Computers and backups - looking for pearls of wisdom from the more experienced

[scubadada]

...If you want to be a nerd look at page 5. If you want to be a diver who’s not thinking “hmm, is the deep and long enough for 50/80 or will 50/85 be ok?” see page 8...

Hi @KenGordon

Are your page numbers referencing the PDF link discussing Fused RGBM?

I've always kind of liked this document because, on page 8, it shows the potential penalty for violating certain RGBM principles. I've always wondered how multiple violations work, are they additive in the M-value reduction or what? Most discussions of RGBM do not discuss these specific reductions at all.

So, what is the M-value decrease after a 60 min SI for a repetitive dive, following a reverse profile, on day 5 of a 7-day liveaboard trip? What if you also had a somewhat faster than normal ascent? If additive, it looks like the penalty might be >37%. I've always wondered. I wrote Suunto a number of times but did not get a reply.

 

[rongoodman]

I see lots of marketing speak describing what remain, at the end of the day, proprietary algorithms.

 

[KenGordon]

Abacus and tables backup issue solved with Deep6 Excursion. Looking forward to its full tech firmware.

Now I just need to get the Shearwater primary, but its not yet a pressing enough need. Though this is now the second 'backup' I've bought for my 'future' Shearwater. But both good choices I think.

I'll buy that different algorithms work just fine. But the prevalence of GF is enough of a tilt that way for me.

I’d rather have the abacus, two even.

 

[MichaelMc]

I’d rather have the abacus, two even.

Hmm. That seems a strong statement coming from roughly "Shearwater are not the only computers, others work just fine as well." I'm not a cross computer (abacus) expert, so I'll allow any experts to pursue that.

 

[boulderjohn]

I watched a DAN hour long presentation from an Anaesthesiologist who is also a tech diver. I'm sure you know more than he does.

Dr. Simon Mitchell is certainly one of the most reputable people in the field.

One caution that I am surprised was not mentioned (unless I missed it) is that the video is about decompression controversies on decompression dives, not NDL dives. Some people will insist that there is no difference, but there is a significant difference. Simon gave me considerable help for my own article, and in our discussions, he made it clear that he sees a big difference. In fact, he would not help me on my planned (then) article on NDL dives because he said there is not enough good evidence out there on which to form an opinion, and he had not formed one.

Will adding deeper stops or spending time deeper during ascent hurt you the way it apparently does on decompression dives? There just isn't enough solid information about that out there. Does it help you? Maybe. We just don't know.

In NDL diving, the GF low number does not matter. The GF high number is all you have to worry about. If you use a Shearwater, it has a feature called SurfGF, which is a very nice way of telling you what your GF would be if you surfaced right then. When I am doing NDL dives, I use that to tell me when to end my safety stop most of the time, except when I am with friends and have to wait for them to count off their 3 minutes.

 

[boulderjohn]

Of course any backup to your DC is using the PADI RDP tables and planning using that if you have to. You do not need to rely on a computer as a backup when you can just use the RDP.

As has been noted repeatedly in the past, if you are doing a series of multi-level dives, it is very likely that you have violated the table limits, so using tables as a backup for later dives is not a viable option.

 

[nanohawk]

Dr. Simon Mitchell is certainly one of the most reputable people in the field.

I thought he had a great presentation. His video came up when I was looking for something that explained the difference between bubble and gas tissue models. While I realize the info isn't directly applicable to NDL I still found it interesting and informative.

In NDL diving, the GF low number does not matter. The GF high number is all you have to worry about. If you use a Shearwater, it has a feature called SurfGF, which is a very nice way of telling you what your GF would be if you surfaced right then. When I am doing NDL dives, I use that to tell me when to end my safety stop most of the time, except when I am with friends and have to wait for them to count off their 3 minutes.

SurfGF sounds like a great feature. I will look it up when I get the computer out of the box and start setting it up.

 

[scubadada]

...In NDL diving, the GF low number does not matter. The GF high number is all you have to worry about. If you use a Shearwater, it has a feature called SurfGF, which is a very nice way of telling you what your GF would be if you surfaced right then. When I am doing NDL dives, I use that to tell me when to end my safety stop most of the time, except when I am with friends and have to wait for them to count off their 3 minutes.

The GF high does not take a safety stop into consideration. When I asked Shearwater this question, they told me it is because the safety stop is not required. So, if you set your GF high at 85, leave depth right when you are up against your NDL and do a normal ascent directly to the surface, you will be close to the GF high of 85. Any safety stop will decrease your surfacing GF to a value below your GF high. When you log your dive to the Shearwater Cloud, you can see your surfacing GF for any dive under computer summary:

As @boulderjohn pointed out, SurfGF reflects instantaneous transport to the surface. When you follow Surf GF at your safety stop or last deco stop, it will decrease further during your final ascent. I make sure my last 15 feet of ascent are over at least a minute. The actual GF, GF99, increases very significantly over this final ascent:

 

[KenGordon]

Hi @KenGordon

Are your page numbers referencing the PDF link discussing Fused RGBM?

I've always kind of liked this document because, on page 8, it shows the potential penalty for violating certain RGBM principles. I've always wondered how multiple violations work, are they additive in the M-value reduction or what? Most discussions of RGBM do not discuss these specific reductions at all.

So, what is the M-value decrease after a 60 min SI for a repetitive dive, following a reverse profile, on day 5 of a 7-day liveaboard trip? What if you also had a somewhat faster than normal ascent? If additive, it looks like the penalty might be >37%. I've always wondered. I wrote Suunto a number of times but did not get a reply.

Yes, it looks like about 25% to my eye but really it doesn’t matter exactly, it is just more conservative than called for by the raw dissolved gas model. We teach people to avoid short surface intervals. This is good practice. It might not suit boat captains in Cozumel but longer SIs are a good thing. Similarly taking it easy if doing a load of days. The pure dissolved gas models just have the slow compartments which really do next to nothing.

Hmm. That seems a strong statement coming from roughly "Shearwater are not the only computers, others work just fine as well." I'm not a cross computer (abacus) expert, so I'll allow any experts to pursue that.

I have a poor opinion of the makers of the Excursion. I think the algorithms are secondary to the rest of the user experience. That particular computer has a crazy UI and the company selling it just made a rubbish excuse of it not being a technical computer. Compare that with basically all the other single gas computers and how they behave (sensibly) and I have to doubt their judgement. Their USP is cheap ZHL16 GF but ZHL16 GF is really for deco dives so this just illustrates how silly this obsession with algorithms is.

I know I can execute dives with a slate and two timers. I would much rather have buddies with two computers than one and a timer as that forces us to the limits of their slate So it is likely we will be ascending earlier than necessary.

 

[scubadada]

Yes, it looks like about 25% to my eye but really it doesn’t matter exactly, it is just more conservative than called for by the raw dissolved gas model. We teach people to avoid short surface intervals. This is good practice. It might not suit boat captains in Cozumel but longer SIs are a good thing. Similarly taking it easy if doing a load of days. The pure dissolved gas models just have the slow compartments which really do next to nothing...

In my experience, the SIs in Cozumel have been relatively long. My last trip with Aldora had an average SI of 1:40. On the other hand, my SIs in SE FL probably average about 45 min, sometimes as short as a half hour. Of course, it depends on your next dive. I nearly always use my NDL planner to ensure I have enough bottom time. I'm sure these habits are geographical, yours are likely quite different.