Complicated decompression theory question.

[declan long]

Hi Guys, first of all thanks for your replies. Please accept that i have read deco for divers so i don't need that recomendation and i am not interested in this shold have been covered in my training, just answers please.

one - I have a shearwater petrel dc, when i go into deco on air when it gives me a ceiling is this the m value of my fasteest tissue just to clarify or is it with the conservitism.

two - for air dives with light deco i also take my sunnto gekko, i know this dc uses rgbm but why is the ceilng differnt and shollower, surely an m value is that value and i expect the suunto to be conservitive, can you expand on this please with some info?

three - i have been told when i was having the debate about using eight v hundered percent o two that using either gas i can do the six m stop at five foour or three without difference so is this true and why does this affect off gassing?

four - why does my dc go red when i ascend past my stop and not the ceiling, what is the harm in this?

thanks
declan

 

[LowVizWiz]

1. http://www.shearwater.com/wp-content/uploads/2013/07/PetrelManual_1_2_0.pdf (Pages 13, 14, 17 & 29)

I suggest you read those pages and follow all of the recommendations on page 29. Including the recommendation to read Erik Barker's Clearing up the confusion about Deep Stops and Understanding M-Values and specially the recommendation to not use the system until you understand it.

I'm really not trying to be a jerk or mean to you, but you should not be doing deco dives without the clear understanding of what the ceiling depth is and what the harm in exceeding it is.

 

[flots am]

one - I have a shearwater petrel dc, when i go into deco on air when it gives me a ceiling is this the m value of my fasteest tissue just to clarify or is it with the conservitism.

two - for air dives with light deco i also take my sunnto gekko, i know this dc uses rgbm but why is the ceilng differnt and shollower, surely an m value is that value and i expect the suunto to be conservitive, can you expand on this please with some info?

You'll never get two computers from different manufacturers to show identical data.

three - i have been told when i was having the debate about using eight v hundered percent o two that using either gas i can do the six m stop at five foour or three without difference so is this true and why does this affect off gassing?

I have no idea exactly what you're asking, except that you seem to be asking about the difference between 80% and 100% O2 at different depths The differences should have been clear after reading Deco for Divers.

four - why does my dc go red when i ascend past my stop and not the ceiling, what is the harm in this?

I have no idea what "going red" means. You should read your computer's manual to find out.

I would also suggest that you stop riding your computers for deco dives, and absolutely don't do any more deco dives until you completely understand what your computer(s) is/are saying. Riding the computer makes dive planning unnecessarily difficult and gas planning nearly impossible.

What happens when you have 20 minutes of deco and 5 minutes of gas?

Try vPlanner and a wrist slate and a bottom timer. You'll know everything about your dive before you get wet.

flots.

 

[rongoodman]

Your Suunto is using a proprietary algorithm which they have labeled as Suunto RGBM--there is no way to understand why it does what it does because they have never made the information available.

 

[lv2dive]

Hi Guys, first of all thanks for your replies. Please accept that i have read deco for divers so i don't need that recomendation and i am not interested in this shold have been covered in my training, just answers please.
...
four - why does my dc go red when i ascend past my stop and not the ceiling, what is the harm in this?

There is a difference between a "ceiling" and your planned stop depth. If you exceed your planned stop depth the computer flashes red to alert you that you have violated a stop. From the Shearwater manual:

The next selection is CEIL With this setting, as long as the NDL time is 0 (you have a decompression ceiling), the raw ceiling will be displayed instead of the NDL This is the equivalent of the ‘Man on a rope’ It will show your ceiling without it being rounded up to the next even 10 foot or 3 meter stop Please note that there is very limited information on the effects of following a continuous ceiling instead of stopping at stops and only moving up to the next stop when the stop has cleared

It is the author’s opinion that all stops should be honored It seems intuitive that if you have bubbles, and you stop, you give the bubbles an opportunity to be resorbed If you continuously ascend, the ambient pressure is continuously reduced which prevents bubbles from shrinking Because of this belief, the computer will give one MISSED DECO STOP message during the dive and one after the dive, and will flash the stop depth and time in red as long as you are above the stop depth It will use the increased gradient though, and your calculated off-gassing will be faster than staying at the stops.​

BTW it took me about three minutes to find Shearwater Petrel manual online and look that up.

What is "the harm in this"? I learned to plan my dive and dive my plan - and I try to hold my stops within a foot or less of target. I am not clear why anyone would want to NOT do that?

 

[Dr. Lecter]

Paging Dr. Darwin, paging Dr. Charles Darwin...

 

[Subcooled]

Both RGBM and VPM are bubble models that both have "a hundred parameters" to adjust. Just take a look at V-Planners advanced settings... You can change the parameters, critical radius, and others, and still be running the same model.

Why different stops? I don't know. Maybe doing a slower ascent vs. staying longer at shallow depth is not a always big deal? Follow one computer, do not alternate. Actually, plan and agree on the dives deco profile in advance.

 

[HIGHwing]

You're [planning?] executing air decompression dives and using the actual calculated ceiling [in real time?] versus adhering to the computed stops of the dive computer? I admire your bravery to post this practice here on Scubaboard.

 

[DevonDiver]

i have read deco for divers ...I have a shearwater petrel dc, when i go into deco on air when it gives me a ceiling is this the m value of my fasteest tissue just to clarify or is it with the conservitism.

Reading and understanding are two different things. It may prove beneficial to re-read 'Deco for Divers', as it answers these sort of questions quite clearly. I routinely re-cover material in that book; as sometimes things make more sense over time.

As others have mentioned, basic comprehension of deco theory, relative to the information provided by your computer manufacturer in the manual, makes the answer to this question quite apparent.

The displayed ceiling accounts for whatever conservatism you have elected; gradient factors or VPM +? etc etc

The word 'controlling' is best used when considering varied tissue compartments and the creation of a ceiling. The 'fastest' tissue compartment may not be the controlling compartment. Indeed, in decompression diving, it is rarely the fastest tissue that controls.

for air dives with light deco i also take my sunnto gekko, i know this dc uses rgbm but why is the ceilng differnt and shollower, surely an m value is that value and i expect the suunto to be conservitive, can you expand on this please with some info?

There is absolutely no basis to expect two dissimilar algorithms to provide similar, or exactly matching, ascent plans.

Again... with reference to 'Deco for Divers'; Mark Powell clearly explains the fundamental differences between the leading algorithms used in modern dive computers - and further clarifies the important concepts that differentiated them in development.

In particular, I'd suggest you re-visit the chapters/paragraphs that describe RGBM, Buhlmann and VPM modelling in that book...

i have been told when i was having the debate about using eight v hundered percent o two that using either gas i can do the six m stop at five foour or three without difference so is this true and why does this affect off gassing?

Mark Powell provides explanation of several uses of the term 'Oxygen window'. Re-visit the 3rd definition that he explains in the book...

It's been a long time since I took the TDI Advanced Nitrox course (and subsequently, my recollection is blurred with training in several other agencies), but I do have a hazy memory that this question is answered in the AN manual also. You are TDI tech trained right???

why does my dc go red when i ascend past my stop and not the ceiling, what is the harm in this?

I'd suggest ceasing all decompression diving activities until you have ingrained a profound understanding of the answer to this question.

This is not something a qualified technical diver should be asking...

 

[TSandM]

M-values are chosen, not measured or calculated. Different models will use different M-values, which is part of why different models (even if they are based on the same theory) may give different profiles.

I will echo the others. I find the questions you are asking profoundly disturbing, because either the training you have had was not thorough, or you didn't understand it, or you are attempting to teach yourself how to do this. Stage decompression diving is high risk even when it is done with a full understanding of what the diver is doing, and after evaluation by an instructor who passes the diver as both skilled enough, and gifted with enough composure to keep his wits together when all hell is breaking loose. I do not get the feeling you have been through that process.