Riding GF99 instead of mandatory/safety stops

[old frogman]

This is misleading. Every inch you ascent has a compounding effect as it's preventing further slow tissue absorption. It won't just shorten your current stop but possibly avoid some of the stops down the line.

A properly planned and executed staged decompression ascent should not cause fast tissues to on-gas during the stop. However, slow tissue will on-gas to some extent, during the stop. The point is, that slow tissue compartments are not fully saturated at the end of the bottom time and consequently can accept a small amount of additional dissolved gas, during stops, without stress.

Where slow tissue on-gas becomes a problem is when ascending from very deep dives. Consequently, the diver will be spending considerable time in the shallows off-gassing slow tissue.

e.g. dives to 100 ft for 20 mins entails no deco except for a 3 to 5 mins safety stop at 15 ft. plus additional 3 mins for the ascent.

e.g. dives to 200 ft for about 20 min usually entail about 30 mins deco which includes the ascent time.

e.g. dives to 300 ft for about 20 mins may entail from 1.5 hrs to 2hrs for deco which includes ascent time. Most of this decompression will be spent in the shallows.

Fast tissues include brain, lungs, heart and muscles. They are associated with high blood flow which moves dissolved gas in and out fast.

Slow tissues include fat, cartilage, tendons and bones. They are associated with low blood flow which moves dissolved gas in and out slow.

 

[OctopusLover]

W

What you are postulating is a process that hypothetically has advantages in reducing the overall decompression duration. However, in practice, as outlined by other contributors, it is impractical in water. I would add, that in water, it is dangerous as a slight mistake in ascent rate could trigger decompression sickness (DCS).

A form of continuous decompression which also includes stops, is saturation dive decompression. This is conducted in a decompression chamber where, if problems arise, the decompression can be halted and measures taken to resolve the problem. In addition, divers use an instrument (ultrasound doppler bubble detector) which detects micro bubbles in the cardiovascular system. This alerts the divers and the chamber control team to any DCS issues before they manifest into full blown DCS.

Could a safety margin not be built in? E.g your ceiling is whatever it is and continuously increasing, your goal is to stay a certain distance under that.

A simpler way to ask this question is not going all the way to "continuous ascent without stop" and instead just asking why not shorter more frequent stops than are currently planned by the average dive computer.

 

[Nick_Radov]

e.g. dives to 200 ft for about 20 min usually entail about 30 mins deco which includes the ascent time.

I agree with you on the qualitative explanation but quantitatively that particular profile seems pretty aggressive. Maybe it works for you but I think most of us would do at least 5 minutes more deco (assuming standard gasses and limits).

 

[dmaziuk]

Could a safety margin not be built in? E.g your ceiling is whatever it is and continuously increasing, your goal is to stay a certain distance under that.

A simpler way to ask this question is not going all the way to "continuous ascent without stop" and instead just asking why not shorter more frequent stops than are currently planned by the average dive computer.

Herr Dr Buhlmann's theory put forth in Decompression - Decompression Sickness is that a) continuous ascent is not practical in open water and b) 0.3 atm appears to provide a "good balance" between the greater delta-P that drives the off-gassing and the "safety margin" you speak of. Or at least that's my vague recollection.

This all is optimized for the TTS. If you run a bunch of simulations, you may well find that there is no meaningful reduction of TTS with deco steps smaller than 0.3 atm anyway.


PS OTOH you may find that there is, publish your discovery, and make a name for yourself. In which case I want 10%.

 

[gmerick]

Paper was published in 2022:

Ceiling-controlled versus staged decompression: comparison between decompression duration and tissue tensions​

Conclusions
Ceiling-controlled decompression shortens the decompression duration at the cost of higher supersaturation in the faster tissues. While this increase in supersaturation does not lead to a breach of the limits of the decompression algorithm, one cannot a priori state that it does not lead to an increase in risk of decompression sickness. Computer simulations comparing dives using staged decompression and ceiling-controlled decompression and subsequent analysis of the inert gas tensions suggest that the two procedures might be similarly acceptable and thus the matter should be investigated further.

 

[TheCatFish]

Ceiling control deco methods put too much stock into the Buhlman algorithm's ability to predict what is happening in the body. Ascending in 3 meter increments allows time for the predicted off-gassing to occur. The closer you ride a theoretical line, the easier it will be to accidentally cross it even though the numbers all appear within your expected "safe numbers".

 

[old frogman]

I agree with you on the qualitative explanation but quantitatively that particular profile seems pretty aggressive. Maybe it works for you but I think most of us would do at least 5 minutes more deco (assuming standard gasses and limits).

I agree with you. California waters are quite chilly, so extra deco is good practice. The deco times mentioned are purely illustrative.

 

[rjack321]

Could a safety margin not be built in? E.g your ceiling is whatever it is and continuously increasing, your goal is to stay a certain distance under that.

A simpler way to ask this question is not going all the way to "continuous ascent without stop" and instead just asking why not shorter more frequent stops than are currently planned by the average dive computer.

Sure you could ride any GF you want. GF80 as a ceiling all the way up? Sure!

It's still not magically better than staged step deco. A 0.3ATA step keeping you deeper than your chosen ceiling is pretty small in the grand scheme of things.

 

[dmaziuk]

Paper was published in 2022:

Ceiling-controlled versus staged decompression: comparison between decompression duration and tissue tensions​

Conclusions
Ceiling-controlled decompression shortens the decompression duration at the cost of higher supersaturation in the faster tissues. While this increase in supersaturation does not lead to a breach of the limits of the decompression algorithm, one cannot a priori state that it does not lead to an increase in risk of decompression sickness. Computer simulations comparing dives using staged decompression and ceiling-controlled decompression and subsequent analysis of the inert gas tensions suggest that the two procedures might be similarly acceptable and thus the matter should be investigated further.

However, Table 1 looks like if you want get meaningful reduction of TTS, you need much longer and/or deeper dives.

GF Low controls the first stop and the first stop is for the fast tissues. Whether it makes sense to set GF Low to 30 to protect the fast tissues and then "ride the ceiling" to "supersaturate them more", is an interesting philosophical question.