Ratio Deco on 32%

[lloydwatkin]

I've recently read Garf's post of the TFT blog and have been thinking about the whole ratio deco thing just as a backup tool if things go wrong. After reading Scuby's post here as well I thought I may as well ask.

For example, these days when I end up in deco I've got a fair idea of how much I'll be doing from the dive time and/or air pressure remaining. This has come from experience and just 'remembering' the numbers.

So I was wondering if there's a ratio deco for 32%? I've run a fair number of profiles on ddPlan and it basically seems like for every minute at 30metres (on 32%) you do 30 seconds deco, provided the deco isn't too long (<20mins although I haven't tested this fully, plus I wouldn't have enough gas in real life!). If not then I'll just keep running profiles and see what happens when I fiddle with the numbers, but I'd be interested to know if there's something out there.

Cheers, Lloyd.

[Scuttler]

The dark side bekons

[Howard Payne]

Put simply - a fit, normal person (that's you ) has got 30min before going beyond min deco at 30M (average) on Nitrox 32/30. Sometimes referred to as the 30/30/30 rule

For a 30m dive- min deco is 1@15m; 1@12m; 1@9m; 1@6m; 1@3m - IE a slow controlled ascent with 1min stops in 3m increments starting from half your ave depth to control microbubbles. Microbubble stops would start at 12m for an ave depth of 24m etc etc

Normally the easiest way to carry this out is as a 30sec stop at the target depth and then 30 secs travelling up to the next increment.

For every 3m deeper or shallower you go than the 30m "setpoint" you add or subtract 5min to or from you min deco limit for the 3m. IE 25min is the min deco limit at 33m, 35min at 27m, 40 min at 24m. This ratio breaks down below about 21m.

For every minute you exceed the min deco limit - you add a minute of backgas deco to your profile IN ADDITION to the min deco profile you would already do. IE a 40 min dive at 30m average depth needs 10 mins of deco in addition to the min deco ascent. The majority of this deco needs to be done at 6 + 3m - usually 6 because its easier to hold a stop in the ocean slightly deeper. Without a gas gradient (ie no deco gas) the overall shape of your deco curve should be exponential.

For the example there - I would probably do 1@15; 1@12m; 2@9m; 6@6m and then a slow ascent of 3min between 6m and the surface (sea conditions allowing - perhaps all at 3m if its rough). I appreciate this isn't totally exponential - but trying to hold a stop close to the surface when its choppy isn't ideal as I'm sure you've found.

The cleaner way of decoing on longer 30m range dives is to use O2 at 6m - this halves the amount of deco you need compared to backgas - but I strongly don't recommend you try that without appropriate training.

Several things to note:

Your depth is average depth during the dive - not max
Don't overlook the slow transition from 6m to the surface - it where the biggest pressure gradient is and it'll greatly improve the way you feel after a dive.
Clearly this is for Nitrox 32 - not Nitrox 21!
I would probably do 5min from 6m to the surface (instead of 1@6m and 1@3m) as min deco (ie more) if I was doing multiple min deco dives on multiple days
You need to move quite smartly up from the bottom to your first "bubble control stops" at around 15/12m (depending on depth) - IE 9m/min - dawdling around in the deep part of your ascent is effectively just adding to your bottom time.

My guess is that on your single 15 - even with your good SAC rate Lloyd - you'll be struggling to get into any real deco due to limited bottom gas. A twinset is really where the possibility of any real deco in the 30m range becomes likely

IF YOU ARE IN ANY DOUBT ABOUT YOUR ABILITY TO EXECUTE ANY OF THIS EASILY - GET PROPER TRAINING AND/ OR JUST USE A COMPUTER. I WON'T PUSH YOUR WHEELCHAIR - BUT I WILL EAT YOUR GRAPES!

HTH - ATB

[Garf]

nicely explained chimp.

[GLOC]

If Howard can understand and explain it, I think most other people could too

[Howard Payne]

Perhaps a GUE Instroketor could just check that over for me and pick holes in it if there are any - just for the sake of accuracy and clarity Particularly bearing in mind the fact that most of the time - we haven't got a bloody clue what we're doing.....

[lloydwatkin]

Originally Posted by Howard Payne

Perhaps a GUE Instroketor could just check that over for me and pick holes in it if there are any - just for the sake of accuracy and clarity Particularly bearing in mind the fact that most of the time - we haven't got a bloody clue what we're doing.....

I'm not an instructor (obviously) but one major flaw is that you used the word normal to describe me

Thanks for the info though, Lloyd.

[gwilson]

can i jump on here with a question i was asked.

if have a stage with 60% in what is the fraction of gas at 13 metres, i know 32% is 10 but cant remember how to work it out.

[Garf]

Originally Posted by gwilson

can i jump on here with a question i was asked.

if have a stage with 60% in what is the fraction of gas at 13 metres, i know 32% is 10 but cant remember how to work it out.

the fraction of what gas?

[GLOC]

Originally Posted by gwilson

if have a stage with 60% ....

What is 60%?

60% not recognised, does not compute, brain overload...