Marathon/Skinny Dipper/Sigma Modeling?

[Marek K]

All--

I'm getting back into scuba now, after a break of 14 years. (Yes, I'm taking a refresher course!!) I got my AOWD in 1987, still using the old U.S. Navy-based PADI Dive Tables. Quite a difference from the current RDP!

Just before I quit, I bought a used Sherwood Sigma II computer, that I never ended up using. For all intents and purposes, it's the same as the Orca Skinny Dipper II and Marathon. I want to keep/start using it now, and was wondering what sort of modeling my computer uses... old or modern?

According to one web site, the Marathon -- at least -- uses a "Modified Haldane" algorithm. Is that consistent with the current RDP?

I've been checking out the dive planning sequence my Sigma II goes through at start-up, and it looks like the depth/bottom times it allows are very consistent with the RDP... Max bottom times end up somewhere in the middle of the RDP's "Safety Stop Required" times.

I can't cross-check the surface interval modeling in my living room, of course; but it looks like I should be safe with this thing. I don't want to get into an argument on validity of the current RDP models, and I don't intend on pushing the limits here. But is that a valid conclusion?

As long as the battery compartment doesn't flood on me...

Thanks!

--Marek

 

[Charlie99]

According to one web site, the Marathon -- at least -- uses a "Modified Haldane" algorithm. Is that consistent with the current RDP?

I've been checking out the dive planning sequence my Sigma II goes through at start-up, and it looks like the depth/bottom times it allows are very consistent with the RDP... Max bottom times end up somewhere in the middle of the RDP's "Safety Stop Required" times.
.... ...
I can't cross-check the surface interval modeling in my living room, of course ....

Your logic is sound. The PADI RDP is also Modified Haldane, aka neo Haldanian, aka dissolved gas model, as are almost all dive computers. Even the majority of "RGBM" computers, such as Suuntos are just modified Haldane algorithms that tweak the dissolved gas limits in response to things like fast ascents.

The USN tables are also Haldane, but with more liberal limits than either the PADI RDP (aka Spencer aka DSAT) limits. PADI limits are used in most hockey puck computers (Oceanic, for example). Buhlmann models are also commonly used in computers. It's limits are similar to PADI -- a few minutes more at some depths, a few minutes less at others.

The PADI/DSAT model assumes non-reciprocal offgassing -- even the fast tissues will only offgas with a 60 minute halftime after surfacing. IIRC, USN and Buhlmann don't do this.

One other item to be aware of is that the recommended ascent rates have been reduced to 30 fpm or less, while your old computer may be set to alarm for greater than 60 feet-per-minute. It's a good diving practice to further control your ascent/offgassing by doing a couple minutes stop about 1/2 up, and then 3 minutes at 15' or so.

 

[Marek K]

Charlie--

Ooh, good point about the surfacing rate, and about the additional mid-way safety stop (in addition to 3 min at 15 ft).

In addition to the Sherwood Sigma II, I just mail-ordered a bottom-of-the-line Oceanic Veo 100 computer (starting out slow, trying to keep the expenditures below wife-will-kill-me level!)... I plan to use one of the computers, and give the other to my 14-year-old son who's just going through his first OW course. I plan that we'll be each other's dive buddies for a while.

From what you're saying, the 15-year-old Sigma II and the brand-new Oceanic will be based on the same Modified Haldane model? So, along with the RDT, we'll all be on the same sheet of music, except for maybe the surfacing rate? That's great... and we'd be surfacing together in any case.

Thanks for the info!

--Marek

Your logic is sound. The PADI RDP is also Modified Haldane, aka neo Haldanian, aka dissolved gas model, as are almost all dive computers. Even the majority of "RGBM" computers, such as Suuntos are just modified Haldane algorithms that tweak the dissolved gas limits in response to things like fast ascents.

The USN tables are also Haldane, but with more liberal limits than either the PADI RDP (aka Spencer aka DSAT) limits. PADI limits are used in most hockey puck computers (Oceanic, for example). Buhlmann models are also commonly used in computers. It's limits are similar to PADI -- a few minutes more at some depths, a few minutes less at others.

The PADI/DSAT model assumes non-reciprocal offgassing -- even the fast tissues will only offgas with a 60 minute halftime after surfacing. IIRC, USN and Buhlmann don't do this.

One other item to be aware of is that the recommended ascent rates have been reduced to 30 fpm or less, while your old computer may be set to alarm for greater than 60 feet-per-minute. It's a good diving practice to further control your ascent/offgassing by doing a couple minutes stop about 1/2 up, and then 3 minutes at 15' or so.

 

[O2Addict]

They use the same basic model, but the newer computers have been tweeked to create a more detailed model, based on the same theory.

My view is that I relied on my Skinny Dipper in the 80s, I never got bent so why not rely on it now!!

Just make sure that it is accurately recording depth. I have actually moved to Oceanic/Aeris computers as I needed Nitrox.

 

[Charlie99]

From what you're saying, the 15-year-old Sigma II and the brand-new Oceanic will be based on the same Modified Haldane model? So, along with the RDT, we'll all be on the same sheet of music, except for maybe the surfacing rate? That's great... and we'd be surfacing together in any case.

The short answer: Just go diving, have a good time, and surface according to the more conservative computer.

Longer answer: "Modified Haldane Model" only means that they use the same method of bookkeeping ---- tracking depth and time and using that to calculate the assume dissolved N2 loading in compartments with various halftimes.

Both the old US Navy tables and the PADI RDP are based on Modified Haldane Model, but they have different allowable limits of N2 loading for the various compartments.

If the initial NDLs for the Sigma II are close to what is on the PADI tables, that simply means that the model limits for the computer and the table are approximately the same. IIRC, the skinnydipper and the Marathon used the US Navy/Workmann limits. The Sigma II might be close to PADI numbers not because they used the PADI/DSAT/Spencer M-values, but instead because the manufacturer decided to make the computer more conservative by reducing the USN limits a bit. The end result is pretty much the same.

On a practical basis, you could have a buddy with a computer using a Buhlmann model (yet another Modified Haldane Model) and another one using the PADI/DSAT version of Modified Haldane, and any difference would be swamped out by 1) extra fudge factors added by the computer manufacturers, and 2) slight differences in the depth/time profiles of the two divers.