Varying Dive Computer Decopression Obligations

[Unregistered User]

I recently made a dive @ GSR going into Devil's Ear and taking the mainline (via DPV) back to the Mainland jump and into Mainland itself. Maximum depth reached was 110'. Upon exiting the cavern zone via The Ear, my bottom time was 1h 31m.

My AL80 stage had an actual analyzed mixture of 32.2% and my back gas (PST D120s) was analyzed at 31.8%. My Oxygen deco bottle is filled straight from a rented K bottle from PraxAir and is as close to 100% as can be determined.

I was wearing an older Cochran Commander Nitrox Computer and a newer Cochran EMC-20H 3 Blend Trimix Computer throughout the whole dive. Bottom mixes for both computers were set at 31.0% with a single deco mix of 95% slated to be used for calculation after 35 minutes had elapsed and a depth of 23 feet was reached. All three (effectively two) mixes for the EMC-20H were set with He @ 00.0%. The Commander's conservatism factor was set at 0, while the EMC-20H was set at 05%.

I had been monitoring both computers throughout the dive and remember when turning the dive in Mainland, the Commander was displaying a Deco obligation of 0:06 @ 20 feet and alternately displaying the total Deco obligation of 0:36.

At the same time, the EMC-20H displayed a deco obligation of 0:06 @ 20' while alternately displaying the total Deco obligation of 0:10.

During the hasty exit on the DPV, I never looked at the Commander again, only checking my bottom time and deco obligation on the EMC-20H (it was on my non-scooter wrist) once at the beginning of the cornflakes. It showed 0:10 @ 20 feet and total time of 0:16.

After retrieving my deco bottle, I began ascending through The Ear and upon reaching a depth of 21-22 feet to begin my deco, the Commander was reading 0:12 @ 20 feet and a total deco of 0:56, while the EMC-20H displayed 0:13 @ 20 feet with a total of 0:16. This significant difference had me perplexed, as I have done numerous multi-mix and/or semi-deep (>170 feet) dives with the EMC-20H and have never had any DCS symptoms - acute or otherwise. The commander is vintage 1998 or as late as 2001, while the EMC-20H was manufactured in February this year. Both had brand new batteries installed specifically for this dive.

Questions:

1) Has the algorithm used by Cochran been drastically altered since the Commander's manufacture? If so, could it be construed that the older algorithm was excessively conservative?

2) Certainly, a number of other divers within this forum have probably made this exact dive with a similar- if not identical- mix. Would you mind sharing your deco times along with the type of DC you used?

---My body fat ratio is extremely low and I am a smoker (3/4 pack per day) in otherwise good shape, enough to hike doubles to Friedman's or carry 500 feet of rope to the top of Thunder Hole. I would think that my lungs would be less efficient and incapable of off-gassing/transpiring inert gases than that of a non-smoker.---

3) As a general rule, would it be more likely for a non-smoker with slightly more body fat to exhibit signs of DCS compared to that of myself while doing the dive I described above?

I never exhibited any signs of DCS on this particular dive (or ever during ANY dive), but seeing how the EMC-20H got me out of the water so quickly I was concerned.

Thanks

[Kelly Jessop]

I've never owned either,but I do remember diving with people with a Commander,and having excessively long deco obligations,especially if it was a multi-dive. It wouldn't be uncommon in the 90's to see reels at the surface with the Commander tied to the other end to finish the deco,and retireve later. The other things you mention are variables that can effect deco and dcs,but there is no real clear relationship,only inference.

[skip]

I used to dive the commander and yes it always gave very long deco times. and yes, I've left it hanging on the end of cave string to do the deco on its own! I quit diving it cause it never agreed with any other computer by a long shot. Then it began to disagree on the current depth and time at depth (once I really got to comparing it). I tried to talk to cochran about it, but they were so unpleasant and offensive to deal with I vowed to never buy anything from them again.

skip

[Unregistered User]

Yes, I've seen the "Cochran Clothesline" myself on occasion, as well. The main thing I did not like about the Commander was that it showed the deco time as if it were "air" and NOT the mix you were actually using. Thus you could put the screen right next a dedicated bottom timer and see the deco time countdown much quicker than the bottom timer counted upward. The ratio was dependent on the gas mix, but if it showed :10 @ 10', you could guarantee that your actual deco time (as elapsed) was closer to :07, or thereabouts.

I bought the EMC-20H to replace my older Cochran Nemesis IIa integrated computer. It had 3 mixes (no trimix, though) and worked great because it recognized the pressure drop in your back gas and thus calculated deco that way. The programming wand and contacts on the computer became unusable and Cochran discontinued servicing it. They did, however, give me a half-price discount on the new EMC-20H with the return of the Nemesis and I have had plenty of contact with them that was cordial and informing. The only conversation they did not reply to was when I questioned the wisdom of the naming of that particular computer (Nemesis). I would have chosen something more marketable such as "Idyllic Utopian Bend-Be-Gone" or something along those lines. Though "Nemesis" was the spirit of divine retribution against those who succumb to hubris (arrogance before the gods), I could see the other alternative meaning "to give what is due" to be more appropriate.
Good to see that I'm not the only one who thought the deco time was exorbitant!

[Sludge]

What perplexes me is giving you time at twenty feet and then just a few minutes at ten. Every deco profile I can remember seeing is twenty minutes or so at ten feet after requiring time at twenty.

[Unregistered User]

Perplexing, indeed. I haven't asked Cochran about this but aim to in the very near future.

Perhaps this scenario is the equivalent of a 0:05 stop @ 20' with a subsequent stop of 0:15 @ 10'? Does Cochran like giving you deeper, longer stops vs. what most of us have experience seeing on their dive computers?

I used to dive with a buddy 13 years ago and remember doing that exact dive many times with roughly the same mix and incurring a deco schedule with 0:15 @ 20' and another 0:35 @ 10' (thereabouts). He had a Commander and I used a Cochran Captain air computer with tables before I bought the Nemesis. Even with the Captain, the shallower stops were ALWAYS longer than the previous deeper stops. btw-I hit a 60' 0:03 ceiling with the Captain diving Little River (the well and deep section) on air once. You don't even want to know what time the 10' stop called for. Let us just say that the vomiting from the cold (I had on 3mm wetsuit) was more perilous than sloughing off 0:40 of deco...

This probably means that I have spent literally DAYS doing unnecessary deco.

[KarsticGator]

What perplexes me is giving you time at twenty feet and then just a few minutes at ten. Every deco profile I can remember seeing is twenty minutes or so at ten feet after requiring time at twenty.

I read an article... cannot remember the details, or even the publication, but they discussed the physiological advantages of longer 20' stops than longer 10' ones?

[DA Aquamaster]

I used to own an EMC-20H and I noted it tried to get me out of the water faster than anything else on the planet.

I spoke to someone from Cochran about it a few years ago at a trade show and was advised by the person there that he did not actually recommend using it at a conservatism of less than 30%.

I found that if I was trying to get run times in the same ball park as divers with other computers I had to use a setting in the 40-50% range.

IIRC it showed time at your first stop but also showed total time to surface based on the gas you were currently on - in effect not considering the deco gasses that were programmed until it actually switched to them in sequence, so at depth when still on backgas it showed ungodly long tts.

[Kelly Jessop]

I spoke to someone from Cochran about it a few years ago at a trade show and was advised by the person there that he did not actually recommend using it at a conservatism of less than 30%.

.

I understand without conservatism,it is running raw Buhlmans.

[gschaut]

I've killed a lot of brain cells in the past decades. But I think I remember something about a set of (French?) tables that did a reverse deco profile? Deeper stops were longer, with each shallower stop shorter?

Ring any bells?