Purchasing your first dive computer. A non-technical approach.

[dmaziuk]

If your ambient pressure is ever 0, you probably don't need to worry about whether your dive computer is going to lock you out.

Yes, precisely. I assume you know perfectly well that any subroutine operates over a finite range of inputs.100 atmospheres might be another ambient pressure where the computer says bye-bye. And when it's theoretically infinite, it's limited by the hardware. E.g. a deco calculating subroutine may have your gas loading not fit into 8 bits, or however many there are in a given device.

You have calculated that the user is bent and needs to go to a chamber. Now they're outside of the design/operational parameters for your deco subroutine, so pop quiz: what do you do? -- Theoretically you could try to program in in-water recompression I guess... but without any handle on what the user's symptoms actually are, I don't really see how much use that could be.

 

[dmaziuk]

My anecdotal experience is that people who steadfastly claim that the algorithm really doesn't matter are either saying that it doesn't matter for "other" people, or they are saying that it hasn't been an issue for them, but it turns out they fall into the category of mostly diving an AL80 in relatively shallow water. As opposed to multi-day, multi-dive days using a 100 or larger and diving often past 100'.

Oi! I resemble that remark! (The shallow Al80 dives, that is.)

If you're looking for a first computer and you're dead set on being in a cave course a year from now, I'll be the first to say "buy a shearwater".

 

[dmaziuk]

According to the manual, the Zoop will add a mandatory stop if your ascent rate even momentarily exceeds 12 meters/min and, if you violate that stop, it will penalize your NDL on the next dive. So you're flailing that is not really an actual ascending could cost you NDL - No thanks.

RGBM computers tend to penalize you for skipped safety stops in general, so don't violate the stop. I also wonder what their definition of "momentarily" is because it has to "smooth out" the readings -- this is the "flailing" part where raising computer hand while coming up causes "momentary" ascent rate violation that is clearly a false positive.

 

[scubadada]

Which is probably in part because many of them never dive anything but an AL80 and also generally do mostly relatively shallow dives, where NDLs are quite long.

My anecdotal experience is that people who steadfastly claim that the algorithm really doesn't matter are either saying that it doesn't matter for "other" people, or they are saying that it hasn't been an issue for them, but it turns out they fall into the category of mostly diving an AL80 in relatively shallow water. As opposed to multi-day, multi-dive days using a 100 or larger and diving often past 100'.

Stuart"s comments are not wrong, but they are incomplete. I have been diving a long time and have an RMV of a bit less than 0.4 cf/min. An AL 80 is perfectly sufficient for all my dives in Jupiter, West Palm, and Boynton Beach, FL. Sometimes I run out of gas, sometimes I am limited by NDL. But, I am never limited by a NDL dictated by an overly conservative RGBM-based decompression algorithm. The same applies to proprietary algorithms based on
Buhlmann ZH-L16C like PZ+ or the very conservative Deepblu Cosmic.Dive whatever computer you want, but be aware that there are major differences in the very safe decompression algorithms. All commercially available decompression algorithms have shorter NDLs compared with the Navy NDLs.

 

[Lorenzoid]

Many, many divers never get past their gas consumption being the main limiting factor in their run times. NDLs never play a roll in their diving. Others of us are concerned about reasonable, predicable NDLs for our dive planning.

As I know you are using a Shearwater computer, I would be interested in hearing what GF settings you are using for recreational dives.

You're absolutely right. Along the lines of what Stuart pointed out, there are an awful lot of divers out there doing 50-minute DM-led dives with Al 80s on reefs that are relatively shallow, like 60 to 80 ft. max, with the average depth of the dive being way shallower than that. Reading SB, one might get the impression that most divers own their own steel tanks, dive on deep wrecks, spearfish, etc. I simply pointed out that Suuntos are pretty ubiquitous out there in the world of vacation divers. They are certainly not for everyone, but they may be fine for a whole lot of divers. A whole lot of divers seem to have no issue with them. Suunto sells a lot of computers.

I am using 40/85 on my Shearwater, I believe. And I think if I were still using my old Suunto with the dives I am increasingly doing, I might very well run up against NDLs sometimes. For example, in Cozumel I have taken to diving those steel 120s that Aldora offers. But I spent many years blissfully doing the kind of easy vacation dives I mentioned above and rarely felt my dive was cut short because of the Suunto's NDLs. If someone sees himself as that kind of diver, then Suunto's algorithm needn't be a consideration.

 

[scubadada]

You're absolutely right. Along the lines of what Stuart pointed out, there are an awful lot of divers out there doing 50-minute DM-led dives with Al 80s on reefs that are relatively shallow, like 60 to 80 ft. max, with the average depth of the dive being way shallower than that. Reading SB, one might get the impression that most divers own their own steel tanks, dive on deep wrecks, spearfish, etc. I simply pointed out that Suuntos are pretty ubiquitous out there in the world of vacation divers. They are certainly not for everyone, but they may be fine for a whole lot of divers. A whole lot of divers seem to have no issue with them. Suunto sells a lot of computers.

I am using 40/85 on my Shearwater, I believe. And I think if I were still using my old Suunto with the dives I am increasingly doing, I might very well run up against NDLs sometimes. For example, in Cozumel I have taken to diving those steel 120s that Aldora offers. But I spent many years blissfully doing the kind of easy vacation dives I mentioned above and rarely felt my dive was cut short because of the Suunto's NDLs. If someone sees himself as that kind of diver, then Suunto's algorithm needn't be a consideration.

Thank you very much for your reply. 40/85 is pretty similar to Oceanic PZ+. 45/95 is more similar to DSAT. The mapping is not perfect and is somewhat depth dependent. I have been diving a Dive Rite Nitek Q as backup for about the last 110 dives to get more experience with Buhlmann ZH-L16C and DSAT.

When my Oceanic VT3 finally dies, I will likely migrate to a Perdix AI or some other similar computer that is available at the time.

Good diving, Craig

 

[Joneill]

RGBM computers tend to penalize you for skipped safety stops in general, so don't violate the stop. I also wonder what their definition of "momentarily" is because it has to "smooth out" the readings -- this is the "flailing" part where raising computer hand while coming up causes "momentary" ascent rate violation that is clearly a false positive.

Right - but if your flailing caused a mandatory stop that, for some reason, you did not complete - you end up getting penalized on NDL on the next dives for a false alarm.

After 3 days of diving on Air recently, I had to watch NDL on deeper dives the 4th day. For example, on this one, I got distracted and down to 1 min before I ascended (and it hit 0 as I ascended, but quickly went positive). Lesson learned for me.

Same on this dive 4 days into our GC trip last year - was down to 5 minutes and ended up surfacing with lots of air as I could not stay on the reef as it was too deep for my NDL (and not much fun hanging way above the bottom).

 

[gfaith]

RGBM computers tend to penalize you for skipped safety stops in general, so don't violate the stop. I also wonder what their definition of "momentarily" is because it has to "smooth out" the readings -- this is the "flailing" part where raising computer hand while coming up causes "momentary" ascent rate violation that is clearly a false positive.

I call misinformation or "BS" you pick. Quote from zoop manual.

3.2.1. Recommended Safety Stop
With every dive over 10 meters the instrument has a three minute countdown for
the recommended safety stop, to be taken in the 3 - 6 meter [10 ft - 20 ft] range.
This is shown with the STOP sign and a three-minute countdown in the center
window instead of the no-decompression time (Fig. 3.9.).
The Recommended Safety Stop, as the name implies, is recommended. If it is
ignored, there is no penalty applied to the following surface intervals and dives.

 

[Joneill]

I call misinformation or "BS" you pick. Quote from zoop manual.

3.2.1. Recommended Safety Stop
With every dive over 10 meters the instrument has a three minute countdown for
the recommended safety stop, to be taken in the 3 - 6 meter [10 ft - 20 ft] range.
This is shown with the STOP sign and a three-minute countdown in the center
window instead of the no-decompression time (Fig. 3.9.).
The Recommended Safety Stop, as the name implies, is recommended. If it is
ignored, there is no penalty applied to the following surface intervals and dives.

Read the next section, 3.2.2 Mandatory Safety Stop: "...When the ascent rate exceeds 12 meters/min [40 ft] momentarily or 10 meters/min [33 ft] continuously the micro-bubble build-up is predicted to be more than allowed for in the decompression model. The Suunto RGBM calculation model responds to this by adding a Mandatory Safety Stop to the dive. The time of this Mandatory Safety Stop will depend on the severity of the ascent rate excess..."

"...If you continue to violate the Mandatory Safety Stop, the tissue calculation model is affected and the dive computer shortens the available no-decompression time for your next dive."

 

[gfaith]

I understand that. I have exceeded ascent rate for a short time and had a 1 minute Mandatory Stop, but that was not what he said .. It is still BS..