Long but shallow profile, what kind of training do I need

[eelnoraa]

With single tank, my bottom time is limited by gas mostly. With double, I found my bottom time is limted by MDL if the dive site is at the 70-90 ft range. Let's say I have no plan to go below 100ft, what kind of training/class should I take to gain deco knowledge? GUE R3 says 130ft/15 minutes and T1 says 170ft/30min. But what if I want to do 100ft/60minutes or 80ft/80min kind of dive, will I be able to do it with R3 knowledge? or do I need T1?

 

[rjack321]

T1, really. 60 mins at 100ft on 32% is about 30mins of backgas deco and 10-12mins on 100%. But R3 only allows you 32% as a deco gas. So basically you end up with 30mins of deco.

(disclaimer, I have not read the GUE R3 standards in a long time)

 

[PfcAJ]

I'd go for T1. At a minimum, your card will say 50% and 100% o2, and you really need o2 for a dive like that. Any other gas is sub-optimal for that kind of profile. During class, I would definitely make your goals clear to the instructor.

 

[decompression]

For those type of profiles I looked closely at the best types/mixes of gas, single or multi deco bottles, cns% and otu's.
It's been awhile since I've had a look at GUE standards but I'm familiar with TDI, ANDI and UTD. Most of the "tech 1" courses would work, as they give you capacity (you're in doubles already) and deco (50-100%). Helitrox (UTD rec 3) would give you a very desirable mix used with a single 50% deco bottle. If you're in contact with an instructor, let him/her know your goals and they should be able to come up with a plan. For your own info, you can cut your profile (ideco or v planner etc) and see the details then checkout The websites of the courses and see which interest you.

You could also do min deco on back gas but that adds a lot of deco time so instead of a 60 min dive it could be a 90+ min dive.

 

[PfcAJ]

Switching to 50% at 70ft following a 100ft dive is kinda silly...

 

[rlynch]

T1, do a switch to 100% for deco.

 

[decompression]

Not do diverge from the original post, the difference for a 100' dive on 32% and doing deco on 50% or 100% is a few minutes of deco time, a minor rise in OTU's and a slight drop in CNS %. I do like the ability of accessing my deco gas at 70' versus 20' in case of an emergency. There are MANY ways of doing this profile, it's a case of what fits best for the op as far as training is concerned. Maybe give the reason why a switch to 50% is silly and how it would affect the op's decision. That could be helpful.

 

[PfcAJ]

Deco time with 50% is about 1.5x the time for o2 (60min at 100ft, 32% bottom gas, VPM+2), with 16mins for oxygen vs 25mins for 50%. Up the dive time to 80mins and you end up with 23mins of deco with o2 and 36 mins of deco with 50%.

Oxygen is almost always a superior choice for deco, as it has no inert gas. From a 100ft dive with anything but the most absurd bottom times, your ascent doesn't really slow down a whole lot till 40-30ft, so why dillydally deeper (on-gassing) if you don't need to? Getting off the bottom and onto a gas with NO inert component just makes sense. Adding 50% makes sense if you're doing a 21/35 and up type dives as it really helps with min gas requirements and gets you onto a richer gas in a point in the ascent where it makes sense. Even then, I prefer to end the dive on o2 because of its lack of inert gases for all but the shortest of bottom times (20-30min).

 

[decompression]

Thanks PfcAJ, you have a wealth of experience that I like to learn from. Appreciated.

 

[decompression]

From ideco;
100' for 60 min on 32% with 50% deco;
79 min dive time, 40% CNS, 87 OTU'S

With 100% deco
76 min dive time, 45 % CNS, 94 OTUs

With no deco gas, deco time is 25 minutes for a total of 85 minutes.

Off gassing starts at 60'

I don't have V planner on my phone but with the above profile I'd choose 50% for the sake of a few minutes and the safety. PfcAJ, for my info, is your focus on inert gas a SOP for all dives, regardless of depth/time? Thanks. I might have to take a deeper look (excuse the pun).