RGBM has three factors that change the population of bubbles if you do bounce dives, reverse profiles, or repetitive dives (see “Decompression Theory” by B.R. Wienke). RGBM keeps the volume of gas in bubbles below a threshold value by controlling pressure gradient (difference between dissolved gas pressure and ambient pressure) and time.
I'll give this a try. Melanie, I will try to answer your questions directly and in a non-judgmental way.
Deco Stops
Mr. Carcharias is right. I have a Mares Puck that uses RGBM. I have seen it demanding inordinate amounts of time after a repetitive dive. I did plan that dive and was fully aware that the time it was requiring was not reasonable. After the dive, I went back to my manual and found out that under certain conditions, it will lower gradient factors which in turn will extend your required stops. The manual doesn't say much more than that. In other words, it does not tell you by how much or how this will affect a dive.
The end result is that you end up with the questions you are posing. How in the world am I supposed to plan for a dive when I have no clear indication exactly by how much the computer will extend my stops. It all boils down to the fact that this computer was not meant to plan and execute decompression dives. Simply stated, you don't have all the required information to do that. My Mares Puck has a "planning" feature but that feature only tells you what your NDL will be. It will never tell you what a deco obligation will be for a hypothetical dive. In other words, again, the algorithm as it is implemented in Mares Puck is not for planning or executing deco dives. The manual plainly says it and there are no features to plan and if you try to plan, as you did, to start ascent when you have x mins of obligation, the results are not predictable because the thing is changing gradient factors (ie. changing conservatism) on you. I suspect it is constantly changing gradient factors on the fly, but there is no information of exactly by how much, which makes this algorithm implementation unpredictable for deco obligations and you don't want unpredictability in your diving. Changing gradient factors on the fly would explain why time seems to go slower for the wretched gizmo.
Deep Stops
My Puck also has deep stops. The screen will NOT tell me about a deep stop until I reach the deep stop depth. I think this is crappy, or at least confusing design. I'm not sure if there is an audible alarm for it, but still, audible alarms with all the noise of open circuit scuba are so unreliable. Maybe the computer considers the deep stop to be more or less "optional". Before I get to the deep stop depth the screen will tell me that I have a deco obligation either at 20' or 10'. But as I get to ~43' the deep stop text lights up and a 1 minute countdown starts. If my bottom depth was 150' or deeper I can expect another deep stop at ~70'. I am not very familiar with planning deco dives on the RGBM algorithm to judge whether this deep stop strategy is logical or not.
The Good News
Does this make your computer useless for deco dives. Absolutely not. You can still use it in bottom timer mode for the execution part of the deco dive. You just cannot use it to plan a dive -- even if that planning is as simple as saying "I'll start ascending when I get 20 mins of deco obligation". You can even use it in regular mode and let it throw a tantrum when you surface and it still wants you to stay an additional 40 minutes under water. I do it like that all the time. One thing I do love about my Mares is that it will not lock me out completely if I disregard it's wishes -- it will only lock me out to bottom timer mode, which is all I really needed from it in the 1st place.
If you are still persistent in doing deco dives, you'll have to find another way to do the
planning part of it. You CANNOT safely do a deco dive if you don't plan it first. The good news is that you really do not need to buy a $1000 dollar computer to plan and execute a deco dive. There are many desktop computer software packages that you can use to plan your dive adequately. Some of those are even free:
MV-Plan the Java Dive Planner
Let's use a dive similar to the one you talk about in this thread as an example:
Settings: GF=30-80
=========================
DESC:130ft for 02:09 [ 2] on Air
DIVE:130ft for 27:49 [ 30] on Air
ASC : 60ft for 02:20 [ 32] on Air
DECO: 60ft for 01:40 [ 34] on Air
DECO: 50ft for 02:00 [ 36] on Air
DECO: 40ft for 04:00 [ 40] on Air
DECO: 30ft for 06:00 [ 46] on Air
DECO: 20ft for 09:00 [ 55] on Air
DECO: 10ft for 22:00 [ 77] on Air
Gas estimate based on Dive RMV =0.7, deco RMV =0.6cuft/min
Air : 151.0cuft
Oxygen Toxicity - OTUs: 31 CNS: 13%
Discipline
You write down the above plan in something you can take with you under water and only use your Mares to get to tell you what is your depth and dive time, nothing more. From the example above you know that you will do a 2 minute deep stop at 50 feet and you will be leaving 50 ft at exactly 34 mins into your total dive time. As you can see, you have to be VERY disciplined to do this type of deco diving. There is no computer adjusting the plan on the fly in case you failed to make it to the stop on time. And, honestly, you shouldn't depend on a computer to do that for you.
Gas Requirements
You can also see from the example above that the overall total time of the dive will be 77 mins and you will use up
151 cu ft of gas. A BIG HP 130 steel tank will only hold 130 cu ft. This means that for this dive you will need more than one tank -- so you see that wearing doubles is not just because they look cool. Could you get away with just wearing twin Al 80's? Well, an Al 80 doesn't really hold 80 cu ft. In reality it holds 77.4 cu ft. Therefore twin Al 80's will hold 154.8 cu ft. You will not have enough gas for you and your buddy if he experiences a regulator malfunction at depth. Also, what would happen if you get momentarily swept by the current and you have to swim hard for 3 mins to get back on track. The hard swim duplicated your gas consumption and now you don't have enough gas to complete the dive. Which brings us to the next topic:
Gas Consumption -- Measuring RMV aka SAC aka SCR
The above example assumes you have a gas consumption rate of 0.7 cu ft per min per atmosphere. How do I know if that is representative for me? Well, I have to measure my own consumption rates because they will most certainly vary from person to person. Not only do you have to have a clear idea of how much gas you consume during regular diving, but it is also very important to know how much gas you use up during psychologically and/or physically stressful situations. It will also be a good idea to know the consumption rates of your buddy, because you are a source of emergency gas for him.
In Conclusion
You cannot reliably use your Mares to
plan deco dives. There are other non-expensive ways to adequately plan deco dives. You can keep on using your Mares in a bottom timer capacity to execute dives. The examples I give above are grossly oversimplified and for illustrative purposes only. They are not meant to replace proper (formal or otherwise) instruction on decompression diving.
I hope this helps.