Matching Dive Computer Algorithms

[divinh]

I have a Cressi Giotto. I also have a Sherwood Vision. I've read that the Giotto is more conservative than the Vision. I've also read that it's best to leave the dive computer's alone and not bother to try to "match" the algorithms, i.e. set them to have similar conservatism or liberalism. I would be fine being limited by the Giotto for its conservatism. But... if I really wanted to match them up similarly, how would I do it? There's the Dive Plan, which would be a start to match them, but how would I get them to show what subsequent dives would be like?

CRESSI GIOTTO

CRESSI RGBM algorithm. New algorithm created through a collaboration between Cressi and Bruce Wienke, based on the Haldane model, integrated with RGBM factors for safe decompression calculations in repetitive multi-day dives.
Tissues: 9 with saturation half-times between 2.5 and 480 minutes

SHERWOOD VISION

• Modified Haldanean based Pelagic DSAT, algorithm.
• No Deco limits closely follow PADI RDP.
• Decompression in agreement with Buhlmann ZHL-16c and French MN90.
• No Deco Deep Stops >> Morroni, Bennett.
• Deco Deep Stops (not recommended) >> Blatteau, Gerth, Gutvik.
• Altitude >> Buhlmann, IANTD, RDP (Cross).
• Altitude corrections and O2 limits based on NOAA tables

 

[Jay]

Post #8 in this thread How to properly assess the relative conservatism of various algos/computers for Rec divers? should help.

 

[uncfnp]

The only way to know is to dive them together. Both computers use modified algorithms so its near impossible to predict how they will actually approach non-square, multi dives. Almost certainly the Cressi will be the most conservative over repetitive dives but may actually be close or even more liberal the first dive of the day.

So, set both to their most liberal settings, go dive and see what happens. If needed you can then adjust the most liberal for the day to the next conservative setting then dive that and see what happens. Just make sure that you are always following the most conservative computer for each dive.

The problem is you can never get them exactly the same so if you get them too close you will have to monitor both computers every dive to ensure one does not go into deco first and you fail to notice. So best choice is to get them close but have one always slightly more liberal so you can set it and forget it.

 

[Centrals]

RGBM will introduce various factors on how the dive progress as well as afterward, eg. reverse profile, quick ascent, short surface interval etc etc.

 

[divinh]

For the table @Jay_Antipodean posted from Scubalab...

2016 has both the Sherwood Vision and the Cressi Newton, which runs the same algorithm as the Giotto.

NDL times: (best I could do without being able to add a table)

SV CN
17 15 Dive1
20 13 Dive2
24 22 Dive3
36 20 Dive4

It sure seems the Vision's DSAT is /much/ more liberal for repetitive dives. By default, the Giotto is set to Safety Factor SF0 (disabled), with options to only go up SF1, SF2, SF3. It seems my only option is to make the Vision more conservative, but I can leave it as is and pay attention to the Giotto.

 

[dmaziuk]

It seems my only option is to make the Vision more conservative, but I can leave it as is and pay attention to the Giotto.

Dive the giotto, keep vision as the backup. The reason making the vision more conservative won't help much is as @Centrals said, the giotto has all those built-in fudge factors that will make it not match the vision's numbers anyway. Well, most likely: you don't know how Pelagic may have modified DSAT, and nobody knows what giotto is running at all.

In theory you should e.g. see less difference by doing longer SI's (60+ minutes), and more difference if doing shorter SS's. But it's probably also profile-dependent...

 

[Jeremy Williams]

A man with two watches...

Segal's law - Wikipedia

 

[CandiveOz]

It seems my only option is to make the Vision more conservative, but I can leave it as is and pay attention to the Giotto.

I think leaving as is and paying attention to the Giotto is your best solution if you want to dive both computers together. As per @Jay_Antipodean table, you own two computers which are at the opposite ends of the NDL scale.

I checked the user manual for each computer online. The Vision has one Safety factor which is the altitude setting 3000 feet more than the default. The NDLs for the Vison are listed in the manual. The Giotto has more Safety factors in addition to the default SF0 but I'm not sure how many. The manual refers to SF1 and SF2 but in one instance refers to SF3. You also mention SF3 so there must be 3 additional Safety factors in addition to the default SF0. Unfortunately the Giotto NDLs for these Safety factor settings are not listed in the manual. Regardless, the initial step would be to try an match NDLs. Since the Vision is the more liberal of the two, you would have to increase its Safety factor. With the Giotto dive planner on the computer, go through each Safety Factor NDLs to determine which setting is closest to the Vision. Of course this makes both computers even more conservative! And even then it doesn't guarantee they will respond the same way during a dive or on subsequent dives.

Although I have never dove a Cressi, reading other posts regarding the Cressi computers, they don't seem to provide much, if any, NDL credit if you ascend to shallower depth's on a multilevel dive. I found this very surprising since it defeats the whole purpose of a dive computer over tables. Cressi seems to be more of a square profile dive computer rather than a multilevel dive computer. The following reference is in regard to the Leonardo but the Giotto may respond in the same way unless the programmers subsequently modified the algorithm. Has this been your experience? I only mention this observation because its another complication you may face in trying to "match" the computers.

The problem is that if you do a "normal dive" , that is go to +/- 100 feet level close to the NDL limit and that you go slowly up, the NDL will not increase like in other computors. It is only when you reach less than 6m (18 feet ) that the Leonardo will start the desaturation process. Most other computors start the desaturation process at much deaper levels.

​

 

[dmaziuk]

Although I have never dove a Cressi, reading other posts regarding the Cressi computers, they don't seem to provide much, if any, NDL credit if you ascend to shallower depth's on a multilevel dive. I found this very surprising since it defeats the whole purpose of a dive computer over tables. Cressi seems to be more of a square profile dive computer rather than a multilevel dive computer.

Maybe you should try asking people who actually dive them instead of reading posts from our resident Suuno-bashers. Of course they reacalculate for your current depth; my Leo won't even't pop up the safety stop icon if I spend enough time above 8 metres or so at the end of the dive.

 

[kelemvor]

A man with two watches...

Segal's law - Wikipedia

A man with three watches on the other hand...

I think if you're going to dive both, and you're diving anywhere near NDL's then you've got no choice but to follow the more conservative. Otherwise, your backup may very well be locked out due to an NDL violation when you need it most.

If you're nowhere near NDL then it doesn't matter.

Personally, I'd try and sell one of the two and pick up another identical to the one I kept.