Deco diving for beginners

[Mark Chase]

There are three types of deco diving...

Table diving
Deco on the fly
Deco via computer

Table diving is what most decompression or trimix divers will use. This is usually where the tables are generated using standard models or by building custom tables on a PC or laptop. Most divers these days will use the latter option and a good selection of soft ware is available to facilitate air, nitrox and trimix diving up to a maximum of 100m depth. Past this depth with any significant bottom time the existing soft where is being pushed to its limits and arguably beyond. New soft ware with the latest thoughts is being developed by Mark Elliot holder of the world depth record.

Ocean Gladiator Inspired Training Decochek Mark Ellyatt Technical Diving


Three basic systems are the bedrock used for deco calculations; there is Bhulman RGBM and VPM. Most if not all the others are variants of these systems. Pure Bhulman works on getting the diver as shallow as possible as quickly as possible and is therefore vary rarely used. RGBM and VPM incorporate deep stops, micro bubble stops and ultra slow ascents. As with dive computers there are those who will blindly plan a dive with little or no understanding of what they are doing and as a result there is a big chance of getting a DCI hit. It is possible to set some software to pure Bhulman and even less than Pure Bhulman. If this is done in ignorance expect to get hurt. There are those who believe it’s the responsibility of the computer tables to stop them getting bent rather than the divers personal dive preparation and execution. This is of course rubbish.

Strangely for such an important and advanced part of technical diving, table diving is cheep. The software ranges in price from free to £200 with some very good stuff available for £40.00. Most software will calculate your gas requirements PP02’s at given depths and CNS loading. The better ones allow variable SAC rates for bottom time and deco and will provide facilities for you to alter the dive profile in terms of how deep you want to start your deco, how shallow you want to finish and how aggressively you want to get there. Having paid for the software you only need a depth gauge and bottom timer to complete the dive.

Links to free deco software:

V planner
V-Planner VPM-B dive decompression software - download

GAP
GAP-Software - Home

GAP User guide
GAP-Software - Home


Table diving is in my opinion restrictive,

You HAVE to follow a dive plan which has been pre determined on dry land. This is OK if the dive is on a well-known site but on a new site you can create tables that do not cover the actuality of the dive. Usually this means doing extra deco unnecessarily as you tend to plan for the worst. On known dives the plan and the profile don’t always mirror each other so generally divers create tables for slightly deeper than the actual dive. This is an inbuilt safety mechanism. As a result some divers use aggressive decompression profiles and get away with it because most of the ACTUAL dive was spent above the planned depth. Using these profiles can prove dangerous on the one dive where the whole time is spent at planned depth or slightly deeper.

Tables must be cut for several time profiles to cover aborting the dive early or overrunning bottom time in an emergency. The extent to which this is done will depend on the depth of the dive. A planned decompression dive to 40m could have tables running from 30min to 60min in 15min blocks bottom time to allow the diver the option of staying on the wreck and maximising his gas or aborting the dive early because its rubbish or there is a problem. On very deep dives it may be necessary to cut the increments down to as little as 5mins. On a dive to 80m a diver staying on the bottom for 20mins has 62 min's of deco but staying 5mins longer increases that to 85mins of deco, 10 min's 105mins deco. So on an 80m dive overstaying your bottom time for 10mins almost doubles the deco commitment. (Figures produced using my standard gas profiles and decompression settings)

I use a four-page deco slate mounted on my wrist to allow space for all the possible profiles on each dive. As you can imagine planning such a range of profiles would take ages using hard tables so a PC can remove a lot of drudgery form decompression diving. Most experienced decompression divers will have produced a range of tables covering a range of times and depths and had them laminated. These will go in a book or be tied together in a pocket and used with a set range of back and decompression gasses. I prefer to custom plan each dive to get a clear mental picture of the dive and decompression profiles before I set foot on the dive boat but that’s my ting. Despite running a computer for my main decompression tool it is still necessary to have a full set of tables ready for the day the computer fails. Computer failure is like rain at Wimbledon. Inevitable. If it doesn’t happen that’s surprising, not the other way round


Deco on the fly

This sounds a great system but little information is available outside Tech 1 trained students so it’s not a good one to discus. Ultimately the system relies on mental capacity and this may be disrupted during the dive. As a result I am not a big fan no matter how simple the math is supposed to be. I have had a dive where a migraine put paid to rational thought and the VR3 paid for its self in spades. Some would argue that the buddy is there in these situations and in my case they would be correct but I always plan dives as if they were solo. There are some restrictions with DOTF in that you must stick to a set back gas and deco gas within a given depth range. Apparently as soon as you start altering the 02 and He content in the plan a new set of equations apply. Not having done the Tech 1 course I am not best qualified to offer advice in this area. However a rough idea of DOTF can be found here:

'Deco-on-the-Fly' - Page 4 - YD Dive Forums & Scuba Community

Post Number 39


Deco by computer

There are very few actual decompression computers on the market. Most computers will only generate shallow stops and have no capacity to cope with helium in the mix. Even using these for Nitrox or Air diving the deep section of the decompression is glossed over or dealt with by controlling the ascent rate. Most are heavily padded to add a big safety margin but divers still somehow manage to get bent using them.

The fact is deco by computer is not just an easy way out. If you rely totally on the computer and have no understanding of what it is telling you and how your body is actually dealing with the off gassing, it will bend you sooner or later. Having a ‘safety setting’ on the computer is another comfort zone thing. This is totally unnecessary if you understand what you are doing. I always have my computer set to 0 safety. That way I can pad the deco my self but have the computer set to get me out of the water fast if the need arises. Obviously fast is more risky but the risk is balanced against the need.

The diver can add his / her own safety by controlling the various stages of the ascent and altering the shallow stops to reduce the body stress. The extent to which this is done is dependent on how the diver feels physically and how well prepared they are for the dive. This really is a bit of deco on the fly that any one should be able to do. The computer will then control the 6m (or 3m) final stop and the to surface ascent is once again, up to the diver.

A step towards dealing with the deeper stops is available on the Suunto Vytec. This alows accelerated deco using two decompression gasses and does offer some deeper stops than a conventional dive computer. This is Suunto's first attempt at a multi gas dive computer and I have no doubt that with a few modifications it will prove to be an excellent tool for divers entering into multi gas deco for the first time. Its BIG draw back is that it offers no dive planning soft where that will allow preplanning of the dive using the Sunnto algorithm. This is a big problem when gas planning and forces a bit of guess work by the diver. I found using Decoplanner on a setting of 20/80GF matched the Suunto within +/- 5mins on 50m dives with 40 to 60min bottom times.

The latest and greatest computers have begun to look at full on deco and the use of Trimix and multiple decompression gasses. The best known of these is the VR3. As divers tend to come and go quickly they can be forgiven for thinking this bit of kit has been around since the dawn of time. In fact it was put on the market in 1999/2000 and its present format was available in 2002. Arguably in 2003/4 some of the final bugs were ironed out. So basically it’s a brand new bit of kit.

This computer is so far removed from what most people dive with that it can at first be seen as over complicated and difficult to use. In fact once mastered (In about a week’s diving) it becomes pretty simple to perform the main functions. Apart from the obvious differences in the use of he in the mix and up to 10 available dive gas’s operating at the same time, the computer has a few major developments. Micro bubble stops (deep stops by another name) are a critical part of the decompression profile and true deep stops near 80% of the average dive depth are included. The other massive step forward is look ahead decompression time. The computer will look at all your available gasses and calculate the quickest time out of the water based on your profile and all available gasses. The next giant stride forward in terms of deco is the computer will credit you for staying longer on the mid section stops and doing a slow ascent. As a result the shallower stops can be greatly reduced. Alternatively the diver can choose to do a no stop ascent. The computer allows the diver to push the maximum window by following the leading tissue compartments as they off gas. This is achieved by a simple graphic of a diver on a shot line moving down away from the maximum decompression ceiling during a stop. The diver can chose to rise slowly as the guide drops and to chase the deco ceiling all the way to the surface.

The big advantage with dive computers is freedom. The diver can chose any profile he or she likes during the bottom phase of the dive and the computer will follow along constantly updating the decompression profile. Tables are based on Max depth as a bail out back up and invariably the VR3 will cut massive chunks of the planned deco due to the multilevel dive calculations. The VR3 diver who has completed the deep part of the dive and is now fining along the top of the wreck can notice a porthole on the bottom and drop 10m and happily spend 15mins digging it out and bagging it up. The computer will cope and the diver is only limited by his available gas supply as to how much he /she alters the dive plan. The diver on tables will have to do extended deco pretending that he spent the whole dive at the max depth or will have to leave the porthole for a VR3 diver to collect

The Table diver on a new wreck who finds the wreck is on its side. He planned to do the dive on the deck 15m off the seabed but the wreck being on its side all the interesting bits are 7m deeper. Unless he has a vast selection of tables to choose from he is looking at doing a lot more deco than may be necessary. The VR3 diver doesn’t care and does exactly the deco that the dive demands regardless of the profile.

The table diver who picks up gas on the day of the dive and analyses it to find that his 18/35 planned trimix is in fact 16/30 due to the rush overnight fill, is in trouble unless he has a laptop or palm computer handy. This would appear to be true for Deco on the Fly as well. The VR3 diver just re programs the gas list and does the dive.

VR3’s are expensive but so is trimix diving. The VR3 on open circuit Trimix costs about the same as the gas for 8 trimix dives. The other way of looking at it is that the VR3 can maximise the bottom time on every dive. So over 8 dives you probably were able to spend an extra dives worth of bottom time on the wreck. Based on that premise it will pay for its self in two seasons of mix diving

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What ever system you chose for decompression diving remember that knowledge is king and the more research you do into decompression and gas mechanics the les likely you are to hurt your self.

ATB

Mark Chase

[Narked@50]

Very impressive Mark

Did you actualy write all this or is it cut from other areas?, it reads like you wrote it.

This is where I have to confess to relying on my computer 100%. For my style of diving it is an excellent tool with no drawbacks I can think of other than failure, and as I am trained, I can deal with that. I don't think it's a substitute for planning a deep dive though, there is no use going to 45m for 30 mins only to look at your computer and have it confirm you didn't bring enough gas for your stops! So all divers should know how to use tables and how to plan dives, depths and air - before relying on their new toy the way many of us do.

Well written Mark

James

[Phoenix]

Excellent article Mark, I just ordered a VR3 from Bang-on today so am looking forward to the learning curve.

Ian

[wreckweasel]

its a good job Im not on here to make friends.

Deco is an inexact science, that is currently being aggressively pushed based upon multiple theories. As an engineer, the testing regimes/controls can at best be described as "basic"

In recent years deco seems to have gone through a number of trends, from "first out of the water" (still prevalent) to "effective deco" (still relatively rare).

There is a lot of bollocks talked about the basis of deco theory, from gas gradient to bubble models. At the end of the day, find what works for you.. be comfortable with it... and be open to discussion/change.

This business of "computers deco you faster" is probably true. having said that, I dont give a flying f*ck about speed of deco, what concerns me is how I feel after the dive (thermal/deco considerations). A damn good mate of mine (who happens to be an excellent diver), regularly exits the water 10-20 minutes before me on dives in the 60-80m range. Fair play to him... good luck.. he gets to bag the first biccies..... but frankly... how much difference is a few minutes??

I spoke to a chap tonight about dives in the 140-160m range, who merges VPM-B and Buhlmann. I laughed.... again.. he's doing what works.

So whats my point? Deco is an inexact science...... don't push it... enjoy your diving and keep diving for many years.

Don't be fooled by glittery toys/deco models... they only represent peoples desire to model whats *really* happening. Do what 'feels' good. At the end of the day.. keep reading.. keep learning... keep an open mind.


/Zak

(just another cruton, floating on the bouillabaise of life)

[Mark Chase]

[quote]

Originally Posted by zaksherlock

its a good job Im not on here to make friends.

I don’t see any thing unfriendly in your reply Zac. It’s a balanced statement from the table’s camp. The article is my own work and therefore bias and I welcome comment and counter argument. It wasn't supposed to be definitive

In recent years deco seems to have gone through a number of trends, from "first out of the water" (still prevalent) to "effective deco" (still relatively rare).

There is a lot of bollocks talked about the basis of deco theory, from gas gradient to bubble models. At the end of the day, find what works for you.. be comfortable with it... and be open to discussion/change.

If I do a 50 -60m dive planned on 20/80GF with the VR3 set to 0 safety and actually stick exactly to plan I will only save a couple of mins with the computer tables over the VR3. This demonstrates that the VR3 algorithm is fairly conservative. However on my first ever dive on the Duke all I knew was that it was a 60m wreck. So I planned for 30mins at 60 with the usual +/-5, 10 15min bailouts. Andrew my dive buddy was on the same gas and same 2-deco mix as I. Having dropped onto the wreck we did the whole dive between 50 and 55m. in the cargo holds. We left the bottom on 30mins as planned and Andrew had 50mins of deco to do on his tables. I had 36mins showing on the VR3.

Andrew was so pissed off he went out and got one the following week.

That had nothing to do with a desire to get out of the water fast it was purely down to the dive not matching the plan. Comparing tables against the computer over all dives since the picture has stayed the same. However on our 100m dives in the red sea the opposite happened. Our first deco stop planned on deco planner was at about 66m. The VR3 surprised us with an 80m first stop. When we got back and analysed the dive our drop to 102m and a forced delay at 90m would have asked for a stop at 75m on deco planner. On that occasion the VR3 added deco for all the right reasons.

This business of "computers deco you faster" is probably true. Having said that, I don’t give a flying f*ck about speed of deco, what concerns me is how I feel after the dive (thermal/deco considerations). A damn good mate of mine (who happens to be an excellent diver), regularly exits the water 10-20 minutes before me on dives in the 60-80m range. Fair play to him... good luck.. he gets to bag the first biccies..... but frankly... how much difference is a few minutes??

I couldn’t agree more

Andrew and I add a 3m/min - 5m/min ascent from the deepest stop to the deco recommendations of the VR3 and regularly insert stops where we feel that the jump recommended on the VR3 is too big. We then clear the VR3 on the 6m stop and spend a further 5mins getting up the last 6m and out of the water. I then prefer to continue breathing 80% 02 on the surface and back on to the boat. Having sat down I will still breathe 80% until de kitted and totally relaxed. This adds surface waiting time (variable) and a minimum 5mins on the boat to the time spent breathing 80%.

As an old fat git I am in no hurry to do most things including deco. However in the light of my experiences with tables in the past I still prefer to use computers to monitor and calculate the dive. I had hoped the article I wrote expands on a range of issues rather than just keeping deco to a minimum.

I spoke to a chap tonight about dives in the 140-160m range, who merges VPM-B and Buhlmann. I laughed.... again.. he's doing what works.

So what’s my point? Deco is an inexact science...... don't push it... enjoy your diving and keep diving for many years.

Don't be fooled by glittery toys/deco models... they only represent peoples desire to model what’s *really* happening. Do what 'feels' good. At the end of the day.. keep reading.. keep learning... keep an open mind.


/Zak

What works works is a statement very well suited to deco theory. Unfortunately to find out what works you have to push the limits. I probably would have been well up for that aged 20 but aged 40 I want to avoid limits like the plague. I hope I was clear when I said no computer model or pre cut table could replace proper dive preparation and understanding of basic decompression theory. If I wasn’t then please read this and note it down because it’s very important.

Computer models, dive computers and PC decompression programs are tools. They can be used by skilled people to craft good clean profiles. Semi skilled people can create workhorse models that usually work and unskilled people can create total f#ck ups. Most of the time the unskilled person will get away with it and you can guarantee when it bites him he will blame his tools.

ATB

Mark Chase

[Andy Phillips]

Over the last few weeks I have been having a very interesting and informative discussion with a a guy who happens to have written and continues to modify one of the foremost deco packages out there (I am deliberatly not saying which). It also happens I am involved, (all be it in a small way) with a group in UCL (University College London) that are doing research into hyperbaric medicine, I have now put the two groups in touch with each other.

These recent discussions have taught me a lot about current theory into decompression. To say that deco is an in-exact science is no longer an accurate statement, in fact, there is a great deal known about what how why and when. Since the advent of doppler testing it has been possible to measure the formation and reduction of bubbles in the blood with absolute accuracy. It is also no accident that the majority of successful deco profiles, no matter what system was used to produce them are remarkably similar.

Deco on the fly is a joke, not because it doesn't work, it does, it is because it relies on the most falible of computers - the human brain, this is not my opinion, but that of far more experience people than myself. Buhlmans with gradient factors is also a bit of a joke, the ability to apply GF is in fact a fix to make the system work. The VR3 applies a further bodge to this by arbitraily adding in deep stops of two minutes, it does put them in the right place though. That being said if you run a profile out from the VR3 and compare it to the VPM-B algorythm it is so close as to be the same. RGBM is useless for deep dives because it is not designed to do them.

If you want the most accurate system that is the closest to current deco and hyperbaric theory/knowledge then follow the VPM-B process.

This being said, it is all just my opinion, me personally, I will continue to use my VR3, it rocks. I know enough to trust it's profile, I also have my own empirical data to go on (I've felt OK so far).

It's late, this is not the best post I have done, but I felt the need to comment, mainly because of the thread on the DIR page about DOTF.

Take care all,

Andrew

[Narked@50]

Originally Posted by imported_andyp

It's late, this is not the best post I have done, but I felt the need to comment, mainly because of the thread on the DIR page about DOTF.

Andrew

Surely you don't mean DIR could be DIW?

James

[Jules]

Mark
This is a really interesting article and covers the basics really well. It's good to see stuff posted that doesnt start at expert level like a lot of the DIR type discussions tend to. It's hard to follow a lot of these if you arent already knowledgable on the subject.
I havent done any deco training yet, hoping to do TDI deco procedures in the autumn. Nice to see some reasoned discussion in reply too.
Thanks guys
Jules

[Janos]

Fantastic thread this, which I'm really enjoying reading, although the deco here is well out the sort of stuff that I normally do (doing a 38m wreck this Monday and for me that's a 'big' dive) but then I'm fascinated by deco theory and a little background reading has changed the way I dive now (always try and do slow ascents etc)

Originally Posted by zaksherlock

Do what 'feels' good.

Zak, can you expand on this a little. After a days diving I always sleep the sleep of the righteous, even if all I've done is a couple of shallow dives in Wraysbury.

How do you know when you have cut it a bit fine?

Cheers,
Janos

[Pierre Farrugia]

Hello Mark,

Good write up! I also think that doing accelerated deco, one has to know what is going on in your body and how to efficcently offgas. Also I think that a gas diver first have to do dives with deco tabels, to learn how to cope with run times, deco gas planning and what a profile should look like. Also backup tabels are always important, once during an extended range dive, during ascent I realise that my pencil deco shcedule is gone , earesed (now I apply selfadhesive tape to prevent this).

Also I dive with a buudy using a Vr3 and we both use tabels cut from proplanner cut 10%/10% but on ascent the Vr3 will sometimes skip the 15m stop, is this normal? If you decide to do the 1m stop as per runtime would you bend the Vr3? this thing is being a little of a problem as we have to do deco seperatly as sometime he has to stop deeper than me and I cannot wait for him due to my schedule!

I also use a Vytec of which I find it usefull, especially as a backup. Mark , did you say Vytec allows for deep stops?

On an other note, I always try to get out of the water in a reasonable time, and I explaine why. I want to know that I am not over staying and at least know what profile I dive. Then at the end of the time I can decide how much safety stop at 6m to add.

Regards

Pierre Farrugia