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Andi We have dopplered numerous air diver who decoed on air they always have substantial bubbles grade 3 +. On EAN 50 there is usally a lesser degree of bubbles and on EAN 70 and above we rareley detect bubbles. On long dives I imagine there is always some degree of bubble formation. By long I mean in excess of an hour at depths greater than 120 fsw. In my opnion to deco on air following an air dive shows a distinct like of understanding of DCI. $20.00 is a hell of a lot cheaper than a ride in a chamber. Tom You wrote: > > >>There is a local TDI instructor here in Ottawa (very good reputation) who >>has recently started decompressing on either surface suplied O2 or 50/50 >>nitrox. He has also stated that decompression times longer than about >>20 minutes on air are unsafe. >> >>I have seen studies that say doing deco on EAN can decrease the incidence >>of DCS by 2-3 times, HOWEVER, I can't find any studies that show what >>the rate of DCS is on long deco on regular Air. >> >>To decompress on pure O2 or on Nitrox is going to cost my wife and I at >>least 20 bucks a dive. This can amount to some substantial bucks. >>What I want to know is: "Is air deco really 'unsafe', or is it just less >>safe." Does anyone have numbers about DCS hit rates with deco on different >>gases? I'm really looking for concrete numbers here, not speculation. > >Hi Andy, > >The DCS risk with most table is not constant over the depth and time range. >The exception might be the USN Maximum Likelyhood tables (NMRI I and NMRI II). > >If you use the USN 1957 tables, the DCS risk is between 2 and 5% within the >no-decompression limits (assuming you spend all bottom time at specified >depth) and reach 45% in the extreme exposure range. > >If you use the DCIEM tables, the risk varies between 1 and 3% within the >normal exposure range and reach 18% in the extreme exposure range. The risk >increase in the no-decompression range and gets a little bit lower in short >decompresion stops dive (less than 10 min. deco). after that, the risk >increase with time. Apparently, the DCIEM table has a higher DCS incidence >between 130 and 170 feet. > >I don't know for you, but for me, 2% incidence of DCS is a awful lot for >recreational diving where no hyperbaric chamber is waiting for me at the >surface. So, using a richer mix allows you to eliminate more nitrogen than >the table assumes (if you are not using an accelerated decompression table). >As well, should you have to bailout in an emergency, loosing the lasts >minutes of decompression are likely to be of less impact as your required >(by decompression model) decompression may already be completed. > >I used to dive the DCIEM air table and decompress on O2 at 20 feet and 15 >feet (I pull my 10 foot stop to 15 feet). On the other side of my in-water >custom reproduced DCIEM tables, I have their Modified In-Water O2 >Decompression table (Table 2M), that I would use if staying in water >represent to high of a risk (in case of hypothermia, or loss of gas, for >example). I have to say that I am often decompressing in less that 50F water. > >Decompressing on O2 has another advantage. The depth of the stop is not >important as long as you stay below your ceiling and you stay above the >Maximum Operating Depth for O2. This is why I do my 10 foot stop at 15 >feet. You should consider taking five minutes air-breaks every 20 or 25 >minutes. > >I once wrote a decompression calculation software which I used to simulate >different decompression efficiency scenarios (it was simulating the Orca's >diving computers). You would be surprised to see how ineficient a stop >becomes if it is done just a few feet below the required stop. This is >likely to increase DCS risk. Since it is really difficult to stay at the >exact depth for a deco stop, decompressing on air seems to increase the DCS >risk. > >A trick to avoid the problem would be to make sure you don't go below the >stop depth after half (or third) the decompression stop time. You must >assure that you don't go above the stop depth for the first half (or third). >Note that this approach is not documented anywhere and is just my own >belief. I don't encourage you or anyone to not decompress at the exact stop >depth when you are on air. > >I have to say that if you are doing stops longer than 20 minutes, I believe >that it is worth to decompress on a more friendlier gas, such as a strong >Nitrox or on Oxygen. However, the use of mixtures other than air require >specialized training and procedures... > >Michel Therrien > >PS: Say HI! to Norman and Louise-Ann for me. >PPS: I will forward my message to Ronald Nishi, DCIEM table author to get >his comments. > > >-- >Send mail for the `techdiver' mailing list to `techdiver@terra.net'. >Send subscription/archive requests to `techdiver-request@terra.net'. >
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