TDI Decompression Procedures

[ianr33]

Well,as you asked for comments......................

If dive #2 is to 80 feet and dive #3 is to 180 feet I would say that that is rather a large step.

Why 80% ?

What do yo do if you can not find the 80% ?

 

[double80s]

Slightly on topic, if anyone is looking for an instructor in the Los Angeles area, my friend Bruce W is teaching Naui Tech. He just brought his website online, but it is still being modified/updated: SoCalTecDivers.com

The Naui course is structured very similar, 150' dives, shoot bags, gas switch, tox diver rescue, gas management and planning. Great dives & education for anyone interested in moving towards advanced diving.

 

[amascuba]

Well,as you asked for comments......................

If dive #2 is to 80 feet and dive #3 is to 180 feet I would say that that is rather a large step.

Why 80% ?

What do yo do if you can not find the 80% ?

Dive's 1 & 2 are going to be at a location called Blue Hole in Santa Rosa, NM and I would assume it's more to get me comfortable with double tanks and what ever skills that are going to be taught.

Dive's 3 & 4 are going to be at a location called Rock Lake outside of Santa Rosa, NM. It's not a huge hole, so losing the 80% would be hard unless you were extremely disoriented at the 30 foot mark. It's mostly clear water at the shollower depth. I would asume that the instructor will tie the bouy off somewhere so that it can't just drift around aswell. I'm with you though. I see no real reason for 80% based on my "computer dives" that I've planned, but that's what he has planned for the class.

 

[ianr33]

I just checked your profile and if accurate your deepest dive is 104 feet.

I dont want to tell you how to dive (or your instructor how to teach) but IMHO going from 104 feet to 180 is dangerous. Before my deco course my deepest dive was around 120. First course dive was to 130,last one to 150. Nice and gentle progression.

Dropping down to 180 before I had ever been in deco before would have played some very nasty games with my head,assuming I was not narked stupid.

The 80% thing is a rather,err.........heated subject.
http://www.subaqua.co.uk/reference/articles/bakers-dozen.shtml

I am not familiar with the site you will be diving so tieing off the 80% may make sense,but most people prefer to carry their deco gas with them if possible (caves excepted)

 

[Diesel298]

i never rely on gas hamging from a line, and always carry all my gas with me...
say your on a wreck and you have to blow off the wreck, or the line parts....
so far this season i have personally seen 3 divers have to surface away from the boat for whatever reason...
they wherer diving NDL limits or had all the nessary gas with them so it was all ok but still.
it happens

 

[loosebits]

I'm surprised you'll be using 2 deco gasses (I did the IANTD course but the instructor threw in a TDI Adv Nitrox card just so I could get the O2). In that range, the benefit of the extra bottle is marginal when weighed against the possibility of grabbing the wrong reg on your first switch. Of course since your 30' bottle wont be with you on your first switch, it's a non-issue but keep that in mind when diving in the future. Looking at profiles in the 150' range using EAN50 vs EAN50 and O2, you only shave a few minutes off the deco but you're hauling two bottles around which is a pain and introduces that added risk.

For the more experienced divers on the board, at what depth/time do you generally start considering using that second bottle?

 

[double80s]

I'm surprised you'll be using 2 deco gasses (I did the IANTD course but the instructor threw in a TDI Adv Nitrox card just so I could get the O2). In that range, the benefit of the extra bottle is marginal when weighed against the possibility of grabbing the wrong reg on your first switch. Of course since your 30' bottle wont be with you on your first switch, it's a non-issue but keep that in mind when diving in the future. Looking at profiles in the 150' range using EAN50 vs EAN50 and O2, you only shave a few minutes off the deco but you're hauling two bottles around which is a pain and introduces that added risk.

For the more experienced divers on the board, at what depth/time do you generally start considering using that second bottle?

I think he was saying that he slung two bottles for practice...swimming, handing them off, etc., he didn't mention doing 2 gas deco.

 

[amascuba]

I just checked your profile and if accurate your deepest dive is 104 feet.

I dont want to tell you how to dive (or your instructor how to teach) but IMHO going from 104 feet to 180 is dangerous. Before my deco course my deepest dive was around 120. First course dive was to 130,last one to 150. Nice and gentle progression.

Dropping down to 180 before I had ever been in deco before would have played some very nasty games with my head,assuming I was not narked stupid.

The 80% thing is a rather,err.........heated subject.
http://www.subaqua.co.uk/reference/articles/bakers-dozen.shtml

I am not familiar with the site you will be diving so tieing off the 80% may make sense,but most people prefer to carry their deco gas with them if possible (caves excepted)

I've thought about this aswell. My Instructor know's that my deepest depth. I'll have more time to talk to him soon so that I'm able to get a better idea about what exactly is going to take place on our check out dives. Personally, I like the DIR philosphy better on depth, which basically say's that from 20 - 100 feet you use 32% and for everything deeper you use helium based mixes. I don't really want to get into a pissing contest over mixes, because that's not part of this discussion. I'm sure that at 180 feet I will be more narced than I have ever been. I wouldn't mind being narced in a controlled environment, but really I'm not a huge fan of deep air. I would be more comfortable within the 130 - 150 foot range I believe. I'll discuss this more with him.

As far as Rock Lake goes. It's not really a "lake" in the sense that we normally associate with lakes. It's basically a deep hole in the ground, which is filled up with spring water. Like I said, I haven't been there yet. My instructor has several dives at this location know's the lake pretty well. For this particular location, I don't think that leaving the 80% tied off to a bouy that bad of an idea. I would assume that we would carry enough 50% to complete the decompression obligation in the event that we aren't able to locate the 80% during our stops. From what I can tell from running different profiles using 80% for deco and the same profile using 50% the difference in run time is only a couple minutes. It would seem likely that to run 80% the same as 50% (with proper MOD) to add a little bit more conservatism, but I'm no expert here as you guys know.

Also. I need to verify with my instructor if he means 180 feet altitude corrected or 180 sea level. Basically at the altitude of the lake when you're at 150 feet the altitude corrected depth is 180 feet.

 

[amascuba]

I'm surprised you'll be using 2 deco gasses (I did the IANTD course but the instructor threw in a TDI Adv Nitrox card just so I could get the O2). In that range, the benefit of the extra bottle is marginal when weighed against the possibility of grabbing the wrong reg on your first switch. Of course since your 30' bottle wont be with you on your first switch, it's a non-issue but keep that in mind when diving in the future. Looking at profiles in the 150' range using EAN50 vs EAN50 and O2, you only shave a few minutes off the deco but you're hauling two bottles around which is a pain and introduces that added risk.

For the more experienced divers on the board, at what depth/time do you generally start considering using that second bottle?

Good point loosebits! On your question; I'm not sure. My instructor and I haven't discussed the dive profile in depth yet. I had asked him a couple questions about the check out dives and he gave a overview of the dive profiles. I'll be talking to him more indepth about the dive profiles when I talk to him next. Thanks for your input.

 

[ianr33]

For the more experienced divers on the board, at what depth/time do you generally start considering using that second bottle?

Well speaking as someone that has done all of 2 dives that used 2 deco bottles I think it is not so much cutting the deco time but rather allowing for contingencies that matters.

Using Al doubles and a SAC rate of 0.6 I find that 25 minutes at 150 is about the maximum that you could deco on backgas in the event of a deco gas failure

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

Dec to 150ft (3) Air 50ft/min descent.
Level 150ft 22:00 (25) Air 1.16 ppO2, 150ft ead
Asc to 80ft (27) Air -30ft/min ascent.
Stop at 80ft 0:40 (28) Air 0.72 ppO2, 80ft ead
Stop at 70ft 2:00 (30) Air 0.65 ppO2, 70ft ead
Stop at 60ft 3:00 (33) Air 0.59 ppO2, 60ft ead
Stop at 50ft 4:00 (37) Air 0.53 ppO2, 50ft ead
Stop at 40ft 5:00 (42) Air 0.46 ppO2, 40ft ead
Stop at 30ft 9:00 (51) Air 0.40 ppO2, 30ft ead
Stop at 20ft 12:00 (63) Air 0.34 ppO2, 20ft ead
Stop at 10ft 23:00 (86) Air 0.27 ppO2, 10ft ead
Surface (86) Air -30ft/min ascent.

Off gassing starts at 107.9ft

OTU's this dive: 34
CNS Total: 12.7%

148.3 cu ft Air
148.3 cu ft TOTAL

In practice I would back this off a bit,so 20 minutes at 150 seems a reasonable maximum for me with 160 cu/ft of backgas and one deco tank. Steel tanks would give you longer