average depth tables???

[Clare Gledhill]

Originally Posted by liamm
In terms of adding time to deco or padding it am i right in thinking that as long as you are at less that 80% of the atas then adding time will not incurr a deco penalty?

David Doolette's presentation at the GUE conference warned that stopping deep (and for the purposes of this answer yes that includes above 80 per cent) is likely to add to your decompression profile and therefore make the deco more aggressive if you do not take this into account when calculating your profile.

Essentially whilst some tissues may be offgassing in this range, some will be on gassing and in non saturation diving these will control your ascent. If you are diving a computer which is calculating your real time profile this will of course be taken into account.

If you are table diving or diving ratio deco or any other method which you calculate yourself it is not as simple as leave the bottom stop counting bottom time. Doolette argued that on an 57 metre dive the first stop which would be safe from on gassing was roughly in teh 20s and he would prefer to see fast ascent rates to that point.

He acknowledged however that this would bring forth other issues and thus balance was needed. That is where familiarity with dive profiles, experience in diving them and being able to calculate average depth can be of benefit - even if in this case it would be extending the deco - and coincidentally but pleasingly mucking up Chasey's arguments and warnings of death and destruction

There is so much about deco we don't know. David and Simon Mitchell's presentations at the conference made me think so hard about my own decompression profiles and my lifestyle. Fitness does have scientifically proven benefits as does excercise prior to diving. Thus my 500 metre swim time is approaching 11 minutes and dropping, the weight I'm lifting at the gym is increasing and my cardio regieme includes at least 40 minutes hard work out in the 24 hour period before I dive. Each one of us can find time to address fitness and it will not just help us in our diving - but in all other areas of life as well.

[liamm]

OK - reaching the point where i might be out of my depth. how far "up" the stops do you still count time into your "bottom time".

so far i've been looking at deco from the point of view of research papers which dont necessarily link to real life diving scenarios. Is there any good books that link the two better??

At the moment its a case of a little knowledge is potentially dangerous so ill probably stick to V planner for now.

Ant way ashes to ashes in on so im off for beer and some mans television ....

[Gareth J]

I have read most of the posts, on this tread.

I think we all need to understand that in the last 10 years, that a higher percentage of divers are diving deeper for longer, the use of gases other than air has become more common.

The end result of this is that we now have Nitrox as the gas of choice for the majority of recreational divers. Nitrox is part of at least one agencies standard diver training package.

In comparison with the 90's, a larger & larger number of recreational divers consider decompression times in excess of 15minutes as normal. 30 minutes is not exceptional. We regularly talk of 60minute decompression times or longer on this forum.

We have also moved to a point where Helium is now being positively encouraged for dives exceding 40m. Helium & poor bouyancy & ascent control are not good bed fellows.

The physiological effects of this are not fully understood.

During the 70's the HSE & commercial divers where not aware of the degree of damage that the 'standard' commercial diving profiles where inflicting. For a lot of those involved in the diving during the 70's it was many years before the full effects where know, even now the why is not fully understood.
It is also worth remembering that since the end of the 70's the amount of research & money spent on Decompression science since has significantly reduced. The Oil companys have reduced there investment & the practice of using 'human' subjects is no longer acceptable.

I am aware that diving has risks, a number of them hidden, some unknown. Deep diving, extended decompression diving & mixed gas have an even greater degree of risk. How much subclinical DCI are 'we' experiencing?

Two years ago (I think), there was a presentation at DOC (BSAC), relating to unusual diving injuries suffered by recreational divers (those not involved in commercial or military diving, as against instructing). Diving Doctors where seeing injuries that historically where only experienced by saturation divers in recreational divers.....

I would prefer to spend an extra 10 minutes in the water per dive decompressing, than find in my old age I have bone necrosiss, or other diving related injuries.

I only consider getting out of the water quickly for;
gas pressure (the lack of),
thermal considerations,
tide,
or something having gone seriously wrong.

The first three should be solved during the planning phase, prior to turning up let alone getting in the water. The last well.....

So, I prefer to be conservative rather than aggressive in my choice of decompression profile!

Gareth

[Mark Chase]

Originally Posted by liamm

OK - reaching the point where i might be out of my depth. how far "up" the stops do you still count time into your "bottom time".

so far i've been looking at deco from the point of view of research papers which dont necessarily link to real life diving scenarios. Is there any good books that link the two better??

At the moment its a case of a little knowledge is potentially dangerous so ill probably stick to V planner for now.

Ant way ashes to ashes in on so im off for beer and some mans television ....

Forget black and white with deco, there is only gray

If your diving to 80m you would expect to be racking up deco big time.

So what happens when your first deco stop is 80m?

Yep, you rack up deco big time.


If you want to see the effects of this than get GUEs decoplanner software. Start putting in dives with the same bottom time and the same gas and just alter the deep stops. To do this set the GF low below 30 and the GF high at 100. Then up the gf low till you get rid of all the deep stops

Set your ascent rate at 10m per min.


As an example a dive to 80m for 40mins

GF 10/100 first stop depth = 60m total deco = 201mins

Now the same dive with 100/100 GF deco (Buhlman tables)

GF 100/100 first stop 33m Total deco 135mins

So by not doing deep stops you reduce the deco by 65mins

The 9m + 6m stop time for a 10/100 is 103mins

The 9m + 6m stop for 100/100 is 83mins


So using Buhlman tables you reduce deco significantly by maximizing the off gassing gradient and not doing deep stops.

However modern thinking is that by adding deep stops you reduce the stresses on the body. This is proven by empirical date from pioneers like Richard Pyle.

Deep Decompression Stops

But if you add deep stops you have to extend deco to compensate.


Then along came VPM. This decompression system wasn't tested on goats and snakes like Buhlman but was apparently done in a petri dish. This system worked on the critical bubble radius. This used a lot of deep stops and strict ascent rates to maintain the critical radius of the bubble. In doing so it reduced the need for shallow stops

This dive using VPM set to zero safety and stops timed to the second comes in at 153mins deco with a first stop depth of 57m

Still not as fast as straight Buhlman but it does have deep stops.

Unfortunately with early VPM a lot of divers apparently got bent. So VPMB came along to fix a few bugs and generally a safety setting of 2 was accepted as sensible. This put the dive time up to 171mins


However my Buhlman software only allows stops of 1min minimum.

Put this restriction on to the VPM and you get 176mins

So now VPMB is within 24mins of the 10/100GF profile on a dive with over three hours deco and still its 40mins longer than Buhlman
On big dives there is another level of VPM.

VPMBE. This extends the shalow stops in a Bhulman stile and the end result on this dive is it gets within 8mins of the 10/100 profile i posted in the begining.

Punching in numbers like this shows patterns in deco and exposes unmistakable truthes. Do deep stops and it extends deco.

Play with deco all you want but in the end the profiles all come out prety much the same. If all you can save on a dive of 240mins its a mear 20mins? whats the point of messing arround on a 90min dive.


Its all very facinating but in the end i have come full circle on it and I just do whats necessary plus a healthy chunk for safety.

ATB

Mark