Dive computers... SO many choices!

[stuartv]

Do you have any idea of roughly what kind of gradients the Oceanic/Aeris run in their Z+ alg?

I do not, I just know it's "more conservative" and I have always been comfortable using DSAT on my Atom.

FWIW, I now pretty much only dive tech computers and use them set to GF 50/80. But, that is partly because I am now trained and certified for deco diving. As such, if I'm out on a recreational charter, I will stay down as long as the NDL on my Atom (i.e. as long as ROUGHLY GF 95/95 would allow), but then do the stops dictated by my tech computer on the way up.

If I were not certified for deco, then I would still be just using my Atom on DSAT and doing a minimum of 5 minutes at the safety stop for any dive deeper than 80 or so feet (if I was down anywhere close to the NDL). I don't have any science to back that up. It just seems like a reasonable precaution, if I have the gas to do it, after doing a deep dive to the NDL of a computer with an algorithm that is known to be very liberal. I like to maximize my bottom time, but there is no reason whatsoever to skimp on safety stop hang time.

Likewise, if I were not certified for deco, but using a computer with GF, I would probably set it for something like GF 75/95, or maybe GF 80/90, and stay down to the NDL, but then do at least 5 minutes at the safety stop (for deeper dives, near the NDL).

All that said, as with anything in scuba, I would start with any computer by being conservative (i.e. not push it to its NDL) and gradually work my way up to using its more aggressive settings and staying down to the NDL.

 

[LandonL]

I think having presets that match Shearwater OC Rec mode is a good idea.

I dive a shearwater for my tech computer at the moment, so I like that idea as well. lol

But, I am unconvinced that it's a problem to give recreational divers the ability to do custom settings. If you simply limited GF Hi to a range of 60 - 95, and, of course, required that GF Lo cannot be set higher than GF Hi, then the most aggressive setting anyone could use is GF95/95, which would make it about the same as an Oceanic or Aeris computer running DSAT. I.e. not "dangerous" for a recreational diver. Shearwater allows a max GF Hi of 99, but I definitely don't see anything wrong with a rec computer being limited to a max of GF 95.

And a min GF Hi of GF 60 (or thereabouts) would keep the diver from setting it so low that they end up with some ridiculous amount of deco obligation by accident (e.g. if they set GF Hi to 40 and then stayed down with a buddy to the NDL of the buddy's computer).

I was speaking more of giving them 0-100 range, which you pretty much summed up why I wouldn't above. Limiting the range to a subset of the total would cover your scenarios, but would be confusing for most recreational divers, as they wouldn't really understand how to use them.

I think for a simple rec computer, having something easy like "0, 1, 2, 3" will be a bit more straight forward for use.

 

[LandonL]

While it's a little off-topic, I'd love to hear what you guys think is the safest gradient set for rec diving. I don't care about aggressive times. I'm a little unsure given your 3 sets because according to that lecture a higher "high gradient" is better, but of the choices given, it also means that the "lo gradient" also ends up a lot higher, which would mean more super-saturation at the surface!

I understand the hi gradient pretty clearly, but don't understand all of the limitations around the lo gradient.

The best way to think of them is that GF Low defines the first deco stop, while GF High defines the surfacing value.

Increase the GF Low - and you get a shallower depth for the first deco stop, decrease the GF Low and your first deco stop would be deeper. If you were to say set your GF low at 30%, decompression starts when the inert gas partial pressure in the leading tissue compartment reaches 30% of the of the way between Ambient Pressure line and M-value line.

Similar principle applies to the GF High - you are effectively selecting a saturation level for the surface. It is basically the NDL.

Throughout the dive, the gradient factor actually shifts along the line between the two points. Because the leading compartment, depth of stops, time at each stop, and other variables can change, two different sets of GF values could result in the same total run time, whereas settings in between could be longer than both.

Matti Antilla wrote a pretty good article covering it all fairly well, search online and you can find it, its probably the best detailed explanation I can think of.

Now to your question - technically the 'safest' GF setting would be lowest numbers that you are willing to dive with, as this would keep your tissue saturation minimal throughout the dive and at the surface, although using incredibly low values for both would create some ridiculous dive profiles. Picking GF values is more about picking what you are comfortable diving with. It's all about your comfort level in relation to the M-line.

 

[stuartv]

Increase the GF Low - increases the tissue saturation you are willing to take on before beginning your ascent, because you are allowing yourself to get closer to the M-line.

Well... technically, the GF High determines when you begin your ascent (if you are observing NDLs). GF Lo just determines where your first stop is once you've gone into deco and you decide when to start your ascent.

Don't believe me? Fire up the dive planner on your Shearwater and set your GF to 99/99 then plan a dive to 130' for 1 minute. It will tell you what your NDL is. Then change the GF to 30/99. You will see that you still have the same NDL - because the GF Hi determines when you have to begin your ascent.

If you go into deco, then you decide when to begin your ascent, not the computer, and the GF Lo just determines where the first stop is.

 

[scubadada]

Thanks @LandonL

I agree with @stuartv, running the same GF presets as Shearwater Rec is easily the best choice for their 1st computer and beats a proprietary RGBM hands down.

 

[BoltSnap]

Entire screen layout:
View attachment 397227
Dive mode example, currently doing a stop:

View attachment 397228

What are the + and - signs on the left and right of the center line? What does the "OK" below these sign mean?

In fig "18" What does the entire line numbers and icon mean (starting with 30FT)?

 

[stuartv]

I dive a shearwater for my tech computer at the moment, so I like that idea as well. lol

I was speaking more of giving them 0-100 range, which you pretty much summed up why I wouldn't above. Limiting the range to a subset of the total would cover your scenarios, but would be confusing for most recreational divers, as they wouldn't really understand how to use them.

I think for a simple rec computer, having something easy like "0, 1, 2, 3" will be a bit more straight forward for use.

Giving them presets and a way to set custom (with intelligent range limits) and a good paragraph in the manual explaining GF seems like it would be very easy to understand. Recreational divers are not ALL morons...

 

[LandonL]

What are the + and - signs on the left and right of the center line? What does the "OK" below these sign mean?

- means left/down
+ means up/right
ok means ok/enter (press both together)

The function is determined by the screen you are on, such as cycling through options in a menu, or setting the O2 percentage in nitrox.

In fig "18" What does the entire line numbers and icon mean (starting with 30FT)?

Your stop is at 30ft, the up/down arrows pointing at each other means get to 30ft and stop here/stay here, for 120 seconds.

 

[LandonL]

Giving them presets and a way to set custom (with intelligent range limits) and a good paragraph in the manual explaining GF seems like it would be very easy to understand. Recreational divers are not ALL morons...

I disagree on the part about it being easy to understand..... I think I would be fielding tons of calls and emails asking for more explanation, help me, why couldn't we make it easier, etc. lol

I don't think all rec divers are morons, I just think its excessive overkill for a $140 single gas computer to have over 1000 possible conservatism settings...

We could have it do trimix, and be multi-gas too, but that isn't exactly the intention with this model, as we really are trying to have a simple computer for recreational divers.

 

[BoltSnap]

- means left/down
+ means up/right
ok means ok/enter (press both together)

This is referring to the left and right buttons?