Why a 120 tank?

[guyharrisonphoto]

For deep dives (130), I like 120s for extra gas in the event I have to share gas on the way up. Just look at a "rock bottom" gas management forumla and you can see the more gas the better!

Otherwise, I like really long bottom times on shallower dives.

 

[fdog]

As an opposing thought, since you are diving HP 100's already, have you considered a HP130? My experience is that they dive just like a 100 as far as trim goes, the two are quite indistinguishable underwater (except for the weight swing from gas). IIRC they're only 10 lbs heavier above water which hasn't been a big difference for me (or my petite wife, either).

If you're doing square-profile wreck dives, where you're NDL limited, it certainly is easy to get into too big of a cylinder. Shore diving is a different story and where a bigger cylinder shines.

I'd crunch the numbers for your working bottom gas needs, and, rock bottom. I wouldn't be able to make a strong case for a cylinder that exceeded that total.


All the best, James

 

[rhwestfall]

I'd crunch the numbers for your working bottom gas needs, and, rock bottom. I wouldn't be able to make a strong case for a cylinder that exceeded that total.

Side note:

As the OP said, his PDC was dancing between "remaining gas" and "NDL" as the controlling factor. I'm wondering what the gas plan was.....

 

[maniago]

Side note:

As the OP said, his PDC was dancing between "remaining gas" and "NDL" as the controlling factor. I'm wondering what the gas plan was.....

It was a square profile as we were wreck diving, so the bounce between the numbers was the expectation, along with not MODing out....not really sure what other kind of profile youre going to do 25miles out in the ocean.

 

[victorzamora]

I think there is a disconnect occurring here. I believe the difference is had planning vs dive profile

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[rhwestfall]

It was a square profile as we were wreck diving, so the bounce between the numbers was the expectation, along with not MODing out....not really sure what other kind of profile youre going to do 25miles out in the ocean.

for me, the words "rock bottom" come to light in gas management/dive plan.... profile is as I would have expected

 

[maniago]

I think there is a disconnect occurring here. I believe the difference is had planning vs dive profile

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Hey, I'm totally about learning, so if I'm doing something blatantly wrong, I want to know.

To be explicit, I used the rule of thirds for gas - freefall to depth, then third out from the anchor line, third back, third up the line with deep and safety stops. Anything left over above 250/300 was also burned at 15' just because. And there was plenty. Am I forgetting something?

 

[BluewaterSail]

I never heard of that rule of thirds. The one that I am familiar with is 1/3 out, 1/3 back, 1/3 for emergencies. What if you had to share gas from the furthest point, would you have had enough for 2 people, who would probably be breathing heavier than normal?

 

[fdog]

Thirds certainly is a valid strategy in some dives. However, it is not always applicable - and a deep square profile dive is one of them.

The objective is usually to get yourself, and your buddy, back to the surface under emergency conditions. This is commonly calculated as a RMV of 1.0 cfm for each diver. Because it's deep, you want to have an ascent that won't bend you.

Just off the top of my head (battlefield calculations), time to ascent from 120' would be (1 minute to deal with the emergency)+(2 minutes to 60')+(1 minute at 60')+(1 minute at 50')+(1 minute at 40')+(1 minute at 30')+(1 minute at 20')+(1 minute at 10')= 9 minutes.

120' is (round numbers) 5 ATA. Surface is 1 ATA. 5+1=6 Sooo...6x9=54 cubic feet that needs to be kept in reserve to get 2 divers to the surface. This would be (54/3) x 100 =1800 psi.

If you search for Rock Bottom you'll find a wealth of information. Hope this helps.

All the best, James

 

[jcaplins]

I never heard of that rule of thirds. The one that I am familiar with is 1/3 out, 1/3 back, 1/3 for emergencies. What if you had to share gas from the furthest point, would you have had enough for 2 people, who would probably be breathing heavier than normal?

I believe the "rule" Maniago is referring to is just a general guideline for recreational, non-penetration, no mandatory deco stop, open water diving where the "furthest point is a max of a 130' straight up to the surface.

I believe the rule of 3rd's you are referring to is more of the technical, possibly penetration, possibly mandatory deco stops, rule that requires a lot more gas planning for emergencies where the surface may be more than 130' away and most likely not straight up.