Question Understanding Decompression Stops: Confusion with Shearwater Peregrine

[inquis]

Do you know any books to learn more about decompression ?

Powell's Technical Diving - An Introduction is a good place to start, as it gives the "big picture" info. For more specific info about the science, his Deco for Divers, 2nd Ed. will be informative.

 

[ScubaFoxx]

Does a deco dive with his peregrine with 0 training.
Goes to roughly 46m on probably air.
Knows it's not smart to go right up so takes his merry time doing so (thankfully).
Then wonders why his computer stopped telling him he was in deco.

First, have you read the manual?
What were you breathing?
Why did you break the recreational barrier of 40m? For testing purposes?

Please buy books on decompression, but keep them tucked away untill after you get some proper training. The risk you take doing a 1 tank deco dive is greater than your life is worth

 

[Njord fr]

Goes to roughly 46m on probably air.

Knows it's not smart to go right up so takes his merry time doing so (thankfully).
Then wonders why his computer stopped telling him he was in deco.

First, have you read the manual?
What were you breathing?
Why did you break the recreational barrier of 40m? For testing purposes?

Please buy books on decompression, but keep them tucked away untill after you get some proper

Thank you for your feedback and concern for safety. Just to clarify, in France, where I come from, the recreational limit for diving on air is actually 60m, so my dive to 43m was within local guidelines.

I was using air for the dive, 2x12liters on air + 2x7liters for the deco at 32% and 40%, and I do have training and familiarity with my Peregrine's manual. Plus, I was accompanied by a well-respected instructor during the dive. I understand that diving beyond 40m can seem risky for you, but in France, it is quiet usual, and with the instructor's guidance, I approached it with caution and a bit of curiosity.

I appreciate the advice on decompression and the reminder to keep learning, that’s why I am here, I already have some resources and plan to get more hands-on training. Safety is definitely a priority for me, and I’ll make sure my future dives reflect that.

Thanks for looking out for fellow divers, for your valuable input, and for your incredibly helpful and considerate response.

 

[inquis]

A safer way to explore the deco realm is to plan an NDL dive according to your normal choice of conservatism (gradient factors). Then change your computer to be extremely conservative (e.g., GF=30/40) but still dive that NDL profile/bottom time. The computer obviously shows you the deco info & warnings. Practice hitting the stops without exceeding them. See the warnings if you drift slightly above the stop. If you have a failure (e.g., blow a stop or equipment malfunction), going to the surface is still a safe option.

 

[tursiops]

A safer way to explore the deco realm is to plan an NDL dive according to your normal choice of conservatism (gradient factors). Then change your computer to be extremely conservative (e.g., GF=30/40) but still dive that NDL profile/bottom time. The computer obviously shows you the deco info & warnings. Practice hitting the stops without exceeding them. See the warnings if you drift slightly above the stop. If you have a failure (e.g., blow a stop or equipment malfunction), going to the surface is still a safe option.

Or, max depth permitting, dive Nitrox but set the computuer for air. Set your backup cimputer for the Nitrox you are actually using, so you have a window of non-deco comfort between the two computers.

 

[dmaziuk]

Related question, what’s the relationship between exceeded ndl and deco? Straight line, i.e. exceed by a minute and a minute of deco or something else? If the latter, why, whats the deco theory behind it?

For the algorithm running in Shearwaters: gas flows to the lower pressure (i.e. on descent: from ambient into the tissue, on ascent: the other way) at the rate of log2 of the delta-P and time. One tricky part is it's the log curve, not a straight line, so it's not a straightforward one-minute-for-one-minute.

The other tricky part is multiple "compartments" with different time quantums. While some of them are over the ambient pressure (off-gassing), others may still be under it and on-gassing. Which of them are "over the NDL", and by how much, will determine your total deco time.

PS. The other other tricky part is gas mixes. Ratio Deco has the relationships you ask about, but they're only valid for certain gas mixes.