Choosing gradient factors

[honeybadger]

Just got my new OSTC yesterday! Looking for thoughts/advice on setting GF. I am fit (weight training 2-3 times/wk), 44 yrs old. Going to the Red Sea in a few weeks for a liveaboard. I'm planning 4-5 dives/day on air or nitrox, diving in the 20-40 meter range. Oh, CMAS ***, 350+ dives (50/year or so), making me am an advanced beginner diver.

About a year ago I implemented the Bülhmann algo, and came up with some profiles which allowed repeat dives to 30 or 40 meters with a surface interval of 1 hour; you could (in theory, and if my implementation was correct) do this profile all day long-- in theory, 20 or more dives in a row with no change to the profile. It worked because I hacked together my own GF approach.

I'd like to set up my GF on the OSTC to allow a similarily packed dive schedule, and am looking for advice on how to do so.

I am considering two approaches. Need advice on implementing each approach and also on how to choose between them.

Theory 1: set conservative values to allow repeat dives with less surface interval. But what is conservative, and what is overdoing it? 30/70? 20/80? 10/70??

Theory 2: set liberal values and pad them manually (by monitoring the leading tissue saturation on the display). Under this scheme, if I need to make a fast ascent, the computer will show me the "safe" profile and I can cancle/postpone the following dive. Again, what counts as liberal-- an upper factor of 90? Am I then looking at 10/90, 20/90, 30/90? (yes, I know how the first number makes the first stops deeper. What I don't have is the years of experience in the water with these numbers=

Thanks for suggestions and wisdom

[Solodiver]

Hope you are still well and alive .

Which factors did you choose and what is your experience with them?

[Tom]

How many dives you can do on 1 day has nothing to do with your GF settings but with common sense, how you decompress, how resistant your boddy is to deco and how lucky you are among other factors ...
With 350 dives you should know that its not the computer that determines if you get bent or not.
I think i would set something like 50/80 to 60/90 with air for gas and use nitrox as margin where the target depth allow for it. And I wouldnt do 5 dives a day.

[dmainou]

Hi mate,

To help you understand GF and therefore be able to choose your own.

You may wish to read this book:

http://www.dive-tech.co.uk/deco%20for%20divers.htm

and read the links on this page:

http://www.decompression.org/baker/home.htm

# username: downloadfiles

# password: decompression1

It is an FTP so you'll need the details above to surf
Hope this helps.

D

[honeybadger]

Thanks, Dmainou, for the link and information. This looks very good.

I also finally got smart enough to just run some simulations on my OSTC. I found a higher initial gradient factor did not add substantially to the total deco time. The main effect was to push the first stop lower.

[dmainou]

I would recommend/advise you NOT to mess with the settings until you understand what you are doing. (leave it at the default settings until you do)

The book in the first link is very good as it is no nonsense no maths approach to the concepts of decompression. Its aimed to the common diver.

The links in baker's FTP go into the math. Erik's code is the base of GF and thus the base of the OSTC.

if you want a quick answer read this:

http://www.rebreatherworld.com/general- ... mmies.html

In a nut shell the high GF controls the conservatism of the shallow part of the dive whilst the low GF controls the depth of the first stop.

Don't go only by this statement. There is a lot more to it so make sure you at least read the link above. Preferably read the book and then Erik's papers. (start with the book)

D

[Alex]

[quote=honeybadger]
About a year ago I implemented the Bülhmann algo, and came up with some profiles which allowed repeat dives to 30 or 40 meters with a surface interval of 1 hour; you could (in theory, and if my implementation was correct) do this profile all day long-- in theory, 20 or more dives in a row with no change to the profile.
[/quote]

Just a word of caution... Buhlmann himself stated that he felt his algo was a bit weak when calculating repeated dives.

Wienke claims to have solved these weaknesses by implementing a folded version of his RGBM algo on Buhlmann. This is the "RGBM" version available on most recreational dive computers today...

That said, the overwhelming majority of my dives in the last few years are rather deep technical ones so I am not an expert in repetitive dives.

Guys like the Counterterror Countermeasures Dive Team (CCDT) do a lot of repetitive diving (they are sort of the GUE equivalent at this sort of diving), so they could be a source of reliable information on the topic.

[honeybadger]

Alex, thanks for adding the note of caution. There are so many things which could have been wrong or gone wrong with my calculations (not that I am careless, but just saying). It was a theoretical exercise, and I only guarantee its safety if you use it for theoretical dives.

I didn't exaclty use Buhlmans algorithm. My synthetic repetative dive profiles were derived from Buhlman's a and b tissue values, but the profiles were based on keeping all tissue N2 saturations under twice ambient pressure. This followed Haldane's suggestion that bubbles only occur when saturation is 2x pressure.

The resulting profiles had short bottom times and equally long ascent/deco times. Long tissues never accumulated more than trace amounts of N2, and the fast tissues were all desaturated after the 1 hour surface interval.

again, IN THEORY