Free-diving decompression tables

[alexunder]

Hi all

I have read some very interesting discussion about DCS in free diving in this forum.

Can anybody refer me to some official free diving decompression tables / guidelines ?

Thanks a lot

Sasha

 

[ILDiver]

Stay on the surface 2 times the duration of your dive and you should be clear of Nitrogen. There are not tables for Deco. The nitogen only has time to get into the "fast tissue" and should clear out fairly quick.

 

[efattah]

Stay on the surface 2 times the duration of your dive and you should be clear of Nitrogen. There are not tables for Deco. The nitogen only has time to get into the "fast tissue" and should clear out fairly quick.

That doesn't work for me, and I would warn anyone against using 'official' tables, as I have followed two different ones and gotten bent many times with each one.

One of the problems with the tables is that they neglect the ascent speed, which is the main risk factor. To avoid DCS you need to average < 1 m/s on the ascent, and preferably < 0.75m/s in the last 20m of the ascent.

 

[ILDiver]

I've never heard the slower last 20m. I've always heard a steady 1m to 1.5m per second. Everyones physiology is different of course. I thought DCS in freedivers happens more in repeated "shallow" dives?

Eric, can you describe your day's dives when you got the DCS hit? I'm curious of the type of dives and how the hits came on.

I'm definately not arguing , I'm asking to learn. I've heard that people have gotten hit while freediving, but never seen any accounts of them.

 

[efattah]

As I have gotten some type of DCS probably 10 times, it would take a long time to describe them all.

However, having had minor to moderate DCS and one treatment in the chamber, I have learned to identify very 'subtle' signs of DCS that allow me to tell if I was on the edge of DCS. With this info I can then relate the times I was bent (even if slightly) and compare those profiles to the times when I didn't get any symptoms. When I post my profiles, people often say that 'wow, I've done that and never got anything...' BUT, maybe they did get a bit bent and just didn't notice the minor symptoms.

My worse even DCS was on the following profile:
20m, 30m, 44m, 48m, 51m, surface swim, 30m, 34m, with about 7-8 minutes rest between dives. Each dive as about 2'10". Ascents from the 40m+ dives were about 1.3m/s.

The most 'gentle' situation I have been able to get symptoms is from two 40m dives, each 1'40" long, 4 minute rest, 1.4m/s ascent.

My 2nd worst DCS hit was from 9 dives, 25-38m, average duration 2'44", 6'30" intervals, with slow ascents of 1 m/s. That's one of only three times I was bent with slow ascents. Another time was after two 70m dives (2'35") with 12 minutes rest.

Anyway, in my case, once I started controlling the speed of ascent and adding an apnea deco stop at 6m, I could do anything I wanted (any depth, any recovery interval) and never get bent.

I have also suffered DCS from a single dive, which throws out any idea of a 'table'. It was at Vertical Blue 2008, after a 62m FRC dive with a very fast sprint ascent (total time 2'12"). Obviously I don't do controlled ascents or apnea deco stops on 'target' competition dives. Two days later I did 67m with a similarly fast ascent, and went on O2 at 3m immediately after, and had no symptoms.

I have also found that if I don't have enough air to do a controlled ascent or an apnea deco stop, then I can surface, hyperventilate for 60 seconds, then dive down again immediately for an apnea deco stop for 60 seconds at 6m, and this dramatically reduces my risk of symptoms.

TylerZ got bent after a 70m dive followed by 3-4 30m no fins dives.

 

[demasoni]

My worse even DCS was on the following profile:
20m, 30m, 44m, 48m, 51m, surface swim, 30m, 34m, with about 7-8 minutes rest between dives. Each dive as about 2'10". Ascents from the 40m+ dives were about 1.3m/s.

Eric, was this with inhale + packing? What were your symptoms and did you go to the chamber for this one?

 

[ILDiver]

Wow, very interesting. Is there are there descriptions of the subtle signs anywhere?

 

[Aber]

Eric, it would be very interesting to hear what kind of symptoms you had in the various situations in the range from where you had "subtle symptoms" to more serious ones. Can these symptoms also be very different from individual to individual (the "subtle" and less serious ones)? I ask because I suspect I have had symptoms like this myself.

 

[alexunder]

That doesn't work for me, and I would warn anyone against using 'official' tables, as I have followed two different ones and gotten bent many times with each one.

One of the problems with the tables is that they neglect the ascent speed, which is the main risk factor. To avoid DCS you need to average < 1 m/s on the ascent, and preferably < 0.75m/s in the last 20m of the ascent.

Hi Eric,
It makes a perfect sense to me.
Shallower the depth during the ascend faster the pressure decreases.
the most significant change is in the last 10 meter : twice !

How do you decrease your surfacing rate during the last 20 meters?

 

[alexunder]

As I have gotten some type of DCS probably 10 times, it would take a long time to describe them all.

However, having had minor to moderate DCS and one treatment in the chamber, I have learned to identify very 'subtle' signs of DCS that allow me to tell if I was on the edge of DCS. With this info I can then relate the times I was bent (even if slightly) and compare those profiles to the times when I didn't get any symptoms. When I post my profiles, people often say that 'wow, I've done that and never got anything...' BUT, maybe they did get a bit bent and just didn't notice the minor symptoms.

My worse even DCS was on the following profile:
20m, 30m, 44m, 48m, 51m, surface swim, 30m, 34m, with about 7-8 minutes rest between dives. Each dive as about 2'10". Ascents from the 40m+ dives were about 1.3m/s.

The most 'gentle' situation I have been able to get symptoms is from two 40m dives, each 1'40" long, 4 minute rest, 1.4m/s ascent.

My 2nd worst DCS hit was from 9 dives, 25-38m, average duration 2'44", 6'30" intervals, with slow ascents of 1 m/s. That's one of only three times I was bent with slow ascents. Another time was after two 70m dives (2'35") with 12 minutes rest.

Anyway, in my case, once I started controlling the speed of ascent and adding an apnea deco stop at 6m, I could do anything I wanted (any depth, any recovery interval) and never get bent.

I have also suffered DCS from a single dive, which throws out any idea of a 'table'. It was at Vertical Blue 2008, after a 62m FRC dive with a very fast sprint ascent (total time 2'12"). Obviously I don't do controlled ascents or apnea deco stops on 'target' competition dives. Two days later I did 67m with a similarly fast ascent, and went on O2 at 3m immediately after, and had no symptoms.

I have also found that if I don't have enough air to do a controlled ascent or an apnea deco stop, then I can surface, hyperventilate for 60 seconds, then dive down again immediately for an apnea deco stop for 60 seconds at 6m, and this dramatically reduces my risk of symptoms.

TylerZ got bent after a 70m dive followed by 3-4 30m no fins dives.

WOW!
According to this, mandatory freediving deco stops could be a good idea

 

[Deepwater]

Get tested for a PFO.

If you want tables , get a custom decompression program you can pay with. I normally half my depth and pause for a few seconds. The pause every few meters for a few seconds.

 

[samdive]

I've also suffered from DCS after freediving and my doctor recommended a flat 10 minute surface interval for any dive to 30m or more (bearing in mind my max is only 40)

You'd learn about this on an AIDA 4 Star course - the info given there is:

Calculating surface time with depth
The depth/5 equals the surface time in minutes
Example:
depth 30m - 6min surface interval
depth 40m - 8min surface interval
depth 50m - 10min surface interval etc…
This applies to all dives. For example, after a 50m dive the freediver should wait 10 minutes before he can safety dive for their buddy

Calculating surface time with divetime
The surface interval equals the divetime x 2
Example:
Divetime 30 sec – 1 min surface interval
Divetime 2min – 4 min surface interval etc...
Common knowledge would suggest to use the ”Depth Table” for dives deeper that 30m and the ”Time Table” for shallower dives

and a whole lot more but you'd need to do the course!

Sam

 

[ILDiver]

I've also heard of deco recovery. Diving down and breathing off a bottle at about 10 feet. I don't recall all the details and don't want to give them and have incorrect data out there.

 

[Deepwater]

Pure Oxygen at 6m is beneficial for this. Your Po2 is at 1.6 and you drive out any inert gas and bubble damage.

 

[Bill]

It may be just semantics but IMHO the amount of O2 is not what is important in decompression. It's the partial pressure of N2. I don't have tables available to check but a local tech diver told me that decompression on 50% O2 is almost as good as on 100%(time wise) and doesn't require special equipment. I've never made a decompression dive so only interested in the freediving aspect.

 

[alexunder]

It may be just semantics but IMHO the amount of O2 is not what is important in decompression. It's the partial pressure of N2. I don't have tables available to check but a local tech diver told me that decompression on 50% O2 is almost as good as on 100%(time wise) and doesn't require special equipment. I've never made a decompression dive so only interested in the freediving aspect.

As far as I know, the highest % of O2 in recompression mixture is below 50% due to scuba equipment limitation (the highest o2 % regulators can deal with).
Anyway there are special tables which allow to calculate maximum exposure time to o2 according to its partial pressures.

Anyway overexposing to o2 can cause very dangerous o2 poisoning which affect central nervous system.

Cheers,
sasha:friday

 

[Deepwater]

Bill, you need to have a open a PO2 window to optimize safe decompression. You friend is correct , the only thing missing from what he said is what depth to use the gas at, in which case 50% would be great at 21m.The Pos would be around 1.56 then which is also good.

The deeper you deco, or start your deco, the better it is for you.

I use GUE standard Deco gases and even in-corporate helium in deco gases for some real extreme limits.

 

[Deepwater]

As far as I know, the highest % of O2 in recompression mixture is below 50% due to scuba equipment limitation (the highest o2 % regulators can deal with).
Anyway there are special tables which allow to calculate maximum exposure time to o2 according to its partial pressures.

Anyway overexposing to o2 can cause very dangerous o2 poisoning which affect central nervous system.

Cheers,
sasha:friday

Sasha,

You can use pure oxygen as long as your regulator is cleaned for O2 use. Once you have a clean O2 reg, you can use it for any mix over 40% Oxygen.
regarding CNS Poisoning, you need to be on heavy pseudoehiphidrines and be exposed to a PO2 of 1.6 for a while to generate hydrogenperoxide in you lung wall in order for you to tox out.

 

[alexunder]

Sasha,

You can use pure oxygen as long as your regulator is cleaned for O2 use. Once you have a clean O2 reg, you can use it for any mix over 40% Oxygen.
regarding CNS Poisoning, you need to be on heavy pseudoehiphidrines and be exposed to a PO2 of 1.6 for a while to generate hydrogenperoxide in you lung wall in order for you to tox out.

in other words breathing pure oxigen deeper than 6 meters (1.6) can cause these symptoms without pseudoehiphidrines

 

[alexunder]

and 79% of other inert gases will create partial pressure of 37.6 atm