Qs About Science of Diving Content

[BlueTrin]

Incidentally that confused a few more people in 2020, I know that confused me when I did the class when it was free in 2020

dive computers and reverse profiles

Coincidentally I just finished that SSI section last night and was also confused about the reverse profile and computers. I rarely use my computer, so didn't give it too much thought. I figured it was something like -- the computer will track nitrogen regardless of your profile, but your body...

scubaboard.com

 

[Dan G]

Going up and down repeatedly on a dive is often called a sawtooth profile, and it is generally considered to be unsafe, although I have ever heard an explanation as to why. I remember looking at my computer profile after diving the Car Wash cave in Tulum. It was one of the worst sawtooth profiles you will see, but I had no choice because I was in a cave. It is indeed typically avoided.

As for multi-level dives, it is indeed better to do the deepest parts of the dive first, since the fact that you are slowly ascending aids in your off-gassing. It is better to have the final ascent from the shallowest part of the dive rather than the deepest part of the dive.

This does not, however, fool your computer, and I have no idea why it would.

Is the premise here that we are ascending through fewer atmospheres to the safety stop when we ascend from a shallower depth? And/or we are spending time off gassing while spending time at less than 1 ata?

Incidentally that confused a few more people in 2020, I know that confused me when I did the class when it was free in 2020

dive computers and reverse profiles

Coincidentally I just finished that SSI section last night and was also confused about the reverse profile and computers. I rarely use my computer, so didn't give it too much thought. I figured it was something like -- the computer will track nitrogen regardless of your profile, but your body...

scubaboard.com

A short scan of that thread and I see all I did was dredge up old news with basically the same people.

 

[BlueTrin]

@Dan G i think it’s fine, it’s worthwhile to revisit topics or there wouldn’t be any threads at all in the forum

But one wonders if SSI should update their materials.

I had sent them an email in 2020 but didn’t get a reply or maybe I got some generic reply …

 

[L13]

A short scan of that thread and I see all I did was dredge up old news with basically the same people.

That was before my time, but it seems I am becoming one of "the same people."

 

[arew+4]

I would feel much better about riding my NDL to the end starting at 90 feet and coming up slow, rather than going slowly down to 90 then right up and out in a race against the clock.

 

[BlueTrin]

That was before my time, but it seems I am becoming one of "the same people."

I recognise you @L13 in the threads

 

[inquis]

Going up and down repeatedly on a dive is often called a sawtooth profile, and it is generally considered to be unsafe

I believe the main objection is that if bubbles form during an an ascent, then compress after going deeper, they can make it past the lungs where later expansion would be dangerous (e.g., to the brain causing a stroke).

Obviously, much depends on the depths, rates, and tissue loadings involved. Following the terrain between 50-60 ft may be different than 100 to 20 to 100 ft. A blanket "don't do it" is the more defensible position. That said, it's not uncommon for divers to "spy hop" for navigation (i.e., briefly surface to sight a bearing) then return to some depth. Or follow terrain up & down. Etc.

Personally I feel the conservatism built into various models preclude this as an issue in my mind.

 

[boulderjohn]

Is the premise here that we are ascending through fewer atmospheres to the safety stop when we ascend from a shallower depth? And/or we are spending time off gassing while spending time at less than 1 ata?

When you do a multi-level dive with the deepest part first, as you ascend to the shallower depths, the faster tissues that were on their way to saturation or saturated will begin offgassing as you ascend. You are well on the way toward safe levels. When you begin the final ascent, you do not have far to go to reach levels safe for surfacing. Your fastest tissues will probably already be there.

If you are first shallow and then descend to the deepest part of the dive, all your tissues will be on-gassing to that deepest ambient pressure before you begin the ascent.

A multi-level dive is similar in effect to decompression stops. You would not do them from shallowest to deepest.

 

[boulderjohn]

I believe the main objection is that if bubbles form during an an ascent, then compress after going deeper, they can make it past the lungs where later expansion would be dangerous (e.g., to the brain causing a stroke).

That's what Wienke argued for why reverse profiles should not be allowed.

I never understood why bubbles that have formed and then shrunk to a smaller size during a later descent would be likely to to do this when the same size bubbles formed at any other part of the dive would not.

 

[ThatDogDontHunt]

That's what Wienke argued for why reverse profiles should not be allowed.

I never understood why bubbles that have formed and then shrunk to a smaller size during a later descent would be likely to to do this when the same size bubbles formed at any other part of the dive would not.

Since every dive starts with a "reverse profile" would that thinking not lead to the requirement to descend as quickly as possible?

Things like waiting under the boat for the rest of the group to get in the water might be considered dangerous?