OP
swimmer_spe
Contributor
I know what you mean.
Remember though, navy divers are usually in excellent shape, where as recreational divers may not be in good physical shape.
That is probably why the difference.
Diving myths taught for safety?
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I know what you mean.
Remember though, navy divers are usually in excellent shape, where as recreational divers may not be in good physical shape.
That is probably why the difference.
Mr Carcharodon
Contributor
Some of the Navy divers were smokers and not so fit, and DSAT did not select for unfit divers. So you may be working with yet another myth or at least only partially correct. DSAT may have pushed the limits back in part because the expectations was that recreational divers did not have easy access to hyperbaric chambers while the Navy divers did. Also it is worth noting that in the DSAT human trials no one got bent. So they established that they were on the safe side of a limit but not where the limit was.
Also...
In comparing the Navy tables and the DSAT tables, don't just look at the individual dives--look at how they treat multiple dives.
The DSAT assumption was that recreational sport divers and Navy divers did different kinds of dives, and different kinds of dives bring on different kinds of decompression needs. The problem with sport divers using the NAVY tables was that the longer individual dive time limitations (usually performed in the Navy by surface supplied hard hat divers) were then followed by surface intervals that were governed by the offgassing rate of the 120 minute compartment. Thus sport divers were doing shorter dives than navy divers and then sitting around on the boats for a very long time before they could get in the water for a second dive because the Navy tables were based on longer dives. The DSAT studies showed that for the kinds of diving that sport divers enjoy, the 40 minute compartment was the controlling compartment for surface intervals. To be more conservative, DSAT used the 60 minute compartment for offgassing and required shorter times for first dives. In summary, that shorter NDL for the first dive is balanced by a shorter surface interval before the second. This was a great help to sport divers.
If you look at the Navy tables with the 120 minute compartment governing them, you will see that the maximum washout is 12 hours. The maximum washout for the DSAT table is 6 hours.
If you violate the limits on the DSAT table and say it is OK because it is OK on the Navy tables, then you need to realize that if you are going to use the NAvy tables to govern that first dive, then you need to use the Navy tables to govern the surface interval for the second dive as well. You can't jump back and forth between the two.
Mr Carcharodon
Contributor
That is a good point about tables.
Of course computers which presumably do the calculations for every compartment used in their model still end up showing large differences in calculated deco time. So at least sometimes it is the details of the model in use and not compartment half times driving the discrepancy. When I end up with 10 minutes of deco and my buddies have 20 minutes of NDL time left it is pretty clear the models are not all that precise.
Of course computers which presumably do the calculations for every compartment used in their model still end up showing large differences in calculated deco time. So at least sometimes it is the details of the model in use and not compartment half times driving the discrepancy. When I end up with 10 minutes of deco and my buddies have 20 minutes of NDL time left it is pretty clear the models are not all that precise.
DaleC
Contributor
I think the models are precise enough but vary when compared to each other. As John said, the most important part is picking a table that reflects ones needs and sticking with it.
With my DCIEM tables there is a whole section that describes how the tables were derived. Unlike the USN tables, that allowed for the possibility of a small percentage of DCS hits, these were created by rolling back the depth/durations until no DCS symptoms were noted.
But again, one has to consider the age and fitness of the diver, as well as the environment. Doing 20 at 100 in 80 degree water is not the same as 20 at 100 in 40 degree water. That same time spent resting is not the same as that spent exerting oneself.
Personally, while I do dive a computer, I prefer to use the 72 rule.
(dive a single St 72 and come up when the air runs out)
With my DCIEM tables there is a whole section that describes how the tables were derived. Unlike the USN tables, that allowed for the possibility of a small percentage of DCS hits, these were created by rolling back the depth/durations until no DCS symptoms were noted.
But again, one has to consider the age and fitness of the diver, as well as the environment. Doing 20 at 100 in 80 degree water is not the same as 20 at 100 in 40 degree water. That same time spent resting is not the same as that spent exerting oneself.
Personally, while I do dive a computer, I prefer to use the 72 rule.
(dive a single St 72 and come up when the air runs out)
I remember back in the late 60s when I finally had to get certified to continue diving. I was in an LAC class taught by outstanding instructor Ron Merker. Almost every time I questioned on of the things he taught based on my knowledge as a scientist, he'd tell me to get down on all fours and kick me in the ass. It really drove home his lessons! Ron was a great instructor and a legend.
I remember when we were doing our Nitrox Cert,it was quite heavily stressed 1.6 Oxygen Partial Pressure = Toxic = Die
Now since then my reading and studies ( especially freediver related documentation ) has shown me it's far more complex than just a magic number being fatal
Now since then my reading and studies ( especially freediver related documentation ) has shown me it's far more complex than just a magic number being fatal
CT-Rich
Contributor
This thread was forwarded to me by a student. My answer to him was, take a course, or don't take a course but ALWAYS, dive within the scope of your training and experience. Audit your skills frequently and be honest with yourself. Video cameras do no lie and leave little to the imagination. Scrutinize and reflect your diving habits. Are you diving solo because you have trouble finding people who will do more than one dive with you? You mentioned having done an overhead dive and a decompression dive. Would experienced divers conduct these dives with you are or you conducting them solo because no one else will or with people who do not know any better? If the answer is either of the later than maybe you need to give this some consideration. I have extensive wreck and technical dive training and experience and there are many divers who penetrate wrecks and conduct decompression dives who do not have the ability to deal with the added potential risks. They often find other divers who do no know any better to undertake such dives with them and put both at risk, while myself and the people I dive with who have extensive training and experience in these environments would never undertake such a dive with someone who is a liability. I have all the time in the world to teach people who want to learn and improve their diving. You can verify with the people in the Victoria dive community but for every dive I do with a student as part of a course, I do at least 3 on my own time to help them gain experience and practice their skills. I have little patience and zero time for knowitalls who endanger other divers. You mentioned that nitrox would make sense for the deep part. If you take a nitrox course, one of the first things you will learn is how nitrox actually limits your depth. Nitrox is most beneficial at depths 100ft and above and for decompression purposes. Maybe you are thinking about trimix?
DONT FORGET TO LOOK AT THE DEFINITION OF WHAT BOTTOM TIME IS for each of the tables. there are differences in total wet time or variations of decent and acent times not counting ect.
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