Saturday, April 27, 2013

Deep Stops explained by Tom Steiner

Interested in knowing more about Deep Stop? here you can download a file with some interesting explanations. Enjoy :-)
 

Deep Stops
by Tom Steiner van den Ouweelen
As a technical diver I am pretty sure that you heard about Deep Stops. Now how does all this work?
First you should know that deep stops have been actually mentioned by Paul-Bert in 1878, so it’s not as new as many might think, I am sure that you all heard about Paul-Bert, just in case to refresh you, he is the one that found out “the Oxygen Toxicity” (CNS), referred to as the "Paul Bert effect". He has shown that oxygen is toxic to insects, animals, mollusks, birds and other animals. Deep stops are also commonly named PYLE stops from Richard Pyle.
The main reason for doing a deep stop is to reduce the critical fast tissue tension and help reduce the incidence of decompression sickness and the fatigue after a dive that is a subclinical DCS.
Different computers and dive organizations tell you to do deep stops at different depth and for a length of 2 minutes.
The 2 minute stop that you will do at the deep stop is the time that the body needs to make one full blood circulation  and this will help reduce the fast tissue tension.
There are different ways used to calculate your deep stops, some computers are using fixed values for different depth, some will tell you to do a stop halfway your depth some will tell you to stop at 80% of your max depth, some have preprinted tables... anyway there are loads of ways to make a deep stop and I am not saying here that they are not right.
What is interesting is to know how they are calculated.
So if I take the first example of the DAN, that recommends you to do a 2 minute deep stop at halfway of your depth.
Example :
50 meters depth = deep stop of 2 minutes at 25 meters
In this example the GVE (Gas Volume Expansion) will be 70%.
Assuming that the initial size of a bubble is 1 on the surface, at a depth of 50 meters (6 bars) the size of the bubble will be 6 times smaller:
1/6 = 0.1666666666666667
You will add 70% on top of it and it will result as follow :
0.1666666666666667 + 70% = 0.2833333333333333

Now you will divide the initial size of 1 (that you have on the surface) by 0.2833333333333333
1/0.2833333333333333 = 3.529 bars
3.529 bars = 25 meters (rounded)
The Gas Volume Expansion percentage will tell you where you will perform the stop. So if you take a Suunto computer it works with a GVE of around 70%, a VR3 with a GVE of around 57% etc… and that’s how they define their deep stops.
I have been using the GVE (Gas Volume Expansion) system now for over 14 years but with a lower GVE for all my technical dives, I use typically 35% GVE.
There are some basic rules you should know, ascent speed from a deep dive is 9-10 meters per minute maximum, your first deep stop is always done on your back gas and not on your deco gas, why that? Well if you have been staying for a while at a PPO2 of 1.4 your body needs to rest a while before switching to a higher PPO2 that will be 1.6, don’t forget your body is a pressure machine, so respect it and listen to your body. Ascent time from your last deco stop to the surface 1 METER per minute.
A good advice, in warm water a PPO2 for the bottom gas of 1.4 is fine, for deco gas 1.6, in cold water a PPO2 for the bottom gas of 1.2 is highly advisable and for your deco gas use 1.4 PPO2.
Repeat the Deep Stop as long as you are more than 10 meters from your first deco stop.
You can round depth to your switch, if your second deep stop has to be done at 22 meters and your switch is at 21 meters,  then you will just switch to your deco gas at 21 meters on your 50% mix, just remember first deep stop is on the back gas.
Let’s take an example with 1 deep stop and a gas switch at the deco stop.
Depth : 50 meters
GVE : 35%
First switch at 21 meters on a 50% mix.
Second switch at 6 meters on O2.
Now in this example I will show you how you can do the calculation easier.
Just divide the pressure, 50 m = 6 bars by 1.35 (one is the initial size and 35 is the GVE that you decide to use)
6/1.35 = 4.44444444444444 bars that equals 34 meters
Now for the second deep stop you will divide 4.44444444444444 by 1.35
And this equals : 3.2921…bars that equals at 22 meters. In this case you will merge the deep stop with the gas switch on your 50% mix, that will become the first deco stop.
Now regarding the GVE, you can use all types of GVE, from 20% up to 70% maximum, that’s my advice. Now honestly the GVE that I would use depends on the type of dive I plan, in good conditions, warm water, no current etc… I would use down to 25%.
35% is the one I use most and works out fine on most dives, in cold water I would rather opt a higher GVE, 45% or 55% as I don’t want to hang to long in the shallow stops due to the cold.
The point is that if you stay deeper for longer time you will on gas your slow tissues while you ascent so you need to find a value that suits you best and therefore you need to try some values.
Never forget that the deep stops have to be integrated in your plan, and on some software’s you need to add the ascent time as well between your maximum depth and your first or second deep stop. Deep stops will also make your decompression time a bit longer at the end, but you will feel better and less tired when you come out of the water.
I hope that this document has been interesting for you and if you wish to receive the Deep Stop generator feel free to contact me and I will email it to you. It’s a free software that runs on windows.
Never forget to seek proper training with an experienced technical instructor and when it comes up to deco, start with it gently and don’t be too aggressive, build Bottom time slowly up.
Wish you some great technical dives.
Tom Steiner van den Ouweelen



Deep stops should be taken to be experimental and should not be used without a full understanding of the inherent risk of Decompression Sickness while diving.
No deep stops of any sort can guarantee that DCS will NOT occur.
The only way to avoid the risk of DCS is to stay on the surface and NOT DIVE.
The author of this article will not be held accountable for any injury caused by the use of a profile generated by this document or by the Software.






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