One Long Deco Dive vs Two Shorter Deco Dives with a longer total bottom time?

[lukeb]

One Long Deco Dive vs Two Shorter Deco Dives with a longer total bottom time?

(I have previously asked a similar question in another unrelated thread, but it was more focused on the boat charter aspect of this comparison. I wanted to expand on it with regards to the actual dive plans themselves, so here goes....)

Since entering the world of deco diving, I keep coming up with the same dilemma when planning dives:

Do I use a larger set of doubles, and plan for a longer single decompression dive.

Or, do I use smaller doubles (e.g ALU 80s), and plan for two slightly shorter deco dives, with a suitable length surface interval, during which I change into a brand new set of tanks.

While there are clearly a lot of benefits to the single long run time dive (one set of tanks, less total deco time, longer single dive duration bottom time). There is however, noticeably longer total bottom dive time by doing the two dive approach.

It seems to me that if you don't mind the hassle of two sets of deco stops and changing tanks, you are rewarded with more bottom time (provided your plan allows a suitable SI).

I have played with this on MultiDeco simulating dive profiles on several wrecks, and it seems to work out the same each time (planning for depths in the 120 - 150 range).

I was curious as to others thoughts on these different approaches as I am aware that most tech dives follow the single long dive approach. What are the pros and cons? Do any of you regularly dive these kind of profiles? At what point does it become more beneficial to plan for the single longer dive?

Note, the dives I am looking at are using Nitrox / air for back gas with a single deco gas (100% O2). I understand that with Trimix and deeper deco dives the single long runtime is probably the only realistic dive planning option. This question specifically relates to lighter technical dives at shallower depths, (less than 1 hour runtime per dive).

Thanks for your input, and just to clarify I am still a newbie deco diver who is simply trying to learn the optimal way to get the most bottom time I can!

 

[tbone1004]

this is a subject that @PfcAJ and @kensuf have both posted about many times on here. One thing that you have to pay attention to in the ocean is the volatility of surface conditions that may need you to get out of the water in a hurry. For that risk factor alone, one long dive is not advantageous since you have a longer individual deco time

 

[lukeb]

Thanks, I hadn't seen it talked about previously. I will do a search for their posts.

 

[runsongas]

with good surface conditions, no logistical hurdles, and a suitable dive site or pair of dive sites, a double dip will get you more bottom time.

the issue is you won't always meet those conditions. you may not be able to get a refill during the SI or be allowed to bring enough tanks. the boat might have a tight schedule it has to keep that won't allow it. the site might not be conducive to a double dip because of a tide or current timing requirement.

 

[100days-a-year]

My typical day consisted of 5 to 7 single tank short deco < 30 MIN RT dives in 120' to 180'. It was hard to justify diving one two or even three longer dives as I got more return on my time by having less deco total and more BT on a given day. Even if I did my deco on backgas.

 

[Celt]

With a single dive you can pick the best time to make the dive in a 12 hour period and if that happens to be low water it will increase your bottom time with less deco and you only have to go there once.

 

[lermontov]

ability to carry x amount of gas is one restraint and personally id do two dive just fro sheer pleasure and less stress. After the first dive which is often a orientation dive you can then go back and focus on a specific area

 

[rjack321]

I would strongly advise switching from VPM to some other model for repetitive dives

 

[runsongas]

@rjack321 I don't think anyone is using/suggesting VPM in this thread

 

[bubblemonkey2]

I can't think of any time when this was the primary deciding factor in planning dives. Are there not other interesting factors that have more influence on the dive planning?

In Truk/Chuuk our schedule (as arranged by the local guides) was one solid deco dive in the morning, back for lunch/fills, and another one later. I.e. your "scenario 2"

If we're diving independently though incl. shore dives, it's almost always "scenario 1," one solid dive (two hours typical) to do whatever we're after and then call it a day. Tides and conditions are only good in certain windows anyway, and who wants to haul all that gear down the beach on anything but a high tide, or more than once? I wouldn't even leave extra cylinders in my vehicle, due to the risk of breakins. Insurance deductibles are usually $500, if they even cover it, and meanwhile your sh!t is broke and missing.