Analyzing a 90 m trimix dive in dahab egypt

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jonas koh tao

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Scuba Instructor
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Location
Dahab Egypt
# of dives
Blue Hole 25th of January 2011. Depth 90 meter – Last dive of Full Trimix Course.

Divers:
Jonas Samuelsson – Instructor
Erik Brown - Student
Zdenek Jankovsky – Assisting instructor

To read the full brief

Support Diver:
Zakaria El Sayed

Dahab Egypt. Eilat Israel, Koh Tao Thailand

Planning a dive using V-Planner (by Erik C. Baker) and Combined Decompression Model – Dive Profile Analyzer – CDM-18 (by Steve Burton)

The dive was a bounce dive to 90 meters with a bottom time of only 2 minutes. Preparation before a trimix dive requires discussons related to many areas. Here we talk briefly about planning a dive using decompression software with the added touch of Steve Burtons CDM-18.

Below are just some points that we discussed regarding our diveplan on this particular day. We are not saying that the below plan is safe and neither is the designers of the V-Planner and CDM-18. You should dive within the limits of your training and remember you use any information on this website at your own risk. The purpose of this little ‘article’ is purely to illustrate some discussion points between team members prior to a mixed gas dive.

Divesite:

We started by looking at the divesite in it self. The Blue Hole in Dahab with an easy access and good visibily combined with a water temperature of 25 degrees and no current made this an ideal site for a 90 meter dive.

We were planning to descent following the contour of the Blue Hole to the entry of the Arch and have a little peak inside (our bottom time did not allow for a dive through the Arch and back). We were three divers going to 90 meters and we had one support diver meeting us at 21 meters during the ascent.

We all agree that the dive site was suitable for the dive.

V-Planner Configuration:
We then looked at the configurations of the V-Planner. We choose a SAC rate of 20 liters/minute both for the bottom gas/travel gas and the deco. We all had better SAC rates than this, but better to be conservative when it comes to gas planning. The descent rate was set at 15 meters/min (also a fairly slow descent rate, but once again a more conservative one). Our ascent rate was set for 9 meter per minute for the ongassing phase of the dive and 6 meters/minute for the off gassing phase (see plan below, off gassing starts at 59 meters and that’s where we slowed down from 9 m/min to 6 m/min.). Our accepted ppO2 for theO2 decompression part of the dive was set at 1.4 ppOO2 for the mix with O2 up to 28%, 1.5 ppO2 up to 45% and 1.6 ppO2 up to 99%. We used conservatism "+2" in the V Planner Configurations.

Bottom gas, 12/60
Oxygen:
We wanted our bottom ppO2 to be well within 1.4ppO2 so we choose a mix with 12% O2 which gave us a pp02 of only 1.19. well within our set limits, all to avoid CNS Problems.
Helium:
We wanted to keep our END (Equivalent Narcosis Depth) low so we chosed a 60% HE mix which gave us an END of 30 meters. HPNS was not an issue being shallower than 100 meter so high HE mix in the bottom mix was not a consideration.
Nitrogen:
So we ended up with a low 28% Nitrogen mix in our bottomgas which resulted in a low END as mentioned above. This amount also relates to ICD considerations during our gas switch at 21 meters. We don’t want to change to a deco gas with a higher Nitrogen level than our bottom gas due to risk of ICD (Isobaric Counter Diffusion - There is still a lot of discussion about prevention of DCS type III caused by ICD, generally accepted rule for a gas switch is that for every 5% decrease of helium can be nitrogen increased by 1%, recent discussion leads to more complex rules for extreme dives but to keep it super simple: basic rule is to avoid any jumps in ppN2 (links to more detailed discussions are available on www.blue-immersion.org)). Our gas planning discussion resulted in the below Deco gas.

Deco gas 1, 50/20
Oxygen:
We decided for 50% mix in our first deco gas. 50% is a gas with a great efficiency ratio of 1:1.32 (compared to Air - 1:1). It’s of course lower than 80% O2 (1:1.37) and 100% O2 (1.1.42) but the advantage is that you can use this gas within ppO2 of 1.6 already at 21 meters making it a very flexible gas.

Helium
To avoid ICD (Isobaric Counter Diffiusion) we mixed out first deco gas with 20% HE to offset the effects of ICD. Remember that we had 28% Nitrogen in our bottom mix. So to avoid ICD problems we want our total amount of Nitrogen (Inert Gas) to be lower after the switch. By having 50/20 mix we only increased our Nitrogen mix with 2% which we decided was within acceptable limits in our planning for ICD avoidance. Our goal is to always avoid any jumps in ppN2. (The planned gas was approved by check in Steve Burtons CDM-18***).

Nitrogen:
Set low to avoid ICD.

Deco gas 2, 100%
Oxygen:
We switched to 100% O2 at 6 meters for an efficient offgassing during the last, and most important part, of our decompression. We also added 3 minutes of breathing 100% O2 during our ascent from 3 meters to the surface. Our ascent rate for the last 3 meters was a very conservative 1 meter/minute. Now we looked at our OTUs and CNS which both were well below accepted limits for oxygen exposure. We had one issue regarding Air Breaks. Our total time breathing 100% O2 was 16 minutes (6 meter and 3 meter stops plus 3 minutes for the added ascent from 3 meter to the surface and 3 minute safety stop on the surface). We decided to take an Air break using the 50/20 mix with less Helium than the bottom mix after 12 minutes on pure oxygen. Due to the low levels of O2 (12%) in the bottom mix and the ICD risk we decided that breathing 70% Helium during the Air break would have increased our risk of ICD due to Helium’s fast on gassing, moreover our bottom mix is hypoxic and at 3 meters would be ppO2 below 0,18. We always plan air breaks for exposure of 100% O2 for more than 15 minutes. So the total time of the dive was actually 58 minutes (total run time: 49 minutes plus 3 minutes added for the ascent from 3 meters to the surface, 3 minute air break (using 50/20 mix during last deco stop at 3 meters and 3 minutes safety stop breathing pure oxygen on the surface).

Brief summary of a planning a trimix dive in bluehole dahab. tec rec trimix course 90 meters |
Travel gas:
We decided to use our 50/20 mix to a depth of 10 meters where we safetly could breath from our bottom mix of 12/60. Using Travelgas did not change the runtime of the dive and did not have any measurable effect on our gases (approx 20 liters less of 50/20 and approx 20 liter more of our 12/60 mix – a 10 meter descent using 15 meters/minute and breathing 20 liters per minute plus minus a few liters)

Bottom time:

We set, as mentioned earlier, our descent rate at 15 meters/minute so to our descent to 90 meters took us approx 6 minutes. So we decided to only stay for 2 minutes at 90 meters due to gas requirements.

Gas requirements for this dive were:

2712.6 ltr Trimix 12/60
629.7 ltr Triox 50/20
364.2 ltr Oxygen
3706.5 ltr TOTAL

If we decided to stay a few minutes longer we would no longer have the required reserve to conduct the dive safely. We discussed bringing larger tanks than our 11 liters twins but decided for a shorter bottom time instead.

Lost gas:

We were three divers carried our tanks during the complete dive so we made a calculated assumption that the likelyhood of all three of us loosing both our 50/20 and 100 mix was very slim but anyway we decided to use support diver for this dive whom we met on our return at 21 meters and he was carrying one tank of each deco gas we were using on the dive.

No failures - normal dive - OK
2712.6 ltr Trimix 12/60
629.7 ltr Triox 50/20
364.2 ltr Oxygen
3706.5 ltr TOTAL

Lost or missing deco gas: 50/20
4164.6 ltr Trimix 12/60
19.9 ltr Triox 50/20
474.0 ltr Oxygen
4658.5 ltr TOTAL

Lost or missing deco gas: 100
2786.2 ltr Trimix 12/60
1155.6 ltr Triox 50/20
3941.8 ltr TOTAL

Lost or missing deco gas: Both
5767.7 ltr Trimix 12/60
19.9 ltr Triox 50/20
5787.7 ltr TOTAL

In Dahab you are required to have a support diver (s) for any dives deeper than 100 meters. Our dive depth was 90 meters. We did not need a support diver to assist us with tanks during our decos either. We only had two deco tanks. But its good practice for deeper dives and a support diver its always a great backup if everything else would fail. Someone with extra gas and a fresh mind couldn’t not hurt was our reasoning.

Here is the full plan we used that day. The plan was checked using Steve Burton’s excellent software The CDM-18 which I recommend to every technical diver to use after planning your dive using a deco software.
CDM-18 find if there is any problems with the dive plan in regards to ICD and DCS. There is a link further down to Steve's website if you are interested in buying your own version.

V-Planner 3.89 by Ross Hemingway,
VPM code by Erik C. Baker.

Decompression model: VPM - B

DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0m
Conservatism = + 2

Dec to 10m (0) Triox 50/20 15m/min descent.
Dec to 65m (4) Trimix 12/60 15m/min descent.
Dec to 90m (6) Trimix 12/60 15m/min descent.
Level 90m 2:00 (8) Trimix 12/60 1.19 ppO2, 25m ead, 30m end
Asc to 59m (11) Trimix 12/60 -9m/min ascent.
Asc to 48m (13) Trimix 12/60 -6m/min ascent.
Stop at 48m 0:43 (14) Trimix 12/60 0.69 ppO2, 11m ead, 13m end
Stop at 39m 1:00 (16) Trimix 12/60 0.58 ppO2, 7m ead, 10m end
Stop at 36m 1:00 (17) Trimix 12/60 0.55 ppO2, 6m ead, 8m end
Stop at 33m 1:00 (18) Trimix 12/60 0.51 ppO2, 5m ead, 7m end
Stop at 30m 1:00 (19) Trimix 12/60 0.48 ppO2, 4m ead, 6m end
Stop at 27m 2:00 (21) Trimix 12/60 0.44 ppO2, 3m ead, 5m end
Stop at 24m 2:00 (23) Trimix 12/60 0.41 ppO2, 2m ead, 4m end
Stop at 21m 2:00 (25) Triox 50/20 1.54 ppO2, 2m ead, 15m end
Stop at 18m 2:00 (27) Triox 50/20 1.39 ppO2, 1m ead, 12m end
Stop at 15m 2:00 (29) Triox 50/20 1.24 ppO2, 0m ead, 10m end
Stop at 12m 3:00 (32) Triox 50/20 1.10 ppO2, 0m ead, 8m end
Stop at 9m 4:00 (36) Triox 50/20 0.95 ppO2, 0m ead, 5m end
Stop at 6m 5:00 (41) Oxygen 1.60 ppO2, 0m ead
Stop at 3m 8:00 (49) Oxygen 1.30 ppO2, 0m ead
Surface (49) Oxygen -6m/min ascent.

Off gassing starts at 59m

OTU's this dive: 50
CNS Total: 24.1%

2712.6 ltr Trimix 12/60
629.7 ltr Triox 50/20
364.2 ltr Oxygen
3706.5 ltr TOTAL


Now when we agreed to the plan and checked and double checked that we were within ppO2 limits, that we had no or limited risk of DSC and ICD, that we had enough gas for the complete dive including lost gas scenarios. Our END of only 30 meters made sure we would not suffer from any considerate narcosis. The plan was signed and sealed.

The dive was conducted with the plan on a wrist slate as a primary and as backup we all had the plan written together with emergency scenarios in our wetnotes. As backup to the backup we carried mixed gas

The dive was conducted perfectly according to the plan and its was an amazing dive!

Dive safe and please share with us any comments that could improve the above diveplan. We would like to thank Steve Burton for tips and hints regarding ICD and Bruce Kenofe who provided us with ANDIs deco gas efficiency chart.

Jonas Samuelsson
Technical Diving Instructor Trainer
Teamblueimmersion@gmail.com
www.blue-immersion.org

Dahab Egypt, Eilat Israel, Koh Tao Thailand

Sources:
Steve Burton www.scubaengineer.com
ANDI Oxygen Efficiency Ratio

Written by:Jonas Samuelsson with team Members Zdenek Jankovsky and Erik Brown,
Team Blue Immersion, www.blue-immersion.org
 

Attachments

  • deco model 1.pdf
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The pp02 increase seems a bit too steep, from 0.41 to 1.54 during first gas switch. Why not three deco gases, to keep the pp02 at a more balanced level during switches?

It seems a lot of precaution was done to avoid ICBD (which is debatable anyway like you said) and ppN2 spikes, but the pp02 looks to me like more of a possible issue in a dive on depth like these.

Thanks.
 
How much backgas gas did you reserve to get an OOA buddy from 90m up to 21m? ~14mins at an average depth of 40m with 2 stressed divers...?

Personally, I would not bother with 2min bottom time dives but to each his own.
 
How much backgas gas did you reserve to get an OOA buddy from 90m up to 21m? ~14mins at an average depth of 40m with 2 stressed divers...?

Personally, I would not bother with 2min bottom time dives but to each his own.

A Single AL80 with the appropriate back gas should be good enough to be used for OOA for this type of dive. I must add that your first encounter with backup divers should then be at around 40m or deeper with additional deco/back gas.

I agree with the BT, maybe it was part of a training dive?!?
 
How much backgas gas did you reserve to get an OOA buddy from 90m up to 21m? ~14mins at an average depth of 40m with 2 stressed divers...?

Personally, I would not bother with 2min bottom time dives but to each his own.
"To each his own. . ." Rjack --why don't you show a little oceanographic curiosity & world savvy, and do a simple search before offering up one of your usual clueless & trite opinions?

2 minutes at 90m for a training dive in the Blue Hole of Dahab sounds fine, since all the interesting features of the reef structure & marine life seem to start at the arch/tunnel at 56m and up through to deco depth ranges. A better question would be, did they traverse any length ("small peak inside") of the 26m length of the arch as well as planning a minimum gas reserve for this?

To answer your original question Rjack [why didn't you calculate it out and enlighten us all yourself?], figure they would need around 6300 litres to get an Out-of-BackGas Buddy from 90m to the first deco stop at 21m, which would require another aluminium 11 litre stage bottle of bottom mix 12/60 together with at least double aluminium 11 litre backgas tanks of bottom mix for each teammate. This totals roughly 6600 litres of bottom mix for each of the three-man team. Better option would be larger capacity doubles (15 to 17 L) and the use of at least one more intermediate deco mix starting at 36m (35/35 trimix) --minimum gas reserve from 90m to 36m would now be roughly 4000 litres.
The Blue Hole is notorious for the number of diving fatalities which have occurred there earning it the sobriquet, "World's Most Dangerous Dive Site" and the nickname "Diver's Cemetery". The site is signposted by a sign that says "Blue hole: Easy entry". Accidents are frequently caused when divers attempt to find the tunnel through the reef (known as "The Arch") connecting the Blue Hole and open water at about 52 m depth. This is beyond the PADI maximum recreational diving limit of 40m and the effect of nitrogen narcosis will be significant at this depth. Divers who miss the tunnel sometimes continue descending hoping to find the tunnel farther down and become increasingly narced.

The "Arch" is reportedly extremely deceptive in several ways:

It is difficult to detect because of the odd angle between the arch, open water, and the hole itself.
Because of the dim lighting and the fact that most light enters from outside, it appears shorter than it really is. Divers report that the Arch appears less than 10 m long but measurements have shown it is 26 m from one end to the other.
There is frequently a current flowing inward through the arch towards the Blue Hole, increasing the time it takes to swim through.
The arch continues downward to the seabed which is beyond view and there is therefore no "reference" from below.

Technical diver passing under the Arch.Divers who resist the temptation of the Arch and remain within their training and limitations are in no more danger than on any other Red Sea dive site. However, the Arch has proved irresistible for many and thus the divesite is considered unsuitable for beginners and a potential trap for even experienced divers.

http://en.wikipedia.org/wiki/Blue_Hole_(Red_Sea))
 
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"To each his own. . ." Rjack --why don't you show a little oceanographic curiosity & world savvy, and do a simple search before offering up one of your usual clueless & trite opinions?

For the risk, effort, logistics & cost, I thought a BT of 2 minutes was quite odd. Even for a training dive with lots of other stuff to see. But sure, each to their own.

I might also have asked this sort of question;

How much backgas gas did you reserve to get an OOA buddy from 90m up to 21m? ~14mins at an average depth of 40m with 2 stressed divers...?

To elicit a discussion about things like more appropriate deco gases, size of tanks, stage bottles, etc. Something like this perhaps;

Better option would be larger capacity doubles (15 to 17 L) and the use of at least one more intermediate deco mix starting at 36m (35/35 trimix) --minimum gas reserve from 90m to 36m would now be roughly 4000 litres.

Of course I might be clueless & trite, or I might think there are better ways to plan such a dive. On the other hand, I may just prefer to give people a hard time.
 
The OP didn't say where the safety diver as meeting them and also didn't mention an OOA (backgas) scenario despite listing various lost deco gas scenarios. He asked for opinions and rock bottom (along with the role of a safety diver in it) seems to be lacking from his plans.

If you only do 2min dives in training does that mean you're only qualified to do 2min dives?
 
I don't think it makes sense to comment the 2 minute aspect, I'll just write that one to 'for training purposes' or whatever thing.

However, other questions are valid enough and would be interesting to hear from the OP.
 
http://cavediveflorida.com/Rum_House.htm

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