Technical Diving - deco planning, trimix planning and general tech dive planning

[eelnoraa]

Recreational dive computer's deco mode is for emergency use only. If you accidentlly exceed your NDL, the it kind of guide you to the surface and minimize the DCS risk. And it ussually lock you out for further diving for a period of time. It is to tell you "your screw up, no more diving for you". So never plan to do a deco dive based on your rec computer. In fact, never plan to do a deco dive unless you have the appropriete training, whether it is the deco procedure, failture handling, ascent speed controlling ........

 

[boulderjohn]

And it ussually lock you out for further diving for a period of time. It is to tell you "your screw up, no more diving for you".

Suuntos will not lock you out if you follow their guidance. They will just give you longer and longer surface intervals. If you jump in before they think it is a good idea (suggested surface interval), then you will get a really short no deco time on that dive. If you go to the surface without completing the required time and don't go back down very quickly, then they will indeed lock you out for 48 hours.

 

[MXGratefulDiver]

Hi

i have a few questions about technical dive planning if that ok. i appreciate your time and aswers - thankyou very much.

OK, but before I get started I'm going to make a recommendation that if you're truly interested in learning this stuff outside a classroom, start with Mark Powell's book Deco for Divers ... it's the best resource I've yet found for the recreational diver making a transition to planned decompression diving.

1) when you do a deco dive as soon as you reach deco does your computer tell you to ascend to your first deco stop? (because if you was deliberatly doing a deco dive then surely you wouldnt want your computer to tell you to ascend until you was ready to)

No .. that's not how it works. Your computer won't tell you when to ascend ... it will give you a "ceiling" ... meaning don't ascend shallower than this depth ... and a minimum time you should remain below that level. As you continue your dive, those numbers will change based on how deep you remain and how long you remain there.

o 2) how do you plan your deco dives? - how do you decide @ what depths to make your stops? do you use deco planning software? (if so which version), how do you decide which deco gasses to take (eg 80% o2 ), do you write your deco plan on a slate? when swiching mix underwater do you always watch your buddy switch and do you not swich gas @ the same time? also how do you know how long to stay @ each deco stop? - sorry 4 all these questions

OK ... in order or asking ...

- I plan my deco dives by running V-Planner on my home or laptop computer and plugging in my predetermined descent rate, bottom time, ascent rate, and planned gas mixes ... and letting the program give me a dive plan. I then compare this to a set of "rules" collectively known as "ratio deco" ... and based on both my own and my team's preferences, may change the shape of our ascent profile to suit our preferences.

- I prefer using a standardized set of gases based on planned maximum depth ... those would be 25/25, 21/35, 18/45 and 16/55. Those will basically accommodate any depth I'm interested in going to.

- I will write my plan on wetnotes, which I keep in my pocket. But for the most part, I won't need to refer to them unless it's a dive profile I'm not familiar with ... or unless for some reason we have to break the plan.

- Me and my team mates will ALWAYS verify that we're switching to the correct deco bottle ... unless we're doing a dive that only requires us to each carry only one deco mix. We alternate ... one watches while the other switches ... and vice-versa.

3) when diving with trimix do you use a trimix computer? - if so which one if you dont mind telling me and if not what do you use?

I use a Liquivision X1 dive computer, which runs the same V-Planner software I initially used to plan the dive ... as well as a backup bottom timer. The X1 runs in computer mode, but I'm running the ascent profile off our plan ... the X1 is just a sanity check. If it's off the plan by more than a minute or two, then I know something wasn't planned right. The X1 is trimix compatible.

A word on trimix computers ... they're not all the same. In fact, you need to decide how you want to run your profiles BEFORE buying a dive computer ... if in fact you choose to go that route. My first trimix computer was a Dive Rite Nitek-HE ... and I bent the bloody thing on every single dive. It runs an algorithm that penalizes you for deep stops ... and I prefer doing deep stops. I finally sold it to someone who prefers the "bend and treat" method of diving, which can get you out of the water quicker for some dives ... but my attitude is it's not a race, and I'd prefer making those deeper stops and being nice to my faster tissues.

That was said earlier in the thread regarding Suunto's, and it really isn't accurate. These computers will send you right to 10 feet, or rather normal safety stop depth, for minor deco obligations. For the most part, so will tech computers. If you incur serious deco, they will stop you much deeper, depending upon their algorithms.

A number of years ago, before I had any tech training, I went to 180 feet on the San Francisco Maru on air with a Suunto Cobra (I know, I know.), which is about as recreational as computers get. I don't remember at what depth it put my first stop, but it required a series of short stops until I did a massive stop at 15 feet.

I once racked up about 30 minutes of deco obligation on my Suunto Vytec doing some air dives at the dive park in Catalina (nitrox is bloody difficult to come by on that island) ... and it never gave me a ceiling deeper than 10 feet.

... Bob (Grateful Diver)

 

[boulderjohn]

A word on trimix computers ... they're not all the same. In fact, you need to decide how you want to run your profiles BEFORE buying a dive computer ... if in fact you choose to go that route. My first trimix computer was a Dive Rite Nitek-HE ... and I bent the bloody thing on every single dive.

I bought a used one cheap intending it for other purposes (too long to explain) and discovered that the real problem is that it hates helium. I once used it on a dive that was within recreational limits. We had originally intended to go much, much deeper, but that dive was canceled for weather, and the only tanks we had for the dive we did instead had a big dose of helium in them. My buddy had an X-1, and I had the Nitek HE. My buddy's computer said we could get out with just a normal stop, and the Nitek wanted me to stay down until the following Tuesday.

 

[mathauck0814]

Hi

i have a few questions about technical dive planning if that ok. i appreciate your time and aswers - thankyou very much.

1) when you do a deco dive as soon as you reach deco does your computer tell you to ascend to your first deco stop? (because if you was deliberatly doing a deco dive then surely you wouldnt want your computer to tell you to ascend until you was ready to)

Some recreational dive computers will assume that you accidentally exceeded your NDL and will send you up to some depth (10-20 ft) with a few minutes of decompression obligation - so far as I've seen on those units, if you ignore the recommendation the amount of decompression that you rack up will continue to increase until you oblige it. Other recreational computers are smarter and will insert deeper stops. Read your manual and know how your specific computer will respond to this situation.

2) how do you plan your deco dives? - how do you decide @ what depths to make your stops? do you use deco planning software? (if so which version), how do you decide which deco gasses to take (eg 80% o2 ), do you write your deco plan on a slate? when swiching mix underwater do you always watch your buddy switch and do you not swich gas @ the same time? also how do you know how long to stay @ each deco stop? - sorry 4 all these questions

I look up the dive in v-planner and make sure that I am carrying sufficient gas (and types of gas) for the dive in question. Once I'm satisfied with the gas planning, then I copy the decompression schedule into my wet notes along with a longer/deeper plan. I use this as a contingency plan in case my computer fails.

3) when diving with trimix do you use a trimix computer? - if so which one if you dont mind telling me and if not what do you use?

I have the Shearwater Predator. It's got dive planning software similar to v-planner built in and has a very simple interface to manage gas switches.

Some people will advocate the use of a bottom timer, which is valid but frankly has all of the same penalties as diving tables instead of a computer (what you end up with is shorter bottom times and longer decompression schedules than is usually necessary). It's a great way to learn and a fabulous secondary approach to a good technical diving computer (my $0.02).

The obvious bit here is to get the appropriate level of training/mentoring/experience before you take any of the advice you get on this board and attempt to put it into practice - there is no substitute for time in the water with someone who knows more than you do.

 

[TSandM]

I just want to reiterate that the critical part of technical diving isn't the deco . . . there are a lot of tools to figure that out, and honestly, if I dove my X1 and did exactly what it told me to do with no planning at all, I'd probably come out fine.

The danger is gas. Having enough gas for the bottom time; having enough gas for the deco, and having enough gas for the contingencies like delays at depth, lost deco gas, etc. Although the risk of death from DCS does increase rather markedly as the depth-time product goes up, people have survived pretty horrendous omitted decompression (read The Last Dive for the story of Bernie Chowdhury's adventures in blowing off deco). But almost nobody survives breathing water at depth.

 

[RTee]

Computers...I use two. A Suunto Vyper in Gauge mode and a Shearwater Predator. Once in gauge mode, the Suunto will remain in that mode for 48 hours after the dive so you need to be darn sure you want to use it in that mode. Once in gauge mode, it does not provide me with any NDL or deco ceiling(s). My Predator runs on Buhlman algorythm and is a multi gas (air, Nitrox and HE) programmable computer (up to 5 gases). Gas can then be selected throughout the dive as part of the gas switching process. The Predator will back up the tables I carry with me. It will provide me NDL then time to surface (following any deco schedule I may have created based on my diving profile and gas up to this point in the dive) as well as a Time to surface + 5 (based on same considerations) but modified to take into account an additional five minutes at the present depth (which could be good for contingency).

Tables...mines (planned profile + contingencies asbrought up by Devon) are cut from MV Plan which also runs on Buhlman Algorythm based on selected gases. So far I have not taken any helium diving related courses therefore I just dive air and various levels of nitrox up to 100 O2. I either print and tape the tables to my slate or transfer the info on duct tape which I put on my slate.

As others have said, tables and gas management go hand in hand. Based on the various numbers I plug into the software and the gases I am planning to use, the software will tell me gas usage of the various mixes. To that I will add the reserve I will require for either myself or a team member.

Deco gas...plenty of opinions out there. Some will swear by 50 % and 100 % while others will prefer 32 - 40% followed by 100 % based on personal preferences and/or gas availability. It is very interesting to plug various mixes into your software and see the effects it may have on the deco. In certain cases, the difference might just be few minutes but depth and duration of stops will be somewhat different. Gas switching...others have already covered the importance of doing it properly.

 

[Tortuga68]

I finally sold it to someone who prefers the "bend and treat" method of diving...

"Bend & mend" has a nicer ring to it

 

[MXGratefulDiver]

I bought a used one cheap intending it for other purposes (too long to explain) and discovered that the real problem is that it hates helium. I once used it on a dive that was within recreational limits. We had originally intended to go much, much deeper, but that dive was canceled for weather, and the only tanks we had for the dive we did instead had a big dose of helium in them. My buddy had an X-1, and I had the Nitek HE. My buddy's computer said we could get out with just a normal stop, and the Nitek wanted me to stay down until the following Tuesday.

Funny ... I used those exact same words to describe the one I had ... how can someone build a trimix computer that penalizes you heavily for using helium?

On the other hand, the guy I sold it to loves it ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

Some recreational dive computers will assume that you accidentally exceeded your NDL and will send you up to some depth (10-20 ft) with a few minutes of decompression obligation -

This is a misconception that gets people in trouble sometimes ... the computer doesn't "send" you anywhere ... it gives you a ceiling. Algorithms vary as to what it does in the intermediate depths, but in all cases the computer isn't telling you to go to that depth ... it's telling you DO NOT GO SHALLOWER than that depth until your deco obligation has been cleared.

I was diving with a fellow on a wall a few years back when he suddenly took off up the wall without any warning. Thinking he might have run out of air or something, I took off after him. We hit the top of the wall and rather than continue directly to the surface, he took off up the slope toward shore. Now ... we had just gone from about 120 feet to about 40 feet in maybe a minute, so I made a decision that I wasn't going to follow him to perdition. At that point, I stopped for a minute, then proceeded slowly toward where he'd disappeared. By the time I got to about 25 feet he was swimming back down toward me, indicating that we should head back to the wall. I shook my head and tossed a thumb.

At the surface I asked him what that was all about. His answer was that his computer told him to go to 10 feet for 3 minutes ... and so he did ...

It's important to understand what the information on your computer is telling you ...

... Bob (Grateful Diver)