Reddit mentions of Deco for Divers

Inb4 Deco for Divers is one of the most commonly suggested books to read. Mark breaks it down really well for us non-chemistry students whilst still being a very technical book. It will also leave you with lots of avenues for research.

I don't have the instructor notes to hand so, from memory this is for two reasons:

1. The brain's initial reaction to the increased PPO2 (I can't remember the exact details of that but can look them up later)
   
   
2. The passage of O2 into the nitrogen bubbles causing a marginal increase in bubble size. This will be temporary and will be offset by the elimination of the nitrogen from the bubbles due to the change in pressure gradient.
   
   If you want a better explanation of the theory behind this, I highly recommend reading Deco for Divers by Mark Powell

I'd reccomend reading http://www.amazon.co.uk/Deco-Divers-Decompression-Theory-Physiology/dp/1905492073/ref=sr_1_1?ie=UTF8&qid=1345549204&sr=8-1 if you want the basis to make a well reasoned choice about this. As a doctor I'm used to rolling my eyes at "layman" explanations of medical problems, but this book does a very good job of it.

7 hours should be a good enough margin in most cases imo. It all depends on your nitrogen loading. Going by the multi-compartment way of approaching decompression problems the logical reasoning would be that the faster compartments are the ones where you will have the most effect of exercise. Increased bloodflow to muscles will help clear them more rapidly of N2 (thus increasing the nitrogen pressure in your blood) but shouldn't have much effect on the fatty tissues.
On the other hand headaches and itching skin are signs that you have light DCS symptoms..

(disclaimer, not a certified diving doctor)

Wow, I guess it's not your fault. Education about dive computers needs to be much better. That said, the tables aren't to be fully trusted either.

The algorithms designed (from which the tables derive) are based on testing, mostly with young fit divers. That got the values by sedning people down and seeing when they reported symptoms. Oh - he got really sick, guess we shouldn't send them down that long. That's how they original (in a few methods) got the m-value - basically, top end of super-saturation various tissues in your body can take. Even then, in testing they found that 80% of DCS incidents occurred with a super-staturation less than 80% of the M-Value - meaning these were very conservative dives but still exhibited symptoms. Even without symptoms silent bubbles (i.e. compressed gas without symptoms) are always present. That line is said to be "a solid line through a fuzzy grey area"

By design the value you get from your computer is how long most people can dive without signs of DCS - that's not 100% and depending on the day it could be you. It's just probability.

Also, one thing to consider, with technical computers we can tune the algorithm conservativeness with limiting super-saturation of the tissues and requiring more off-gassing. When I take that computer diving in warm water I usually scale back the conservation a little since I'm not working that hard - even on this less conservative setting it's usually about 10-20% shorter NDL for me that what you'll find on a rec computer. I dive enough that I scale back my risk even when doing easier dives. I've seen several times, mostly minor, with by-the-book dives leading to symptoms.

Some sources:
http://dspace.rubicon-foundation.org/xmlui/bitstream/handle/123456789/10152/VDC_2012_10.pdf?sequence=1
http://wrobell.it-zone.org/decotengu/_downloads/mvalues.pdf

Highly recommend:
http://www.amazon.com/Deco-Divers-Decompression-Theory-Physiology/dp/1905492073

This is a really good question and something I have long pondered myself. Now that it has been mentioned, I think it is a good question to ask DAN - perhaps something they can address in Alert Diver.

My theory is that it probably can increase risk of DCS, particularly when you are loading your tissues with longer halftimes heavily and when you crack your joints on ascent. My reasoning behind this is as follows (and I will try to make this a short as possible).

There is the assumption now in many decompression models (of note, the VPM and its variants & the RGBM) that there are micro gas nuclei in our tissues at all times, even before we go diving. Bubble decompression models aim to limit the growth of these bubbles by letting them offgas at depth before they reach a critical volume, past which they will only grow causing symptomatic DCI and/or alternatively limiting the number of bubbles in our tissues that reach a certain volume. If you're interested in the exact parameters of these models, I would suggest a book by Mark Powell called Deco for Divers. Anyway, I digress.

My theory is that since something like synovial fluid is likely a very poorly perfused tissue receiving little blood flow to its adjacent tissues, so it is going to take a long time for any decent amount of N2 (or He) to accrue in it. That said, if synovial fluid was relatively saturated with diving gases and then you went and popped that joint, a bubble would mechanically form from the cavitation of the popping and with that bubble being surrounded by relatively saturated fluid, there may be little in the way of a gradient for it to transition back into the dissolved state. Effectively, it would be a bubble beyond the critical volume which would only continue to grow - particularly if it occurs during ascent.

Now keep in mind that this is entirely hypothesis and I have no empirical evidence to back it up. I have cracked knuckles and toes and elbows many times while diving. In fact, I have also observed that my joints tend to have a faster "recycle-rate" while diving than they do on the surface (i.e. the time between when I can crack them again is decreased when diving.) This makes sense - the pressure should help redissolve that CO2 bubble more quickly. Nonetheless, I have never been bent (knock on wood) with or without joint cracking.

I would imagine that cracking your joints, if it does in anyway increase your chances of DCS, probably only does so very slightly, and that chance is probably only increased in the unlikely circumstances that you are diving with mixes containing fast-diffusing gases like He and/or bringing your body to full or near-full saturation (saturation diving).

That said, I have no medical degree and this is purely conjecture. But that is my theory... definitely something I have thought about a few times before. Good question!

I would strongly suggest the book Deco for Divers.

Buhlmann (ZHL-16C is the current iteration, I think?) is a standard deco algorithm, all the compartment values for different gasses and M-values are published. Most computers also implement gradient factors on top of Buhlmann. Here is a primer.

VPM is another algorithm, but it's on shaky ground when applied to mixed gas diving. RGBM is an extension of VPM (Suunto computers use RGBM).

This is probably also something you should read.

If you want to support different gas mixes, you need to be able to let the user specify the % of O2, N2, and He in each mix (I don't know if the M-values for H2 are readily available, but almost no one uses H2 outside of deep commercial diving and medicine), and you will have to treat each gas separately in the decompression model.

And please don't set the limit of the number of gas mixes to something stupid, like two (looking at you, Oceanic).