Computer says ...10' Deco Stop 3 minutes?

[digital_steve]

You can get mark's book direct from him and it'll be quicker.
Mine got here in a week. Although i got mine just after he released it...

 

[DA Aquamaster]

Scubapro (before they were paired with Uwatec under the JWA umbrella) used to have computers that were notoriously conservative in the 80-100 ft range, especially on repetitive dives, and that did not make them popular with divers who dove with more liberal computers.

I never had issues with either Pelagic hocky pucks (Aqualung and Oceanic) or with a Wisdom as both tolerated deep stops very well and would usually clear a few minutes of deco with a slow ascent and deep stop long before you reached the 10' ceiling.

In my opinion, a computer that stacks on more deco time with a 2-4 minute long 1/2 max depth safety stop is not a good computer to have. Some computers are just not intended for intentional deco use and are not deco friendly at all. In their zeal to save you from yourself they create psychological pressure to quickly ascend to the ceiling depth and consequently subject you to what is literally a bend and treat deco model. Not a good thing at all.

Reading some of the odder fine print in the manual might help you determine what is going on. The Sherwood Wisdom manual for example indicated that it used what were essentially US Navy table limits once you got into deco rather than the algorithm it normally used and if you missed that line, you missed all of what it said about a different deco model. So in deco mode it was very liberal compared to the Palm VPM (VPMa) and DPlan tables I cut and used for deco. That was a good thing as the computer always cleared before I did and served more or less as a bailout option if I had to surface quicker.

I still use my Cochran EMC-20H the same way (as a backup to a computer generated profile) although I have the conservatism upped to 70% to keep it semi close to the DPlan profiles I normally use as the EMC-20H is I suspect the most liberal computer on the planet. In my opinion you'd have to be stupid to dive an EMC-20H with the conservatism set to less than 30%. It is a good example of why it is important to have a basic understanding of deco theory and various models before relying on a computer, as both overly liberal and overly conservative approaches to computer modeling have their drawbacks and pitfalls.

In general, it is nice to be able to adjust the conservatism of your computer to match a buddy's computer to ensure you both stay in the same ballpark, but it can take several dives to get them more or less consistent with each other.

 

[gcbryan]

I recently completed a dive with max depth about 105' with most of the dive ranging between 85' and 100'. I was diving with 2 computers - a Tusa IQ-800 and an older Oceanic Data Plus. The Tusa put me in Deco while I was still in a comfortable range on the Oceanic. Both computers on SF-0. My instructions on the Tusa were to do a 3 minute Deco Stop at 10'...that depth seemed strange to me as that is shallower than even a normal Safety Stop. On my ascent I decided to do a Deep Stop for a minute at 50'. While doing this Deep Stop my Tusa started adding time to my 10' Deco Stop ..and in fact actually got up to a 5 minute Deco stop at 10'. After my minute expired at 50' I decided to do a slow ascent to about 20 - 15' and see what adjustments my computer might make. Once in that range my computer was still telling me to make a 4/5 minute Deco Stop at 10'.... so I ascended to 10 ' and hung out and sure enough I was able to wait out the 4 minutes (a big rock ledge aided in this) and then my computer gave me a 3 minute Safety Stop.
This is only my 10th dive with this Tusa IQ-800. The manual sheds no light on this. Does this seem normal to anyone out there? Anyone with any light on this? Thanks!

Your questions have basically been answered now I guess but I'll add...

Go to the dive planning screen on your Tusa and plan for a dive to 100 fsw. If it's like my Tusa IQ-700 it will give you 16 minutes. Now do the same for your Oceanic computer. Again, if it's like my Oceanic Veo 200 it gives you 19 minutes.

They are both based on Buhlmann but each manufacturer tweaks things a bit. Neither will give you credit for deep stops as it's based on Buhlmann.

A 10' ceiling is standard for short deco obligations then it goes to 20' and so on.

To directly answer your question, yes, it's normal. People who routinely do deco dives plan their dives and wouldn't be relying on either of these computers except in gauge mode.

 

[Mr Carcharodon]

I would second the recommendation for reading Mark Powell's book "Deco for Divers" from acqupress (2008) to get a better idea of where the numbers are coming from.

Also for those of you in Southern California Michael Powell (not Mark Powell, but rather ScubaBoards Dr. Deco) is giving a seminar on advanced decompression physiology. From the synopsis:

Starting with a historical perspective of decompression research, this course covers aspects of gas exchange; oxygen and oxygen toxicity; and Doppler bubble detection. Most of the program will focus on bubble development and growth; decompression sickness; gas phase research; bubble control; and the influence of diving decompression research on space decompression and vice versa.

The seminar is at the Wrigley Marine Sciences Center in early April.

 

[Hatul]

I suspect your Tusa computer and many others are similar to Suunto, which is what I understand. When it shows a deco stop of 10 feet, it's really a ceiling-- you must stay below that. There is also a floor, which is the depth as which you start to decompress. So really the computer is telling you to ascend to the decompression zone between the floor and ceiling. It does not mean that you have to ascend to 10 feet.

The deep stop is not built into the old Suuntos and your Tusa, but apparently the new Suuntos do deep stops.

Adam

 

[DA Aquamaster]

Check the manual to confirm the statement above. Some computers will not give you a true second for second credit for deco if you are more than a few feet below the deco stop ceiling. For example if you have a 10' stop and do the stop at 15' it may require you to do 1.5 minutes of deco to reduce the deco obligation by 1, and may require 2 mionutes to eliminate that same minute if you do the 10' deco stop at 20'. In my experience, those are usually the same computers that end up adding deco time on a deep stop.

 

[Hatul]

Check the manual to confirm the statement above. Some computers will not give you a true second for second credit for deco if you are more than a few feet below the deco stop ceiling. For example if you have a 10' stop and do the stop at 15' it may require you to do 1.5 minutes of deco to reduce the deco obligation by 1, and may require 2 mionutes to eliminate that same minute if you do the 10' deco stop at 20'. In my experience, those are usually the same computers that end up adding deco time on a deep stop.

You are right, that the closer you are to the ceiling, the shorter your decompression. Nevertheless if his deco obligation is short and he stays close to the ceiling it's not a big issue unless he's running out of air. And he does not need to fear that he must be at exactly 10 feet to satisfy the computer decompression.

Adam

 

[psychocabbage]

I have the TUSA IQ 700 (model right under yours).
Last year when we dove the Spiegel Grove, it made me stop at 50 and did a countdown. and we also did the safety stop as well which it did the countdown for.

I have not messed with its conservation factor at all.. I prefer more conservative. Rather live to tell about than see it once for a bit longer and never see it again.

 

[vlkr]

i don't have your computer but my guess is that 10ft was merely a ceiling. 20 ft was probably the floor. the closer you are to the ceiling without going above it, the more effeciently you are off gasing. kind of like playing blackjack. try to stay just below 10 but if you go above it, you have to go back down and start all over.
sorry. i did not read 2nd page before posting. the suunto owner said it better than i.

 

[Hatul]

i don't have your computer but my guess is that 10ft was merely a ceiling. 20 ft was probably the floor. the closer you are to the ceiling without going above it, the more effeciently you are off gasing. kind of like playing blackjack. try to stay just below 10 but if you go above it, you have to go back down and start all over.
sorry. i did not read 2nd page before posting. the suunto owner said it better than i.

Yes, that's exactly correct. In fact in my Vyper manual they advise against decompressing at 10 ft even if the computer shows that, because it's too difficult to maintain that depth.

Adam