Results 1 to 5 of 5

Thread: Shallow Deco

  1. #1
    Moderator ST-Forum Mod PACKRMAN's Avatar
    Join Date
    09/19/2008
    Location
    TULSA-ish, OK
    Age
    49
    Posts
    1,193
    Images
    33

    Shallow Deco

    I have been told by multiple people that you CANNOT go into Deco shallower than 30'. As I understand it you are essentially off gassing faster than on gassing. Some of those people are what I consider "pretty salty". Recently on a trip to Coz my computer put me into Deco twice at less than 25'. It showed a 2 minute stop at 10' both times. I did slip under 20' after the first one for an instant and the second one took me totally by surprise.

    I am using an Aeris Elite T3 set conservatively. The dive was dive #4 of a multi dive multi day trip. I have the graph of the dive downloaded but I am not smart enough to post it here.

    So what do you guys think? Is it just the conservative mode? The guide was using a Suunto and did not go into Deco. Was I told incorrectly? Did I misunderstand? What did I leave out?
    Originally from the Great White North, I give you...

  2. #2
    Megalodon
    Join Date
    11/12/2007
    Location
    Wichita, KS
    Age
    58
    Posts
    14,155
    Images
    13
    Hey Packrman, Theoretically, you can go into deco on shallow dives. However, it's such a long period of time that most (note I say MOST) rec divers aren't close. However, you still do get nitrogen loading. It took a bit to find, but this site has excerpts from tables that include 10'-30': http://www.tdconline.com/tables/table.htm

    Long and short of it is that surface interval depending, you stack up some rather interesting residual nitrogen times. If you did 4 60 minute dives to 20' with 2 hour surface intervals between dives, you rack up 148 minutes of residual nitrogen at the end of dive #4, which would be higher on your compluter in conservative mode. So, yes, I do beleive it's very possible for your T3 to put you in deco.

    I once saved a man in Wichita just to watch him dive...(inventor)

  3. #3
    Guppy
    Join Date
    04/25/2012
    Location
    Cedar Park, TX
    Posts
    54
    Quote Originally Posted by PACKRMAN View Post
    I have been told by multiple people that you CANNOT go into Deco shallower than 30'. As I understand it you are essentially off gassing faster than on gassing. Some of those people are what I consider "pretty salty". Recently on a trip to Coz my computer put me into Deco twice at less than 25'. It showed a 2 minute stop at 10' both times. I did slip under 20' after the first one for an instant and the second one took me totally by surprise.
    As recreational divers, we were always told that no table or computer was perfect and to avoid diving to the limits. We understand that, but do we really comprehend that?

    Get into technical diving and you start studying decompression theory. You come across different algorithms and different ways of adding conservativism. M values, tweaking gradient factors, controlling bubble size and so on. Run different planning scenarios and quickly you realize that it's very grey. I can come up with a plan that shows no stops and I can add some conservatism and suddenly it has stops. In reality, is it a deco dive? It's very difficult to say. At the end of the day you just have to see how you feel after and make decisions based on what seems to work for you and what risks you can tolerate.

    So back to your original question... I would agree with your salty friends that dives < 30' have infinite NDL times. Many tables and computers disagree... no doubt added conservatism. Is it possible to get bent from a really long shallow dive? Sure. A slow ascent to 15' with a safety stop and a really slow ascent to the surface should make the changes of that extremely remote.

  4. #4
    To add to what chrpai said, when I got my DM we did a lot of study on decompression theory (I think my instructor wasn't normal). Everything is statistics. Statistically I should never buy lottery tickets because I'm not going to win but I buy them anyways. Statistically, if I stay at 20 feet I can stay there for weeks (assuming I have enough air). When they came up with the different dive algorithms, they used different statistics or added a safety factor. For example, the standard way of doing statistics might be to do a Poisson distribution. Any numbers really high or really low would be discarded. With creating a dive algorithm I might keep the really low numbers (were low is depth and time) but discard the really high numbers. I might even keep numbers which were most likely mechanical error of the measure device. This will make my algorithm even more conservative. I might skew the distribution.

    Bottom line, it is all guess work. It is educated guess work but the statistics might say you have a 99.99999999% chance of not going bent but I'm going to code my algorithm to say you have a 98.27464% chance of getting bent. Just because. This is why Suunto computers are WAY more conservative than other computers.

    Additionally, older computers might be more liberal than modern computers. Advances in doppler radar now allow us to see micro-bubbles that would could not before. When they took the statistics from the original Navy tables, they found what seemed to form no bubbles at all actually formed micro-bubbles too small to measure 50 years ago. Now we can see the Navy divers used for the original Navy tables were actually a 'little' bent. But the effect was SO SMALL that no one ever really saw the effects. However, from a DAN workshop they rationalized that Navy divers were in much better shape than your typical recreational diver. If we look at the worst case recreational diver, we should probably add some serious margin for error.

    Finally, I know people who do diving for charity. They will have a surface team supplying them with dozens for tanks with regulators. They go down to around 15-20 feet with 2 tanks. As one tank runs out, they switch to another tank, empty tank goes up to the surface, new tank comes back down. They do this for hours. I think the record is something like 3 days. They usually quit after 1 or 2 days because eating and getting water logged become an issue. Deco is never an issue on these.
    This signature left intentionally blank

  5. #5
    Moderator ST-Forum Mod PACKRMAN's Avatar
    Join Date
    09/19/2008
    Location
    TULSA-ish, OK
    Age
    49
    Posts
    1,193
    Images
    33
    Thanks Guys, I appreciate the help.
    Originally from the Great White North, I give you...

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Best shallow dive
    By bigman241 in forum Wreck Diving
    Replies: 9
    Last Post: 01-16-2010, 17:27
  2. Logging shallow/short dives
    By paperdesk in forum General Scuba Training Questions
    Replies: 56
    Last Post: 06-30-2009, 00:09
  3. Shallow reef in Islamorada?
    By alanconstant in forum Southeast - Florida
    Replies: 3
    Last Post: 06-20-2009, 19:20
  4. Freediving and Shallow Water Blackout
    By No Misses in forum Free Diving
    Replies: 2
    Last Post: 04-16-2009, 09:47
  5. What is your favourite shallow dive?
    By mitchy in forum Scuba Stories, Comments & Questions that don't fit elsewhere!
    Replies: 39
    Last Post: 04-13-2008, 15:18

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •