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>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.
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