Please help me stop!

[Slamfire]

If you are going to use 2 gases but use neither 2 gas decompression tables nor a computer that can handle 2 gases, then you have relinquished your dive profile to guesswork.

Oooh :shocked2:. That sounds like a personal attack without having all the information about me, what goes through my head or having the particulars about the dive in question. Tell you what. I'll post the profile in question so that it can become a more educated attack and have less baseless speculation (see attachment).

As you can see from the attachment at 31 mins into the dive I have a deco obligation of 8 mins (@ 10ft). I know this is not true because at that point I have been on 50% for 10 mins already. But that's fine, I just chuck into added conservatism and not take that into account. I'm not very concerned with %CNS because this is the last dive of the week for me. I know off the top of my head that the EAD for 50% at 35' is 10'. So I spend 12 extra mins (not 8) at an EAD of 10' or above. I know it is not the same as doing deco on air at the physical 10', but that, added to the extra 10 mins I was on 50% before arriving at the 35ft mark, makes me feel all warm and fuzzy about going to the surface before my computer tells me I'm cleared. So, yes, it is a guess, but I'm stacking a lot of odds in my favor in this guess. I believe I am making a thinking, reasonably logical decision here. After all, computers and their algorithms are statistical models that try to help you make an educated guess that will hopefully not get you bent -- but as we all know there's never a guarantee on that.

There was a recent thread in which a diver described his panic when he went into deco by one minute and did not have any idea that his computer could guide him through the ascent. He had also forgotten that he had been taught what to do on tables in that case in his OW class.

This is one of the points I'm trying to make. There's not need to be so fearful of the NDL line. If you cross it only by a little bit, you'll consequently have a little bit deco obligation that could very likely be satisfied with an appropriately slow ascent. No need to panic about it or flee in a rush from it in what could be dangerous ascent speeds in the attempt to get farther away from that ominous NDL line.

Don't get me wrong, deco is not to be treated lightly and you should get training and knowledge to handle it well. But thinking that because you're too fast at running away from that ever decreasing NDL line you're being more conservative than crossing one or two mins into that line is a fallacy that could easily get you bent.

 

[TSandM]

Slamfire, I think what John is saying, and I would echo it, is that you don't seem to be presenting any coherent method for determining how much decompression time you need to do on your 50%. Your computer is wrong, because it doesn't know you're accelerating your decompression, so what ARE you using to decide how long to hang?

 

[Slamfire]

Slamfire, I think what John is saying, and I would echo it, is that you don't seem to be presenting any coherent method for determining how much decompression time you need to do on your 50%. Your computer is wrong, because it doesn't know you're accelerating your decompression, so what ARE you using to decide how long to hang?

I use the force.

As I said before, went to an EAD of 10' when I had 8 mins obligation and stayed at EAD of 10' or less for 12 mins. It is a guess, but I did stack odds in my favor. To be honest, I originally planned on fulfilling the computer requirement and stay the extra time my computer wanted me to stay, but my buddy thumbed the dive. No dire circumstances, he just wanted to go up and I went up with him. Previously, when I got to 35' I did make a mental note of the time because I know 35' = 10' EAD with 50%. So when my buddy thumbed the dive I knew I had spent 12 mins at 10' EAD or less. Compare that to the 8 mins the computer wanted me to stay down at 10' and I think that I'll be happy to grant the wishes of my dive buddy to go up.

In the past when he's given me a thumbs up I've replied that I still want to stay x amount of mins decompressing and he complies with that. He uses a Tec 2G in bottom timer mode and UTD deco methodologies - ratio deco, min deco...

EDIT: I know that my computer is wrong, but if I follow it while using 50% the error will be on the safe side -- it'll be erring into conservatism. Of course, I just have to make sure there will be no conflicts with exceeding max operating depth for 50%, %CNS, and OTU's (kind of a stretch to get into problems with OTU's).

 

[boulderjohn]

I use the force.

As I said before, went to an EAD of 10' when I had 8 mins obligation and stayed at EAD of 10' or less for 12 mins. It is a guess, but I did stack odds in my favor.

Just for fun, I ran your dives through V-Planner, guessing 1 hour for the surface interval.

Here is what that program would have given you for your second dive:

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

Dec to 92ft (1) Air 50ft/min descent.
Level 92ft 42:10 (44) Air 0.78 ppO2, 92ft ead
Asc to 70ft (44) Air -30ft/min ascent.
Asc to 40ft (45) Nitrox 50 -30ft/min ascent.
Stop at 30ft 3:16 (49) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 25:00 (74) Nitrox 50 0.79 ppO2, 0ft ead
Surface (74) Nitrox 50 -30ft/min ascent.

EDIT:
That was with the last stop at 20 feet. You can save time doing your last stop at 10 feet.


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

Dec to 92ft (1) Air 50ft/min descent.
Level 92ft 42:10 (44) Air 0.78 ppO2, 92ft ead
Asc to 70ft (44) Air -30ft/min ascent.
Asc to 40ft (45) Nitrox 50 -30ft/min ascent.
Stop at 30ft 3:16 (49) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 6:00 (55) Nitrox 50 0.79 ppO2, 0ft ead
Stop at 10ft 16:00 (71) Nitrox 50 0.65 ppO2, 0ft ead
Surface (71) Nitrox 50 -30ft/min ascent.

Off gassing starts at 67.6ft

OTU's this dive: 41
CNS Total: 34.9%

114.3 cu ft Air
24.0 cu ft Nitrox 50
138.3 cu ft TOTAL

 

[Slamfire]

Just for fun, I ran your dives through V-Planner, guessing 1 hour for the surface interval.

Just to join you in the fun I ran the dive through vplanner using a bottom time of 18min (your mistake is that your putting 44 minutes bottom time -- look at my attachment bottom time was only 18, overall run time was 44)
Here's what comes up.
V-Planner 3.43 by R. Hemingway, VPM code by Erik C. Baker.

Decompression model: VPM-B

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

Dec to 92ft (1) on Air, 60ft/min descent.
Level 92ft 16:28 (18) on Air, 0.79 ppO2, 92ft ead
Asc to 70ft (18) on Air, -30ft/min ascent.
Asc to 20ft (23) on Nitrox 50.0, -10ft/min ascent.
Asc to 15ft (24) on Nitrox 50.0, -10ft/min ascent.
Stop at 15ft 0:46 (25) on Nitrox 50.0, 0.73 ppO2, 0ft ead
Asc to sfc. (26) on Nitrox 50.0, -10ft/min ascent.

Off gassing starts at 61.5ft

OTU's this dive: 19
CNS Total: 6.8%

47.9 cu ft Air
10.7 cu ft Nitrox 50.0
58.6 cu ft TOTAL

As you can see I did a much more conservative profile. I like conservatism. It is my friend.

 

[boulderjohn]

Just to join you in the fun I ran the dive through vplanner using a bottom time of 18min (your mistake is that your putting 44 minutes bottom time -- look at my attachment bottom time was only 18, overall run time was 44)

My misread. Sorry.

 

[Slamfire]

My misread. Sorry.

no prob. It's been an interesting conversation.

 

[Walter]

I was trying to simplify a more complex explanation.

Sorry, I missed that. It's usually easier and simpler to just say what you mean.

The truth is that 1 minute of deco will probably be satisfied by a normal safety stop. It might be satisfied before the normal safety stop, provided a good ascent profile.

True.