Multiple decompression dives/day- planning

[Dive-aholic]

It's rare for me to do more than one dive a day unless I'm teaching. I prefer to do one long dive than 2 shorter dives. It's the setup and tear down that tire me out. When I'm teaching we do a 2 hour surface interval for non-helium dives and 3 hour for helium. When deciding that I ran several mock profiles and decides that was the best approach in terms of maximizing off-gassing and minimizing the CNS clock. Ideally, if my students and I had unlimited time and resources I would prefer to do a longer course in terms of days. I'm not at my top game on dive 2 and I'm sure my students aren't either. Don't get me wrong, I'm fine doing a 2nd dive. I just prefer not to.

 

[Doppler]

Doubling up decompression dives is more or less a given when teaching. I prefer to run one long dive but the logistics often negate this option.

Long SIT and particular attention paid to 24-hour CNS limits as per NOAA (Dr. Hamilton)

 

[JS1scuba]

Multiple decompression dives in a day become pretty normal for me when running projects, expeditions, and training. But it does take a fair amount of planning.

Repets on trimix will require 2.5-3.0 hours of surface interval time. I will use the same decompression schedule but drop the bottom time back by 5 min. So a 230 for 30 on dive one now becomes the same 230 / 30 deco schedule but bottom time will be between 20-25 min. This is a standard Hamilton Research Ltd approach to repet gas dives. The 3 hours SI is to reset the CNS oxygen clock. The shorter bottom time is to allow consistency in tables and total run time.

If you ran back to back 230/30s the total run time for dive 1 would be 111 minutes and the repet # 2 would be 117 minutes whereas If you cut dive 2 back by 5 minutes the runtime is 97 minutes. By running the dive with my -5 = same deco as dive 1 method I essentially pad dive # 2 with gas and time but not that much. This method works exceptionally well.

Note that I also fly Cochran EMC20H computers set at 05 conservatism. With these I will always add 10 minutes of oxygen once the computer has cleared. (SOP for me) and the units will clear about 10 minutes before my hard tables will clear due to multi level aspect of computer. I will not change my gas mix in the computer for the repet dive if the gas mix is slightly different. IE topped up mix from 17/50 to 19/30 etc. I just fly it at the lowest oxygen highest helium of the mix series. This pads with gas.

A general rule of thumb for me is to keep the repet run time no more than the run time of the initial dive. This will invariably require dropping back the bottom time or going shallower. If however the job REQUIRES longer bottom time specific methods of gas optimization are needed.

FOR CCR on repets I will keep run time the same but switch to OC oxygen at 20 fsw.

It's extremely difficult to program dive planning software to take these little "field modifications" into consideration. These tweaks come from years of use and experience AND recording data for evaluation.

"Decompression is as much an art of execution as it is an inexact science."

All my decompression planning is done with NAUTILUS DIVE PLANNER that we designed and utilize Hamilton Research decompression models.

Cheers
JDS

 

[lobstah]

No issues with 2 tech dives a day with 90 min interval. It's usually what it comes down to anyway after you move the boat, get bottles ready, warm up, splash in and start a descent.
I also did a fair share of tech dives with 60<SIT<90 min - never had an issue with doubled shallow stops.

 

[rjack321]

I also did a fair share of tech dives with 60<SIT<90 min - never had an issue with doubled shallow stops.

Were you actually doubling the shallow stops? 20mins deco = 10mins deep, 10mins shallow then turns into 10 mins deep and 20mins shallow for a total of 30mins deco?

 

[lobstah]

Were you actually doubling the shallow stops? 20mins deco = 10mins deep, 10mins shallow then turns into 10 mins deep and 20mins shallow for a total of 30mins deco?

Yups. It was back in no more than 30 min/50% days and we were switching to BG once 50% petered out. I don't remember exact schedules anymore. Nowadays I value bull****ting on deck slightly more than extra time in water at 20 feet.

 

[DA Aquamaster]

Note that I also fly Cochran EMC20H computers set at 05 conservatism. With these I will always add 10 minutes of oxygen once the computer has cleared. (SOP for me) and the units will clear about 10 minutes before my hard tables will clear due to multi level aspect of computer.

5% conservatism on an EMC-20H is very agressive and adding 10 minutes of O2 is just a really good idea. Anything under 30% strikes me as pushing it unless you are adding some additonal safety.

 

[JS1scuba]

5% conservatism on an EMC-20H is very agressive and adding 10 minutes of O2 is just a really good idea. Anything under 30% strikes me as pushing it unless you are adding some additonal safety.

my sample group has close to 3,000 dives on emc20 since we beta tested them in 2004. the 05 conservatism matches up very well with the Hamilton Kenyon Bubble Model. it gives a nice deeper ceiling and the shallow tail is not exceptionally long. 30 was way long on the shallow side. By using 05 we control how we want to pad out the back end of the dive. Hence adding the 10 minutes.

But like all tools different craftsmen use them differently. That's what's so wonderful about these things. We each get to bring different experiences to the table.

Cheers
JDS

 

[DA Aquamaster]

my sample group has close to 3,000 dives on emc20 since we beta tested them in 2004. the 05 conservatism matches up very well with the Hamilton Kenyon Bubble Model. it gives a nice deeper ceiling and the shallow tail is not exceptionally long. 30 was way long on the shallow side. By using 05 we control how we want to pad out the back end of the dive. Hence adding the 10 minutes.

But like all tools different craftsmen use them differently. That's what's so wonderful about these things. We each get to bring different experiences to the table.

Cheers
JDS

To some extent I think it depends on the profiles you are diving, but overall I think the EMC-2OH is outstanding if you want a computer that will get you out of the water in absolute minimum time when/if the need arises with a reasonably low probability of getting bent. Nothing gets out of the water faster than an EMC-20H with a low conservatism setting.

But in the real world at anything under 10-15%, I found myself doing exactly what you are doing - padding the O2 stops to build in some additional safety margin. This was especially true for repetitive deco dives. 30% was more the norm, if I did not want to have to manually futz with the profile.

Can it be made to work very well? Absolutely. But the follow on question is should you have to alter the profile it gives you? I decided that no longer made sense for me.

Personally, there were two major reasons I stopped using it. The first was it was a major PITA in some cave profiles. The second was that I much prefer controlling the curve with gradient factors as I can optimize the curve to the type of diving being done - short/deep deco dives versus long but comparatively shallow deco dives.

The latter was also important to me as I normally plan the dive with deco software using G/F's and its nice if the tables and computer are in the same ballpark with no need to pad one or the other.

 

[JS1scuba]

Can it be made to work very well? Absolutely. But the follow on question is should you have to alter the profile it gives you?

You are right in that the EMC20H can give you a large range of conservatism factors. Im sure if we bumped it to .10 that we might not need to pad. I just happen to like to take the option of doing it.

Ive got 12 dives coming up in key west in a few weeks i will run one on .10 and the other on .05 and do a comparison.

I'm not keen on GF changes. Its why i used Hamilton DCAP or the HKBM bubble model we created. The bubble model flows nicely with the EMC20. I like the planning software to run algorithms as they were designed, from that info i can make my own adjustments as needed. But like you that requires having a good experience base to make those decisions.

(actually sitting here right now with Dr. Hamilton in his office in Tarrytown)

Cheers
JDS