TDI Decompression Procedures

[ianr33]

amascuba,

Doing you first deco dive with 2 deco tanks seems unneccesarily complex to me.

 

[amascuba]

I'm not disagreeing with you at all ianr33. I would definitely question the sanity of carrying both gases during the dive. I'll get more information from my instructor the next time I talk to him.

 

[Diesel298]

two decoomixes huh..
thats odd espically for a training dive...
if you do the math you will find that its not that much shorter of a deco switching from 50 to 80
you only gonnade doin a few minn at the 50% stops....
is there a reason for this? Does the instructor have a reason for this (serious question)
or just trying to sell youi another tank, and reg?
plus the big jump in depth your taking very fast..
there just seems to be alot of little abnormalities in this...
granted im not there and getting all the particulars.. and dont personally know you and your abilities as your instructor does..
but im gettin a little but of a vibe here

 

[loosebits]

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

[insert v-planner profile]

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

Yep, I guess with such limited backgas, 2 bottles would be called for. I'm always in 104's and with a cave fill, that's almost 4 80's. Usually for my dives in the 150' range, I use EAN50 for deco as it actually gets you out of the water faster than O2. It seems as a rule of thumb, the higer your PO2 is at depth, the less useful O2 is as a sole deco gas compared to making a switch to EAN50 at 70'.

This brings me to a gripe with v-planner. Say you only use O2 as your deco gas. You cut some tables with O2 and with lost gas. Now, lets say I get to my 20' stop and for some reason discover I have no O2. The lost gas profile in V-planner would have me lengthen my earlier stops that I've already completed instead of telling me what I need to do at 20 and 10' to compensate for being on backgas.

-- lost gas -->

OK 100

150 (20) (20)
70 0 (23)
60 1 (24)
50 1 (24) 1 (25)
40 2 (26) 3 (28)
30 3 (29) 4 (32)
20 3 (32) 6 (38)
10 5 (37) 13 (51)

The lost gas profile is the same profile as diving just backgas and not planning on using O2. What v-planner should do here is assume the 50 - 30' stops are the same and tell me what I need to do on the 20 and 10' stops to get out on backgas. In this example, the deeper stops are pretty close so it probably wouldn't matter but I can imagine other profiles where the deeper stops are a little further off.

 

[ianr33]

From V Planner FAQ:

Top 10 questions

1. Lost Deco planning times do not line up? Is it a bug?
Not a bug. In VPM-B, your ascent is calc'd for the entire profile, before the final plan is generated. i.e. using 80/oxygen (or any deco gas) in a dive, has deco benefits across the whole dive, some of which can be realized ahead of time. Adding deco mix to a dive changes the entire ascent, and each plan is different.
Older programs and table based models are single step models. i.e. what happens next has no affect on what just been done and visa versa.
The tricky part is when you get to deco level and discover the deco tank doesn't work. My suggestion, is to wait out the additional time at the current level, and pick up on the subsequent deco times. Remember, this is for emergency planning, and might never happen to you ever in a whole career. Also you have a margin of extra time above bare minimum req'd deco, so are you still OK.

Agree that for our weenie dives the odd minute does not matter too much. If I had to resort to deco on backgas I would probably pad it a bit anyway for my own peace of mind.

After lugging 104's around in Florida a couple of weeks ago on my Intro Cave course I realised how much I like my lightweight Al 80 doubles for Travis muck dives. !!

 

[loosebits]

Yep, I agree that it isn't much of an issue for us but it still bugs me. The difference in a bug and a missing feature is in the eye of the programmer. Since he acknowledges it, that makes it a missing feature. Either way it's just lazy programming.

 

[matt_unique]

...
Dive 1 & 2: 80 foot decompression dive with out a any accelerated decompression.
Dive 3 & 4: 180 and 150 foot decompression dive with 80% and 50% decompression dive. He basically said that I'm going to carry a stage with the 50% and the 80% is going to be hanging at 30 foot from a surface buoy.....

That will be something....particularly on air. I am shocked the instructor would allow this in a course. I wonder if his plan was to include Extended Range? I too have no interest in Extended Range.

I completed Deco and Adv. Nitrox and they were my best courses ever. I will not dive deeper than 160' unless I'm breathing Trimix. I'm not certified for "the mix" as of yet so 160' is it for me for now. I most often notice the martini affect when I descend below 140'. For whatever reason, that is the magic number for me. I'm not "bombed" at 160' but I do feel it. I will do low task dives on air to 160'. I use 70% as a best single deco gas. If you run the numbers, 70% is the best single deco gas for the 150 range.

--Matt

 

[amascuba]

two decoomixes huh..
thats odd espically for a training dive...
if you do the math you will find that its not that much shorter of a deco switching from 50 to 80
you only gonnade doin a few minn at the 50% stops....
is there a reason for this? Does the instructor have a reason for this (serious question)
or just trying to sell youi another tank, and reg?
plus the big jump in depth your taking very fast..
there just seems to be alot of little abnormalities in this...
granted im not there and getting all the particulars.. and dont personally know you and your abilities as your instructor does..
but im gettin a little but of a vibe here

The instructor hasn't offered to sell me any gear actually. I've talked to him about wings, regulators, tanks, etc. He talked about what he looks for in gear, but never really mentioned any brands other than showing me what he uses. If he was trying to sell me gear he was the worst sales person I've ever met.

As I've said, he gave a basic overview of the dive profiles. I'll get some more detail the next time I talk to him. I maybe misconstrewing the dive profile.

 

[loosebits]

I'm not "bombed" at 160' but I do feel it. I will do low task dives on air to 160'. I use 70% as a best single deco gas. If you run the numbers, 70% is the best single deco gas for the 150 range.

--Matt

70% just sounds funny

I'll stick with 50 or O2 (or 80 if I don't have enough HP O2 in my banks).

 

[amascuba]

That will be something....particularly on air. I am shocked the instructor would allow this in a course. I wonder if his plan was to include Extended Range? I too have no interest in Extended Range.

I completed Deco and Adv. Nitrox and they were my best courses ever. I will not dive deeper than 160' unless I'm breathing Trimix. I'm not certified for "the mix" as of yet so 160' is it for me for now. I most often notice the martini affect when I descend below 140'. For whatever reason, that is the magic number for me. I'm not "bombed" at 160' but I do feel it. I will do low task dives on air to 160'. I use 70% as a best single deco gas. If you run the numbers, 70% is the best single deco gas for the 150 range.

--Matt

I'm with you there. I was shocked at first as well. Especially since the tdisdi.com website say's that depth limits for Decompression Procedures is 150 feet. Before this whole thing get's blown out of proportion though I'll get a better dive plan from my instructor.