BillP
Senior Member
Hi Dr. Deco:
I would like to hear your thoughts on the rationale behind decompression algorithms requiring specific decompression stops and your thoughts on recommended procedures for dealing with missed decompression stops. I know why decompression stops in general, but what is the maker of the table saying when he says, "This dive requires a stop at 20ft for 3 minutes and a stop at 10ft for 23 minutes" rather than saying, "Oh, maybe stop at 20ft for awhile and if you don't mind slow down for a bit as you pass 10ft"?
The reason I ask is many recreational divers are starting to do dives requiring mandatory decompression time according to the decompression algorithm they've selected for their dives. They are diving for fun, and decompression time is usually no fun. Computer divers on vacation who accidently go into deco don't want to spend time at stops. They want to be able to skip the stops and then continue diving the rest of the day. They certainly don't want to re-enter the water to satisfy the deco obligation. To minimize the time at deco stops more technical recreational divers sometimes seem to pick the most liberal deco programs, tables, computers they can find. Then they start skipping deco time. They'll say, "One minute less deco time doesn't matter" or, "This is a way too conservative algorithm and 17 minutes deco is just too much." After skipping mandatory deco time, they want to continue subsequent dives as if nothing had happened. Safe? Why does the creator of an algorithm make deco time "mandatory" rather than just a rough suggestion to be fudged as the diver sees fit? Or do they?
For example, US Navy Table protocol. According to the US Navy Dive Tables, if a diver omits a mandatory decompression stop at 20ft or less and he can return to the water within one minute, he returns to the level of the missed stop, lengthens that stop by one minute, and completes the subsequent stops. If the surface interval is greater than one minute and the diver remains asymptomatic, the diver returns to the missed-stop level and lengthens the stop by 1.5 X (then completes the remaining stops). Alternatively the diver can be treated in a chamber with Navy Treatment Table 5 (135 minutes in the chamber). The Navy tables do not list a procedure for allowing subsequent dives if the above procedures are not followed. The Mplan tech/recreational dive planning software program seems to take a much more liberal view. According to Mplan, if you ignore a deco obligation, but re-enter the water for a subsequent dive at the same time you would have if you'd made the mandatory stop, you spend less time in deco on the second dive (presumably because you've off-gassed faster on the surface than you would have at depth). Very appealing to the recreational diver. Should it be OK for the diver to ignore the procedures for a missed deco stop and just continue diving?
I believe that you've been involved in the creation of decompression algorithms. Why didn't you make them more liberal with extended no decompression limits and shorter deco stops so they would be more fun or convenient? What goes through a decompression algorithm creator's mind when he sets a "limit"? If a recreational diver chooses to ignore the limit set by the decompression algorithm they've picked and skip mandatory deco time, what do you think should be their procedure for subsequent dives? Just continue the day's dives as if nothing had happened? Have longer NDL's and less deco time on the next dive (like Mplan) because you've off-gassed faster on the surface than you would have on the deco stop at depth? Have shortened NDL's or extended deco time on the next dive to make up for the missed stop? Or maybe stop diving for the day to prevent further uptake of nitrogen and take time to see if you develop DCS from ignoring a mandatory deco stop(s)?
TIA,
Bill
I would like to hear your thoughts on the rationale behind decompression algorithms requiring specific decompression stops and your thoughts on recommended procedures for dealing with missed decompression stops. I know why decompression stops in general, but what is the maker of the table saying when he says, "This dive requires a stop at 20ft for 3 minutes and a stop at 10ft for 23 minutes" rather than saying, "Oh, maybe stop at 20ft for awhile and if you don't mind slow down for a bit as you pass 10ft"?
The reason I ask is many recreational divers are starting to do dives requiring mandatory decompression time according to the decompression algorithm they've selected for their dives. They are diving for fun, and decompression time is usually no fun. Computer divers on vacation who accidently go into deco don't want to spend time at stops. They want to be able to skip the stops and then continue diving the rest of the day. They certainly don't want to re-enter the water to satisfy the deco obligation. To minimize the time at deco stops more technical recreational divers sometimes seem to pick the most liberal deco programs, tables, computers they can find. Then they start skipping deco time. They'll say, "One minute less deco time doesn't matter" or, "This is a way too conservative algorithm and 17 minutes deco is just too much." After skipping mandatory deco time, they want to continue subsequent dives as if nothing had happened. Safe? Why does the creator of an algorithm make deco time "mandatory" rather than just a rough suggestion to be fudged as the diver sees fit? Or do they?
For example, US Navy Table protocol. According to the US Navy Dive Tables, if a diver omits a mandatory decompression stop at 20ft or less and he can return to the water within one minute, he returns to the level of the missed stop, lengthens that stop by one minute, and completes the subsequent stops. If the surface interval is greater than one minute and the diver remains asymptomatic, the diver returns to the missed-stop level and lengthens the stop by 1.5 X (then completes the remaining stops). Alternatively the diver can be treated in a chamber with Navy Treatment Table 5 (135 minutes in the chamber). The Navy tables do not list a procedure for allowing subsequent dives if the above procedures are not followed. The Mplan tech/recreational dive planning software program seems to take a much more liberal view. According to Mplan, if you ignore a deco obligation, but re-enter the water for a subsequent dive at the same time you would have if you'd made the mandatory stop, you spend less time in deco on the second dive (presumably because you've off-gassed faster on the surface than you would have at depth). Very appealing to the recreational diver. Should it be OK for the diver to ignore the procedures for a missed deco stop and just continue diving?
I believe that you've been involved in the creation of decompression algorithms. Why didn't you make them more liberal with extended no decompression limits and shorter deco stops so they would be more fun or convenient? What goes through a decompression algorithm creator's mind when he sets a "limit"? If a recreational diver chooses to ignore the limit set by the decompression algorithm they've picked and skip mandatory deco time, what do you think should be their procedure for subsequent dives? Just continue the day's dives as if nothing had happened? Have longer NDL's and less deco time on the next dive (like Mplan) because you've off-gassed faster on the surface than you would have on the deco stop at depth? Have shortened NDL's or extended deco time on the next dive to make up for the missed stop? Or maybe stop diving for the day to prevent further uptake of nitrogen and take time to see if you develop DCS from ignoring a mandatory deco stop(s)?
TIA,
Bill