Algorithms, Conservative Factors, Altitude, Planned Deco - Questions

[Mike1967]

which is okay but not as special as people on here seem to think

Sounds like you have your nose out of joint because you couldn't get one. I have the Perdix ai and there's no question how "special" they are, maybe if you got one you would agree with everyone from all over the world that has one.

 

[CandiveOz]

Hi Mike1967,
I'm not sure if Diving Dubai's nose is out of joint. If you carefully reread his post, you will realise he actually uses a Shearwater as a backup.

From someone who uses an Eon, I can say that just isn't true, I can back that up by the fact that I run a Shearwater perdix as a back up. I regularly dive multi day repetitive diving (monthly at least) With the Eon at -2 and the Perdix at 45/95 (both the most liberal aggressive setting) they are pretty much identical by the amount of Stop time they give and when they both enter deco. Their deco times are near identical too

I personally would have preferred an OSTC to the Perdix (which is okay but not as special as people on here seem to think)

He then goes on to say in a following post...

I'm not saying its a bad machine, and Shearwater have a reputation for first class customer service, often going above and beyond what is expected in the US.

There is no denying that the Shearwater is a very popular dive computer; its just that people have their own preferences to meet their diving requirements. @scubadada for example, dives with an Oceanic because he likes the DSAT algorithm. He understands it, he is comfortable with it and its suitable for the diving he does. And because of his enthusiasm (like yours for Shearwater) I've learned more about the unique attributes of the Oceanic's dual algorithm offering and appreciate the two alternatives it offers. Its not a question of what is better or worse (you are either a Ford or Holden man) my aim in my original post is to gain a better understanding of the dive computers on the market today. This will help me make a better informed decision when I upgrade.

 

[Diving Dubai]

Sounds like you have your nose out of joint because you couldn't get one. I have the Perdix ai and there's no question how "special" they are, maybe if you got one you would agree with everyone from all over the world that has one.

As @CandiveOz pointed out, I do own and use Perdix.

I will admit, I'm a new user. I've only had it for 4 months and only put around 120 dives on it. It's also not been used for any onerous dives. Max 50m and 2 gas switch dives.

However with my "limited" experience I still stand by my comments. It is a fine computer, but it's not the be all and end all. In some ways it's lacking behind my primary machine.

I'm happy to hear your comments based upon your vast experience in diving and using other computers.

 

[CandiveOz]

Why does Scubapro believe increased work load would affect NDL? I understand that gentle exercise might be helpful during deco stops.

Workload at the bottom w/ no exercise at deco stop does the opposite of gentle exercise at the stop and no work at the bottom. Presumably they've programmed for the former.

I like to use the analogy of an airbag in a vehicle. Your car has an airbag. Under normal driving conditions it does nothing. Even if you bump into someone, it does nothing. But get into a major accident; it deploys.

I assume the workload safety feature is like that. Under normal diving conditions your computer does nothing; it functions according to plan. Even with a little increase in workload it does nothing. But if your heart rate starts pumping and you are suddenly breathing heavy due to a strong current, the workload safety feature triggers and the computer shortens the NDL or increases decompression obligation.

The main difference between the airbag versus workload is that the automobile manufacturer has set the tolerance of the airbag to only deploy during a major crash. The dive computer, on the other hand, has to be set by the actual user. Therefore it's imperative to know and set your maximum heart rate based on your swimming/diving (your running rate is too high and not applicable) and adjust the breathing rate sensitivity monitor on the dive computer. The Scubapro/Uwatec has the ability to be set to use only heart rate or to use only breathing rate or use a combination of both.

 

[rongoodman]

But if your heart rate starts pumping and you are suddenly breathing heavy due to a strong current, the workload safety feature triggers and the computer shortens the NDL or increases decompression obligation.

Yes, but why would increased workload cause increased N2 uptake or require a shorter NDL? The amount of inert gas in the inspired breathing mixture doesn't change, so the rate of diffusion remains the same. Are they arguing that increased tissue perfusion secondary to the increased heart rate is the problem? (The role exercise might play in bubble formation is a different issue.)

 

[KenGordon]

Yes, but why would increased workload cause increased N2 uptake or require a shorter NDL? The amount of inert gas in the inspired breathing mixture doesn't change, so the rate of diffusion remains the same. Are they arguing that increased tissue perfusion secondary to the increased heart rate is the problem? (The role exercise might play in bubble formation is a different issue.)

Yes.

see Algorithms, Conservative Factors, Altitude, Planned Deco - Questions and then maybe the original discussion I quote there.

 

[CandiveOz]

@KenGordon
You quote @David Doolette . Alert Diver has a good profile on David which uses a Q/A approach to summarise his views on dive computers, DCS risk factors including workload and temperature which has been the topic of our discussion. It's a quick read and good summary for those interested.
Alert Diver | Researcher Profile: David Doolette

 

[CandiveOz]

Unfortunately the article did not describe by what measure did the workload alert impact the NDL or decompression profile or if there was a delay to allow the diver to reduce his workload before any changes kick in. This specific question remains unanswered.

I wonder if emailing customerservice@scubapro.com might be a way to answer the question if the SP OZ doesn't know?

I did contact Scubapro and they responded by email from the US and I also received a call from one of their technicians in Australia. I have to admit, their customer service response was excellent. The answer to my question; “By what measure or amount does the increased workload impact the NDL or decompression obligation profile” is difficult to answer because by its nature the Scubapro/Uwatec Buhlmann algorithm is “real time adaptive” and responds to a number of variables during a dive other than just depth and time. It takes into consideration temperature, accent rate, breathing rate and heart rate in addition to depth, time, surface interval, nitrogen load from a previous dive etc.

The underlying concern with my question was; due to its adaptive nature, would the computer, for example, ever put a diver into a position where his/her air supply would not support the deco obligation generated based on a sudden increase in workload? This was the real crux to my question. You don’t want any computer to suddenly and arbitrarily change your deco obligation to exceed your gas supply. The short answer is no; the computer will not do that because it calculates RBT (Remaining Bottom Time).

All the variables listed above dovetail into this one number; RBT. Unlike Shearwater’s GTR (Gas Time Remaining), RBT takes everything into consideration and when it hits zero, you still have enough gas to complete all stop obligations and get to the surface with your tank reserve remaining:

The RBT (Remaining Bottom Time) is the time you can spend at the current depth and still have enough gas supply to make a safe ascent and reach the surface with the tank reserve. The RBT calculation is based on your current breathing rate, and it accounts for any existing and upcoming decompression obligation as well as for any temperature gradient in the water. It assumes an ascent at the ideal ascent rate (defined in chapter: Ascent rate). When the RBT reaches 3 minutes a warning is shown. (G2 User manual; page 34).

So in a nutshell, when the computer recalculates the diver’s deco obligation due to an increase in workload, it may hit the 3 minute RBT warning. If ignored, it will then hit the RBT zero alarm. So the diver is never at risk of safely ascending to the surface based on the computer.

In terms of actually quantifying the impact, it actually may not be that dramatic. I found in Asser Salama’s 2nd edition; Deep Into Deco where the author created an algorithm called; ZH-L16D based on the same principles of the Scubapro/Uwatec Buhlmann adaptive program.

Based on his personal communication with Buhlmann and Manx Hahn, my friend Dr. Albrecht Salm gave me some details about which ZH-L16 halftimes would need to be altered to reflect cold and/or an increased workload. Ultimate Planner introduced ZH-L16D, a more conservative model than both ZH-L16B and ZH-L16C that generates more suitable schedules for unfit divers or for anticipated colder and/or more demanding dives. For example, assuming the last stop depth is 6 metres (20 feet), the total run time of a 30 minute dive on air to 45 meter (150-foot) depth would be 73 minutes (ZH-L16B), 84 minutes (ZH-L16C), or 87 minutes (ZH-L16D). (page 23)

In the above example, there is only a 3 minute difference between ZH-L16C and D.

 

[CandiveOz]

I would like to take this opportunity to summarise what I learned from the responses I received from my original post regarding the 4 groups of computers I identified and had questions for. First I would like to thank everyone for taking the time to answer my original questions. I now have a greater understanding and appreciation for the dive computers in the market today.

Oceanic and Pelagic Pressure Systems based computers;
One thing I learned is that decompression tables/algorithms are very “dive type” specific. It is not necessarily true that one algorithm fits all dive types. A good example to highlight this fact is Oceanic and its dual algorithm option. The developers at DSAT focused on multilevel, multiday, NDL dives. This was the basis of the PADI dive tables. As a result, if a diver stays within these limits, no decompression dives can be longer than say another algorithm designed for a single square profile deep dive. To cater for these two different types of diving, Oceanic came up with two algorithms; DSAT and PZ+. I believe it is the only computer to offer this option and you could say it’s it competitive differentiation. My original question was why Oceanic didn’t offer software to determine decompression profiles. @scubadada explained that these computers were geared towards the multilevel multiday diver rather than the deep technical diver and as such the algorithm’s background testing didn’t focus on deep decompression dives. You may have noticed that I didn’t use the word “recreational” to describe Oceanic computers. I would like to get away from the notion that to be considered a “serious” diver you should only dive with a trimix tech dive computer rather than a “recreational” dive computer. In fact, you may be selling yourself short if you use the tech computer on multilevel multiday dives. Use the best tool for the job. Since DSAT has been specifically tested for multilevel, multiday, short surface interval, NDL dives, it can offer more liberal no decompression dives than other manufactures who try to cover a wider spectrum of diving.

Simply Scuba provides an excellent summary with the following link.
Oceanic Dual Algorithm

 

[CandiveOz]

Suunto and RGBM based computers
Suunto incorporates the RGBM algorithm which was developed to minimise microbubble formation. Unlike other scuba manufactures, Suunto specialises only in timing devices and gauges, so resources aren’t distracted in designing new fins, regulators or BCD’s. These computers are quite popular in Australia where I live but there seems to be a love/hate relationship on Scubaboard. People claim that they are overly conservative but that is not what I have observed; other brands can be even more conservative based on the type of dive profile. Thanks to divers like @KenGordon and @Diving Dubai who take the time to address any misconceptions others may have about the Suuntos.

Over the years, Suunto has refined its formula into 3 different versions; RGBM, Tech and Fused. With the latter two, Suunto has offered both liberal and conservative settings. And although the algorithm is proprietary, using its software DM5, one can easily plan decompression dives, change the parameters (deep stops on/off) and compare the different algorithm versions. In my opinion Suunto has addressed any issue I have regarding their computers. RGBM may be proprietary but it’s not in a black box. I wouldn’t have any hesitation in owning or using a Suunto for my diving.

My main question regarding Suunto was to determine if the conservative factors were interchangeable with the altitude settings. (ie is P0A1 = P1A0). Using the standard RGBM the answer is yes. But further investigation (again using DM5) I discovered that this is not the case with the other versions.