'liberal dive computer' for live aboard style high frequency diving

[Subboy]

This is a pretty good overview. For recreational diving, I keep my Oceanic primary (on my wrist) in Pelagic Z+ because (a) my backup only uses Buhlmann, and it is good to keep them in sync, and (b) if I do occasionally go into minor deco, I don't get penalized the way the DSAT algorithm does.


---------- Post added February 5th, 2014 at 04:50 PM ----------



I'm not sure this is true. i sat down with the tables once and tried to construct a sequence of no-deco dives that would end up "breaking the NOAA O2 clock," and could not easily do it. For example, 40 mins (NDL, max BT) on 36% at 90 ft gives you 60% of the allowable 24h O2 exposure, but after an hour SI the 2nd dive only gives you 20 mins NDL at 90 ft (because of 20 mins of residual N2) and that only gives you another 15%. After another hour SI the 3rd dive only gives another 15%, so you'd have to have four dives, all on 36%, all to NDL at 90 feet, to break the clock. On 32%, it would take 9 dives (with 1h SI) to 110 ft (the MOD) at max BT to break the clock. Since 36% is getting harder and harder to find at resorts and on liveaborads, I'm not sure the O2 clock is a primary limiting factor.

---------- Post added February 5th, 2014 at 04:53 PM ----------


See my first answer above. If you expect to go into deco, Z+ will do a better job for you. if you can totally avoid deco, DSAT is fine. Nothing is going to force you to spend a long time at 6m (or more likely 3m) unless you really go into deco, regardless of your algorithm.

Thanks, that's basically what I wanted to know.

 

[KeithG]

I am with TSandM and love the "magic bracelet" concept.

If you plan to dive the same profiles regardless, and are just looking for a magic "grandma" bracelet that will not scold you, you have NOT changed your safety level. You are diving the same profile. Looking for a more forgiving computer does not affect your chance of being bent. It may give you something to blame since it did not beep, but YOU are still bent. The computer really doesn't care.

 

[guyharrisonphoto]

The simple truth remains: the longer you stay down the higher the risk of getting bent. Changing algorithms does not change your personal risk from the profile. The algorithm is just a model, and does not show what is happening in your individual body.

That being said, this thread raises an interesting issue. There are a wide variety of algorithms from very conservative (Suunto RGBM) to liberal (Oceanic DSAT). It seems there are no statistics proving that one algorithm is "safer" than another in terms of divers getting bent more often on one than another. Personally, I use the Buhlman algorithm (Z+ as implemented on Uwatec computers) as it steers a middle course in the range of different algorithms. When I rented an oceanic set to DSAT I was uncomfortable with how liberal it seemed, but that is my personal bias, not the result of any scientific studies.

If all the algorithms out there are "safe" (i.e. have the same acceptable statistical probability of triggering DCS) then there seems to be no "harm" in picking the most liberal one to govern your diving and maximize your bottom time. It does not seem to be a "magic bracelet" issue if they all are safe. The only "sure" way to pick an algorithm is to have doppler studies done on your own body using different algorithms, something that I think we all can agree is impractical.

Still, understand, that picking the most "liberal" algorithm available will put you on the highest probability of DCS risk. In other words, even if they all are "safe" more conservative algorithms will be "safer" than more liberal ones. That is something to really think about with aggressive multi-day deep or long exposures. If any circumstances merited a more conservative approach, those would.

 

[redacted]

Still, understand, that picking the most "liberal" algorithm available will put you on the highest probability of DCS risk. In other words, even if they all are "safe" more conservative algorithms will be "safer" than more liberal ones. That is something to really think about with aggressive multi-day deep or long exposures. If any circumstances merited a more conservative approach, those would.

While, intuitively, that would seem to make sense, it is like saying that one negligibly small quantity is larger than another negligibly small quantity. That is, they are not different once you put it in the category of negligibly small.

Those who are more comfortable with more conservative computer should use the computers they trust. But when you see some of those divers who swear by their more conservative computers talking about accepting "light deco" so they can stay down with other divers, then any perceived safety margin may be lost.

 

[drrich2]

A point that's come up elsewhere is that some boat op.s may 'bench' you for the rest of the day if your computer claims you are in deco., and given the alarms some have, it may be apt to draw attention to itself.

A more liberal computer does not require you to stay down longer; you don't have to push the NDL's. But if you choose to, it's less likely to claim you owe deco., which could make you less likely to have to sit out some dives.

Richard.

 

[guyharrisonphoto]

Richard, a very good practical point. But, if I go into deco on my computer and I make the stops per the computer, the computer clears me and there is no alarm or other indication once I am out of the water, except in the dive log. I think many computers work that way. The alarm is only triggered, and cannot be shut off, if you do not follow the deco stops, so that the computer algorithm shows you as "bent" when you reach the surface. In that case, I would not be doing the next dive anyway, and might have even worse problems to deal with.

Awap, yes, this is what is interesting about the issue. If DSAT is indeed just as safe as other algorithms based on current statistics, then maybe I am being more conservative than necessary. I would never consider going into "light deco" on my Z+ computer just to match a more liberal computer. But, maybe I am unnecessarily penalizing myself by not going DSAT in the first place?

I think I will start a new thread on whether there is statistical or medical evidence regarding the effectiveness of the different algorithms.

---------- Post added February 7th, 2014 at 01:06 PM ----------

I started a thread in the Advanced forum regarding algorithms.

 

[Subboy]

Awap, yes, this is what is interesting about the issue. If DSAT is indeed just as safe as other algorithms based on current statistics, then maybe I am being more conservative than necessary. I would never consider going into "light deco" on my Z+ computer just to match a more liberal computer. But, maybe I am unnecessarily penalizing myself by not going DSAT in the first place?

I think I will start a new thread on whether there is statistical or medical evidence regarding the effectiveness of the different algorithms.

---------- Post added February 7th, 2014 at 01:06 PM ----------

I started a thread in the Advanced forum regarding algorithms.

That is exactly what I was talking about. I am not a medical doctor, so can not argue for one or another, but will chose the one that, being reasonably safe, give me more bottom time

 

[Barefootdog]

Subscribing...

Very good thread.

 

[charlier]

If diving liveaboards the most conservative computer will likely call the dive, assuming sufficient air. Suuntos, considered conservative, are very common computers and often worn by DMs particularly in SEA. I've done numerous liveaboards using a Suunto without issue. Oceanic and Aeris, there may be others, have dual algorythms one more conservative and one more liberal. To be honest I wouldn't get too caught up in liberal vs conservative there are other more important factors that will decide when a dive is over when diving in groups and that's almost always air, or lack of, related. I'm not as young or fit as I used to be so am ok erring on the side of caution and diving a little more conservatively. Decompression sickness is not an exact science by any means and skirting the limits i.e. diving aggressively multiple time during multiple consecutive days, may well be asking for trouble. I think the current deal being offered on the DG03 is a good one. Good luck with your search.

Buddhasummer makes a good point. In SE Asia, good cruise directors will not want you to exceed or push your NDL's. In particular in remote areas, medical help is not easy to find. Many SE Asia guides or divemasters use Suunto and an assortment of other computers and it seems to me that the rate limiting step is usually SAC. Many good guides will not exceed ~30m depth for many reasons - mostly due to gas consumption and possible deco obligations. Mark Powell's Deco for Divers is a good book that also provides understanding of what is known and not known about DCS. Using standard gases (e.g., Nitrox32) and simple guidelines of min deco ascent rates will help mitigate your DCS risk. During liveaboard trips, my wife and I discuss always discuss dive plans, so if we do go to deep recreational depths, we have a solid ascent plan that will minimize our risk.