that may fail. Some computers are capable of handling gas switches during decompression dives but most can't. Of those that can, most are limited to nitrox blends and are unable to handle trimix blends. The very few that can handle trimix have a history of being prone to failure, either from flooding (many decompression dives involved great depths, which places heavy loads on seals) or just plain old buggy programming. They are increasingly reliable but the price for failure on a dive can be fatal and their history haunts them.​

 

[grazie42]

Also, since a lot of (most?) wreckdives have square profiles or at least predictable multi-levels, the added benefit of even an "advanced tmx computer" is fairly limited...

I just got back from a trip where the guide used a VR3 + tables. He tended to get out of the water just a few minutes earlier than those of us who followed tables, which corresponded to something like a 1/30th reduction of decoobligation...Considering that the cost of a tmx-capable VR3 is somewhere around 1200€ I have a hard time justifying the cost...

Most of the other questions have been answered...the ocean is a dynamic enviroment and I will carry all the gas that I consider vital thru the whole dive...ymmv...

 

[beejw]

Thanks everyone for your time in answering my questions. I appreciate it.

I cant wait to do some serious tech-wreck dives

 

[TSandM]

Here's a good book that will give you a start on understanding the stuff you're curious about.

 

[rookers]

And of course... obtaining training with an instructor your comfortable with is highly recommended.

 

[beejw]

And of course... obtaining training with an instructor your comfortable with is highly recommended.

of course. i wouldnt have it any other way.

thanks for the link to the book. I think ill buy it just as an interesting read until im ready to take on the world of techincal diving.

 

[beezwax]

The reason i ask is that ive noticed in my readings, many areas where staged deco is often used, such as diving on deep wrecks, where little or no current is present (such as the bikini attol) that many well trusted computers are not acceptable to be used in these conditions. I did a little research, and all i could come up with was that the acceptable computers all had gas switching capabilties, whereas the others didnt.

It sounds like you're tossing a number of different issues into the same hat. Wreck divers and other open water divers usually prefer to carry their bottles with them and won't drop them. If they stage the bottles, it's usually either at the entrance to the wreck where they can be recovered when exiting or at a pre-determined depth where they plan to switch breathing mixtures.

None of this relates to the reluctance on the part of many divers to utilize computers when doing decompression dives. Many divers don't like to rely on computers to calculate their decompression obligations and prefer either to pre-plan their dives, including decompression stops and gas switches, or they learn how to make the decompression calculations in their heads. In either instance, the role of the computer is minimized in order to avoid relying upon a mechanical device that may fail. Some computers are capable of handling gas switches during decompression dives but most can't. Of those that can, most are limited to nitrox blends and are unable to handle trimix blends. The very few that can handle trimix have a history of being prone to failure, either from flooding (many decompression dives involved great depths, which places heavy loads on seals) or just plain old buggy programming. They are increasingly reliable but the price for failure on a dive can be fatal and their history haunts them.​

Actually the OP is not asking about the relative merits of Computers vs Tables for tech dives. He refers to the Bikini Atoll and why some computers are acceptable for those dives and others are not. I assume he was reading the recommendations from Jim Akroyd, who writes:

"Pay close attention to the computer you are bringing to Bikini Atoll for the Bikini dive program. Most dive computers on the market are not designed for staged decompression diving and will lock-up on you or go out of range. This is because the manufacturers are afraid of the potential liability that can arise from divers performing staged decompression dives without supervision or training."

For his list of recommended and not recommended computers, see this webpage:
http://www.bikiniatoll.com/divemaster advice.html

To answer the OP, you are correct, the recommended computers are those with multi-gas switching capabilities.