When, if ever is it safe for a diver to make multiple trips from the bottom to surface in one dive?

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Nitrox_DiverNY

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Hello all, newbie diver question here.. After witnessing divers make multiple trips from the bottom to surface in about 30ft of sea water, I have been asking myself how much risk this creates in a dive profile and how common is this profile. I assume it creates more DSC risk but wanted to see if anyone here could elaborate on it more. There must be certain divers that dive this way for recovery or for special working conditions.

At what point after ascending from depth (not counting NDL or gas left) is it unacceptable to return back to depth on the same dive? How many times can this be done in one dive?

Lets say If a situation arose where a diver was at 30ft, then had ascended to surface for some visual aid to lets say shore, then decided to descend back down to 30ft for the remainder of the dive, is this a safe action taken by the diver?



Thanks for the feedback!
 
Almost all of my shore diving is of this type/depth. I have occasionally ascended from 30 feet, practicing a CESA at times, in order to check my exact location. I haven't done if from 30 feet more than once on a dive. But, I have done it quite a few times from 15-20 feet during the same dive on many occasions. I get turned around a lot shell hunting and just get lazy as it's just as easy as checking my compass and then I know exactly where I am with relation to the exit area I want. I do 2 shore dives in a day. I have had no problems with this, but who knows for others?
 
Almost all of my shore diving is of this type/depth. I have occasionally ascended from 30 feet, practicing a CESA at times, in order to check my exact location. I haven't done if from 30 feet more than once on a dive. But, I have done it quite a few times from 15-20 feet during the same dive on many occasions. I get turned around a lot shell hunting and just get lazy as it's just as easy as checking my compass and then I know exactly where I am with relation to the exit area I want. I do 2 shore dives in a day. I have had no problems with this, but who knows for others?
thanks for your feedback, so you mean that when shell hunting at 15-20ft that you ascended to the surface then went back to the bottom after you found your exit on shore?
 
Let’s ask the question differently. What is the maximum no stop time for a dive to 30ft?
regardless of the no stop to 30ft time, its about the multiple recompressions from ascending from depth then going right back down to the bottom during the same dive with out SI. Unless your saying that its fine to do a dive like that with in the no stop 30ft time?
 
Yes, it is fine to do a dive like that within the no-stop time, by definition of no-stop time. If you are using a computer (correctly), it will keep you safe on dives like this. With a slow ascent rate, I'm aware of no evidence that repeatedly descending to 30 feet, ascending to the surface, and then descending again for an entire tank can cause DCS... it might bother your ears or sinuses though, especially if you're congested.

The reason it's OK is that neither compression nor decompression happen instantaneously, they gradually occur over time. So it is not just the repeated depth changes that matter, there is a time component: how long you are at depth versus how quickly you ascend. In shallow dives such as 30 feet max, the no-stop time is always going to be way longer than the amount of gas you have, regardless of the profile.

If you haven't read it yet, I recommend picking up the book Deco for Divers by Mark Powell. The second edition (2014) is available for around $30-$40 online. The first few chapters of this book go into detail on the history of deco science, and the physiology of diving.

This type of dive profile is often called a sawtooth, by the way -- if you search for that term, you'll find a fair amount of discussion on it, on ScubaBoard and elsewhere.
 
thanks for your feedback, so you mean that when shell hunting at 15-20ft that you ascended to the surface then went back to the bottom after you found your exit on shore?
Yes, often maybe 5 or more times on a single dive. Brett explains it well. I am lucky as I have never had any problems with my ears and equalize internally with no motions. The up & down thing can affect some people. That is why there is the discussion of instructors doing many CESA's (controlled emergency swimming ascents) with a class of students over a short period of time on a single dive.
I believe the No Stop time for 30 feet is about 2 hours 30 minutes. At about 20 feet (it's either 18 or 22 I think) you could stay down for an unlimited amount of time (with unlimited air of course...) and ascend directly.
 
Doing one or two or a few ascent/descents like that on a tank is not a big deal, but you want to keep a conservative ascent rate.

Do the same thing all day with many cycles, your chance of getting bent goes up. Freedivers can go up and down from 30 feet all day and few have problems.
 
When teaching Sports Diver (10m), Dive Leader (15m) or Advanced Diver (20m) rescue lessons multiple assents are common, but we stop the assent at 6m and go back down. That said there is the potential of 6 assents. DCI is a consideration, any polaris assents finishes the lesson and diving for the day.

Neither tables nor computer algorithm were developed for this type of diving.
 
There are great replies here so far! Aside from the above:

Remember, the biggest relative pressure-change occurs the closer you are to the surface. Take those last 15 feet to the surface slowly, if you're doing it several times to minimize decompression stress. While not risky as ascent, I'd also slow your descent too.

Your chances of injury or feeling sick do increase if you're going between 0 and 30ft a bunch of times. While you may not have to worry about nitrogen, there are a bunch of other compression/decompression injuries that can occur.

--

All that said, there are two scenarios where I've surfaced many times on a single tank/dive:
  • Is one specific location I dive, where I may surface up to 10 times on a single dive/tank, typically staying about 20ft or less, in order to check something on the surface.
  • I've also surfaced a bunch of times on a single tank, while practicing skills, like DSMB deployment, or making adjustments to my BCD/harness/rigging/etc. Also staying 20ft or less.
  • I have felt sick or very tired doing this type of diving, not often, but it has happened.
In these scenarios, I have always surfaced and descended at a slower rate than usual, and try to remain shallow. You may also wish to learn free-diving, if you do this type of dive often. Being able to dive without scuba-equipment is awesome, minimal setup, and a LOT more speed!
 
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