Are some computers unsafe?

[t-mac]

I am writing this based on my experience with several different kinds of computers and their levels of conservatism, plus the reactions I've seen on the part of some SB users to degrees of conservatism. Recently I've used a Suunto Gekko, a Mares Puck and an Oceanic OC1 (DSAT) and have been able to compare the allowable bottom time with basic settings (no conservatism adjustments) and I see a pretty dramatic range. The Gekko is way at one end and the OC1 is at the other end with the Mares somewhere in the middle. Recently on the second of 2 approximately 60-foot dives on air, as the Gekko ran up against the no deco limit, the Mares was still in the upper single digits and the OC1 had about 20 minutes. I knew there were differences, but this seemed much more extreme than I expected.

The Suuntos are reputed to be very conservative and in my experience in using the Gekko it is probably at the outer edges of the range -- whenever bottom time has become short for me, it's the Gekko that is limiting. I've seen a number of people criticize this conservatism and there are also staunch defenders of it. Of course, all of the algorithms are based on models supported by the same sets of data and so it seems to me that there is no empirical evidence that one is safer than another.

So, my question for discussion is whether the proponents of conservatism believe that some computers are so liberal as to be unsafe? I've seen the analogy of walking up to a cliff, but how far back do you really need to be? Am I being reckless by leaving the Gekko at home and following the OC1 because it "gives" me longer dives?

 

[beaverdivers]

Dive Computer today are on the +1 or more side of being conservative.

The EDGE ( cir 1983 ) was a -5 as was the Navy Dive tables.

 

[Mr Carcharodon]

Every dive you make a trade off between going diving and being safe. The DSAT models sometime give you 20 or even 30 minutes more of dive time than RGBM, at least for some dives, with a chance of being bent of 1:2000. If there is a measurable improvement in safety using RGBM I do not know what it is. VPM was updated to make it more conservative, RGBM may need to swing the other way. Of course there will be a lot of people who will say it is wrong, if not immoral, to disagree with the magic box on your wrist even if the answers they give are all over the map.

 

[KD8NPB]

Cochran EMC-20H on "0" is equal to the Navy Tables.

I dive mine at +25.

 

[supergaijin]

I believe computers are designed with a 'type' of diving in mind. I have owned Suunto and the Aladdin Pro.

The typical Suunto is designed IMO for recreational divers who do repetitive long shallow dives over multiple days.

My old Aladdin Pro which I bought in NZ was (I believe) designed for a single deep dive when diving profiles were a bit different (typically deeper) when it was released. Compared to other places I have dived, in Egypt I found 'older' divers- especially videographers using the Aladdin.

Comparing the two on Thistlegorm over two dives was interesting. The Suunto Stinger went to deco with the Aladdin still giving me 12 mins on the second dive.

DCS can occur using any computer as we all know. If I was diving just for me, I'd take the Aladdin as it allowed me longer dive times, but I would take that with the understanding that my buoyancy at safety/deco stops and ascent rates had to be spot on as opposed to 'somewhere in the neighbourhood' with the Suunto.

When I used the Aladdin I was making two stops; 3mins at 6m, 2mins at 3m which I believe was the 'norm' for divers at the time when the Aladdin was brought out. It made sense to me.

Today a 3min SS at 5m is the norm with a Suunto (or equivalent).

In either case, I still try to make 2 stops at 6m and at 3m- especially from dives deeper than 25m. When teaching 'deep dives' I have the student make the first stop at 9m (to confirm control of buoyancy), then move to 6m, then 3m until the computer clears. That still makes sense to me.

Is the Aladdin inherently unsafe? No, but I believe it to be less forgiving if something untoward happened during repetitive deep dives.

 

[Mr Carcharodon]

I believe computers are designed with a 'type' of diving in mind. I have owned Suunto and the Aladdin Pro.

The typical Suunto is designed IMO for recreational divers who do repetitive long shallow dives over multiple days.

This sounds like you are discussing tables which consider only one tissue compartment. Computers calcuate limits for all compartments. So IMO the differences are not due to this.



When I used the Aladdin I was making two stops; 3mins at 6m, 2mins at 3m which I believe was the 'norm' for divers at the time when the Aladdin was brought out. It made sense to me.

Today a 3min SS at 5m is the norm with a Suunto (or equivalent).

The 3 minute safety stop on the Suunto is recommended, but not required. Does the Aladdin prompt you for stops at 6m and 3m or was that something you were doing? The DSAT tables do not require any stops at all for dives above 100 fsw.

In either case, I still try to make 2 stops at 6m and at 3m- especially from dives deeper than 25m. When teaching 'deep dives' I have the student make the first stop at 9m (to confirm control of buoyancy), then move to 6m, then 3m until the computer clears. That still makes sense to me.

Is the Aladdin inherently unsafe? No, but I believe it to be less forgiving if something untoward happened during repetitive deep dives.

The rub is the Suunto would consider a lot of dives done by the Aladdin unsafe. If you skipped mandatory decompression required by the Suunto it will lock you out for 48 hours. And you could easily end up with 10 or more minutes of decompression if you followed the Alladin. At the end of the day the algorithms need to be anchored back to real world diving. It certainly appears that DSAT has a safe record and is better anchored than RGBM.

 

[t-mac]

The rub is the Suunto would consider a lot of dives done by the Aladdin unsafe. If you skipped mandatory decompression required by the Suunto it will lock you out for 48 hours. And you could easily end up with 10 or more minutes of decompression if you followed the Alladin. At the end of the day the algorithms need to be anchored back to real world diving. It certainly appears that DSAT has a safe record and is better anchored than RGBM.

Thanks for the insight. I agree with you and supergaijin. I think computers like the Gekko are great for new divers who haven't fine-tuned their control, but some of the features (like adding 1% to MOD caculations on nitrox) are just paternalistic. Beyond the novice, it is just marketing. There is no evidence whatsoever that one computer is safer than another and I personally believe that marketing yourself as the conservative option is smart because it appeals to our training that we should dive conservatively and, therefore, more safely. It does not mean that those of us who select different brands using less conservative models are being unsafe -- perhaps less safe, but I would be safer yet just sitting on my couch. I guess I would also probably be safer driving 55, but I feel perfectly comfortable driving at 65 -- I have the skills and equipment to handle it and I'd really rather not have my car governed to 55.

 

[Karibelle]

Here's my understanding... for what it's worth.

The two models are based on data from different types of diving. The Suunto uses an algorithm that is based on data consisting mostly of deep, repetitive cold water dives at altitude. The DSAT model (likely the one you used in the OC1, although you could change the algorithm) is based on data consisting primarily of no deco, multilevel, repetitive dives at sea level. The DSAT algorithm maximizes dive time for repetitive, multilevel recreational diving. So depending on what type of diving you are doing should affect your computer choice.

If you use the Z+ algorithm on the OC1, it matches the Suunto RGBM model almost exactly. Put another way, the Z+ algorithm will increase the conservatism of the OC1 by somewhere between 15 and 20%.


Good luck!

 

[drrich2]

Unless & until someone shows a substantially higher rate of DCS with liberal algorithm computers, these points are important to me:

1.) I go to dive, and I don't want my bottom time cut short without a good reason.

2.) I am free to dive more conservatively than the computer demands, yet diving less conservatively could be problematic. Imagine alarms going off and what-not!

3.) Some charter boat operations will bench you if they find out you went into deco. per your computer, from what I understand. Since charter operations & live-aboards are popular for dive vacations & may have paternalistic 'busy body' policies in place regarding this, the Suunto might get you benched when the Oceanic would not. I don't know how apt that is to come up in the real world, but 20 minutes seems like a big difference.

4.) In my view, a computer that takes it upon itself to 'lock me out' for an extended time has a paternalistic control freak design and my money goes elsewhere.

Richard.

 

[mala]

dive computers dont determine the length of the dive.

you do.

if you need longer ,deeper repetative dives then you plan for that with the gas available and the mix.


for me i want a computer to tell me how quickley i can get out of the water if i have too.
its a lot easier to add to stops on a well planned dive than to wonder what corners are safe to cut if a problem occurs.