Good divecomputers recalculate if you do things they don't expect. The worst computers show then '505'.
It is easy to programm the 505, as this is just end of what was expected. But recalculating is the most difficult thing for a programmer.
So if I decide to do a stop at 11 m instead of 12, a good computer will not have problems with that, maybe a 'missed decostop', 'confirm' is shown, but the 505 not.
Remember, deco is no absolute science. There are safe ways to calculate, but the decostops from 3,6,9, 12, 15, 18,21, etc are just made to make it easy for the diver. Every 3m.
Most technical divers do their last stop at 6m and then go up slowly with 1m/minute, or try to stay some extra minutes around 3-4m.
The biggest difference in pressure is found in the last few meters to surface. So this is the reason why some people add a few minutes. Or go up slowly with 1m/minute.
As decompression diving is no absolute science, if you forget for example the 15m stop, just add it to the 12m stop and you will probably be fine.
The thing most divecomputers (the ones without spg sensor) cannot do is calculating if you have enough gas. And this is where things go wrong. Especially in really deep dives on ccr can this be a problem. The bailouts are open circuit. With doing dives over 100m, the problems are not in the bottomgas (nor in oc, nor in ccr), but in the travelgases and decogases.
I have done 1 dive to 110m on open circuit on a twin12 liter with 17 minutes bottomtime. The amount of gas in the twinset was within the safetystandards. But the travelgas and decogases not. For the ean80 as last decogas we needed 2100 liters of gas in the calculation. This means you suck a tank of 2200 liter gas completely empty. This also means you need to work with safety divers. Most people think the limits are the bottomgas, but no, the limits are the decogases and the ean80 or 100% is the most important decogas. So you don't want to loose that or have too less. But the good thing is, to drop 100% oxygen, you just need a rope of 6m. We met in this dive safetydivers at 45m, this was a depth we could reach on bottomgas if needed and don't have too less. We calculated to switch to a travelgas around 57m.
At the end less gas was used than calculated, so no cylinders were drained completely.
But 1 minute longer at 100m is approximately 8 minutes more deco. 1 minute more at 130m means 22 minutes more deco. So if you think it is just 1 minute down, the problems can start on the way up. With a ccr it is easy to do that 1 minute more. You see in the computer the new TTS, and that is doable with the comfortlevel of cold or within the time you paid for using the boat that is waiting for you. But the computer don't show the limit of gas that maybe is needed.
With cavediving the same risks apply on ccr. You only need bailout gas to get out. The worst bailout is oc (if possible you try to use (p)scr bailout and then you use bo gas as a gas extender), so in case of a CO2 hit, you only can do an oc bo. You swim in the cave, shallow cave, no deco. You know you have gas for 400m. Then you must know how long it takes to reach that 400m. Because the time is then the sign that tells you you are now over the limit, probably there are no signs in the cave how far you are in already.
But if you jump in a cave you don't know. The depth can vary. What is now the limit before you have to turn? Exactly, this is where ccr cavers can be on risk as they think we can go a little bit further.
In the past, Suunto had computers that showed deco under the shower. This means they were extremely conservative. The last years, Suunto changed someting in the 'fused rgbm' modell and now they show almost the same deco model as a buhlmann gf 80 in the shallows.
The 'secret' decomodells in Suunto computers where also the reason why technical divers did not like this computers. You could not tweak things and the code was secret. Also RGBM is a bubble modell like VPM and this means deepstops and deepstops are nowadays not used a lot anymore. With buhlmann divers are free to ad deepstops (lower the GF-low), or add more conservatism in the shallow part (lower the GF-high). Since a few years, Suunto added Buhlmann to their Steel computers because divers wanted it.
I don't know if the computers now can recalculate a missed decostop, but this was also very annoying.
I have had students with the '505' message on Steel because of this (or better say of forgetting to switch the computer when switching to another gas). It is a good lesson to let this happen, but it is not that good that they cannot use their computers again for the next dive. Of course you can still do decompression dives with a plan on wetnotes and just 2 bottomtimers, but to pay 100 bucks for an expensive bottomtimer, you are not lucky.
The difference between bubble models and tissue models is that the bubble models state there are gas cores and you don't want that they grow into gas bubbles. With the bubble models you state that there are bubbles and you do bubble manegement.
There are on youtube some videos found about the bubble models (vpm and rgbm) and they show that also if you are more conservative in the shallow part you have more bubbles at surface. In 2010 deepstops were popular, now they are not so popular anymore.
But also other things changed, ratio deco. A very popular DIR-tool around 2010. No computer needed, deco by head, etc. Then it was found that it was ok for bottomtimes around 20minutes at 60m, shorter was more on the conservative hand, but longer got unconservative. Also divers had problems with doing deco by head. So the computer is now an accepted tool.
The standard 30/85 of Buhlmann changed into 30/80, then 40/75 and now sometimes I read that 40/70 or 50/70 is a good conservatism. So over the time decostrategies changed.
I have done dives over 100m with vpm-b+2 with no problems, and also with buhlmann 30/85. But now I dive on 40/75 and sometimes 40/70. I did not have problems, but have seen some dcs incidents (no chamber rides needed) from divers I know (I was not diving with them when it happened), and I want to dive as save as possible.
So doing a longer safety stop, or stay a little bit longer at the last decompression stop, or go up with 1m/minute from the last stop is not bad. It is a proven way to improve safety. But if you have already a lot of bubbles, and the bubbles are big enough to cause problems, it will not make you imune for dcs. Also there were (I cannot find them now directly) videos on youtube that stated this.
Also stay on the loop when you climb a ladder or walk to your car is not bad. Same as breathing the last decogas when doing this when diving oc.
So I understand why people are doing this, it does not make you imune, but it increases safety in most cases. If the weather is bad, it is heavy to hold the anchor line, you are really cold, then it is probably better to get out of the water, and don't exhaust yourself anymore. Skipping deco is always an bad idea of course.
