Doing a deco stop horzontally

[Devil Crab]

Hi
I was told by my dad (capel diver) that it was a good idea to Do deco stops horzontally insted of vertically, why is this?

[Andy Phillips]

Pretty obvious when you think about it, If you are 1.5M tall, your feet would be at 11M and your head at 9.5.

Andrew

[Narked@50]

I also find it more comfortable and good buoyancy practice.

James

[Billy the Kangaroo]

and it provides more a water resistance so if your bouyancy isn't bang on you will rise/ drop much less quickly.

Which is why you may have been advised to "flare" like a free-fall parachuterer if you get an uncontrolled ascent/ descent.

and it looks cool.....

unless you go for the dancing-during-safety-stop strategy which is more entertaining.

[Mdemon]

No opinion either way, just that the tables were calculated originally for Navy divers in the vertical position IIRC. How this would affect anything I can't tell you.

(And the award for "Most Pointless Post" goes to...)

[Mark Powell]

On the shallower stops the depth difference from your height becomes significant.

If you are at your 6m stop and are vertical then your feet are at 7.5m

You can see the affect of this by using a deco programme like V-Planner. In V-Planner you can set the depth of your last stop.

I just planned a dive and set the last stop depth to be 6m. The deco stop at this depth was 11m. Switching the last stop depth to 7.5m (where your feet are) gives a stop of 18m.

Don't worry to much about the actual numbers but the point is that at a deeper depth you will not be offgassing at the same rate as at the intended shallower depth.

[MATTBIN]

Does it make any real difference given the shape of a human? Given that different parts of the body off gas differently as they are formed of different tissue compartments it would be interesting to see if the legs for instance off gassed quicker than the head so the actual vertical diference might not make any difference.

Matt

[PeteM]

Originally Posted by MATTBIN

Does it make any real difference given the shape of a human? Given that different parts of the body off gas differently as they are formed of different tissue compartments it would be interesting to see if the legs for instance off gassed quicker than the head so the actual vertical diference might not make any difference.

Matt

I think the shape may actually contribute to problems, there is a known problem that the blood flow round the body changes and is reduced when it has been supported by water for a while (immersion shock). With the body horizontal the bloody pressure differential between the top and bottom must be minimised which I would think would would be as close to what happened in the chamber when they worked out the tables as you can practically get with in water deco.

Does it make enough difference to worry about? Probably not when you consider the many other variables which will effect the efficiency of the deco, however I always deco horizontally because I figure why not give yourself as much of an edge as possible.

[David Martin]

Originally Posted by Devil Crab

Hi
I was told by my dad (capel diver) that it was a good idea to Do deco stops horzontally insted of vertically, why is this?

As I understand it there are a couple of reasons:

The first and most important is promoting off-gassing.

The gas exchange occurs at the lungs.

When vertical there is approx 30cm between top and bottom of the lungs. This creates a pressure gradient from top to bottom of the lungs. The blood (carrying the inert gas - both dissolved gas and bubbles) flow through the capilaries will be concentrated in the lower third or thereabout of the lungs. The newly inspired gas will tend to stay at the top of the lungs, with the gas which was not expired on the previous breath tending towards the bottom. This (non-expired gas) will have a higher fraction of inert gas in it, which results in poorer off-gassing.

When horizontal there is approx 10cm between top and bottom of the lungs. The blood flow through the capilaries will still be concentrated in the in the lower section of the lungs [equivilent to the front of the upright body], but the effect is much less due to the lower pressure gradient accross the lungs. Also the inspired air occupies what is now the front [equivilent to the top in the upright body] - which contains part of the "bottom" of the lungs according to the pressure gradient.

Thus being hoziontal encourages more of the blood with it's dissolved inert gas to come into contact with freshly inspired gas (which has a lower inert gas fraction than the gas which was occupying the residual volume). This increases the inert gas gradient accross those parts of the lungs which are doing the most off-gassing, thereby increasing the off-gassing rate.


Other points that I can think of are:

When horizontal, you can't fin up or down to compensate for poor buoyancy skills - horizontal forces you to be neutrally buoyant (averaged accross the breathing cycle). Once you master this, it helps you relax - which helps gas consumption, and reduces to some extent the risk of higher ppO2s which are used when "accelerating" decompression stops.

It looks "cool"

It allows you to turn/back-up/move towards/move away from your buddy more easily.

David

[David Martin]

Originally Posted by Mdemon

No opinion either way, just that the tables were calculated originally for Navy divers in the vertical position IIRC.

I believe that this was because much of the work they did required them to wear heavy boots to walk around. Their natural in-water position was thus vertical. It was easiest to organise the deco around this. The stops were "mid chest" ie lung level.

How this would affect anything I can't tell you.

Given that horizontal decompression is likely to be more effective, I'd say that it should add some conservatism. Given that the average US Navy diver is likely to be fitter than the average UK recreational diver, this is no bad thing!

David