Physiology of "saw tooth" diving.

[Bert van den Berg]

Can anyone tell me (us) what the repercussions or physical effects of saw tooth diving (multiple ascents and descents in one dive) are?

I've been diving for 45 years without certification or formal dive instructions and have often "popped" up for a quick look to see where I was before going right back down. After watching me dive for 38 years my wife decided to learn to dive but wanted to learn properly and took a PADI open water class. As a favor to her I took the class with her and learned a few things such as safety stops, not to do saw tooth diving and having a redundant air supply.

I dive almost exclusively on surface supplied air (Hookah) and the maximum depth I've gone to in the past year (90 dives) is 46 feet. How bad is it to spend 20 minutes at 35 feet then go up for a quick navigation check? Perhaps I've been very lucky but all those years of saw tooth diving has not had any apparent ill effects on me.

b.t.w I'm 65 years old, my wife is 64.

Cheers,

Bert

 

[TSandM]

The issue with sawtooth profiles is doing the repeated ascents and then further nitrogen loading yourself. On dives as brief and shallow as you are describing, it's probably not a significant worry at all, unless you are doing it multiple times in the same day.

 

[supergaijin]

'Silent bubbles' are formed on ascents. These bubbles are a normal part of life and do not cause DCS (decompression sickness) directly.... however these bubbles may join to become symptomatic given the right circumstances. Multiple ascents increase the number of bubbles and they may interfere with efficient off-gassing during the final ascent. You would not feel any effects during the first few ascents as DCS doesn't usually manifest itself until a good 15mins after surfacing unless it was an extreme case. As you get older, your reduced circulation will probably have more and more influence on effective off-gassing of Nitrogen.

Tests were done on actual humans were they were saturated with Nitrogen at an equivalent pressure of 10m and then the pressure was released suddenly... with no ill-effects. Haldane found during his studies that test subjects (usually goats) were not affected by rapid decompression equivalent to 'half' the maximum pressure.

"Hence it seemed to me probable that it would be just as safe to diminish the pressure rapidly from four atmospheres to two, or from six atmospheres to three, as from two atmospheres to one."

It has also been said to "Never assume an unexploded goat is evidence of a good decompression profile".

DCS has occurred in swimming pools from multiple ascents which theoretically should not be deep enough for this pressure ratio of 2:1. Nowadays a certain heart condition called PFO is sometimes the cause. Many humans have PFO and live noraml lives- but throw in some rapid pressure changes and PFO is a bit more of a concern.
www.diversalertnetwork.org/medical/articles/Patent_Foramen_Ovale

On scuba it is also possible that you will be going a bit deeper than your profiles on a hookah- be cautious of taking your past experiences of popping up for a lookaround and then going back down. The rules-of thumb still apply to ascents- don't go faster than your bubbles.

 

[DivemasterDennis]

The fact is most of my dives have an element of the "sawtooth profile," especially when the topography of the site involves up and over movement. However, these are not repeated ascents to the surface. A dive planned to 60 feet often goes 60-45-55-60-50-40-60-40, and etc. I have never had or heard of others having an issue with such movement. In contrast, repeated ascents to the surface are NOT a good thing. I stress with students that aspect of scuba that is not for those with attention issues: you submerge, you stay submerged, and you ascend at the end of the dive. Sometimes the dive is 30 minutes, and sometimes it is an hour or longer, and we STAY DOWN ( absent emergency) for the duration. Even on shallow dives, 35 feet or less, repeated surfacing is to tempt the "unexploded goat" supergaijin refers to above. I am aware of those whose navigation skill level occasionally requires a "compass check" done by surfacing, looking for the boat, and getting a heading. I am not in favor of those, or any interval surfacing except for emergencies. Each ascent should be slow, and each should involve a safety stop. Do three ascents properly and you spent a fair portion of your air and bottom time doing something other than the planned dive. So, saw tooth profiles varying depth without surfacing? That's ok. Repeated trips to the surface during the dive? Not ok.
DivemasterDennis

 

[Cosimo_Zaretti]

It does concern me that on guided shore dives, the DM will often surface with a diver who's had to thumb early, make sure they're ok with the exit then dive down to look after the rest of the group. On any given weekend there's always going to be some muppet who manages to run out of air when everyone else still has 130Bar, so the DMs seem to be making a couple of ascents on almost every dive. Admittedly most of these dives are probably only to 10 or 12metres, but the practice still bugs me.

 

[AfterDark]

We dived like for years until someone figured out it might not be a good idea. I used the news as a reason to sharpen my nav skills so multiple acsent wouldn't be required. For quite awhile now I've been surfacing once at the end of the dive. All these years you've done this and you are ok. What does that indicate to you?

 

[Bert van den Berg]

Thanks everyone for your comments.

I'm used to navigating boats but without an underwater GPS it's sometimes a bit difficult to compensate for current when swimming a compass course

The fact that I'm still OK after years of saw tooth diving indicates to me that either I've been very lucky or the shallow depths to which I dive has prevented problems or both.

Cheers,

Bert

 

[Citizen]

The fact is most of my dives have an element of the "sawtooth profile," especially when the topography of the site involves up and over movement. However, these are not repeated ascents to the surface. A dive planned to 60 feet often goes 60-45-55-60-50-40-60-40, and etc.

This is a really interesting point. I agree that no dive strictly follows a 'standard' profile - most involve an element of 20-17-16-17-16-18 (metres) etc. So what exactly are the tolerances I wonder? I.e. if a plan calls for a depth of 15m, and you vary between 14 - 17m; is that an acceptable tolerance? Does that count as 'sawtooth'? Is going to ten metres and then back to 15m acceptable..?

Most training would say avoid any ascents and re-descents but there must be an element of tolerance... (even if just to account for different parts of a diver being at different depths!) IIRC this is not discussed specifically in any training...

Perhaps tolerance guidelines could be (or have been?) calculated based on pressure changes caused by variance in MSW, nitrogen absorption and elimination rates, etc..?

 

[tamas970]

These are thumb rules. If there is a certain topography/habitat you want to follow, you will do the sawtooth anyway, just allow for a bit longer safety stops. As the story of the "not exploded goats" says, halving the pressure is fine - which means: 30m to 10m should not be a serious issue if you keep the ascent rate limits. Being more conservative, I'd stay with a maximum 25% pressure drop (30m to 20m).

This is a really interesting point. I agree that no dive strictly follows a 'standard' profile - most involve an element of 20-17-16-17-16-18 (metres) etc. So what exactly are the tolerances I wonder? I.e. if a plan calls for a depth of 15m, and you vary between 14 - 17m; is that an acceptable tolerance? Does that count as 'sawtooth'? Is going to ten metres and then back to 15m acceptable..?

Most training would say avoid any ascents and re-descents but there must be an element of tolerance... (even if just to account for different parts of a diver being at different depths!) IIRC this is not discussed specifically in any training...

Perhaps tolerance guidelines could be (or have been?) calculated based on pressure changes caused by variance in MSW, nitrogen absorption and elimination rates, etc..?

 

[DCBC]

There are always lots of theories, but there have been no deaths attributable to such a profile that fall within the tables, to the best of my knowledge. When I was at DCIEM, we tested repetitive dives having multiple ascents within the same dive ( in the process of refining the DCIEM recreational tables). No additional increase in MBR was detected by Doppler ultrasonic testing. The predicted maximum bubble radius (max BR) attained can be used as a measure of the risk of decompression sickness (DCS). This isn't however definitive, as subsequent research has showed 'bounce diving' may be a factor at non-recreational depths.

What it comes down to is to dive within the tables, in a reasonable manner (make allowances for age, effort, water temperature, etc.) I don't think that "bubble pumping" is anything that you have to be concerned about for the type of diving you seem to be doing.