Large tanks question

[Dirty-Dog]

On a related note...
We're diving with Aldora in Dec. and I'll be taking advantage of their HP120's. However, I'm unclear on the weighting issue. Yes, I know how to do a check, but I like to plan ahead and at least get close.

How much weight do people typically drop from their belts when going from an AL80 to a steel HP80 (shore dives) HP100 (the ladies) and HP120 (the group hoover - me)?

Diving from the mainland in May, I wore 10lbs with a dive skin. I assume the waters might be a bit cooler in Dec, perhaps mandating a change to my 3/2 full. How much difference is there in water temps from Dec to May?

If an AL80 is -1.8 full and +3.2 empty, and I change to an HP120, which is -11 full and -2 empty, I'm at -11.8 full and -6.8 empty using the AL80. Changing to the steel, does this mean I'd need about 5lbs to match the empty bouyancy?

How much more bouyant is a 3/2 full likely to be?

 

[Tunaman68]

Here in Cali, I drop 5 lbs when moving from an al80 to my HP steel 100. Some will say up to 7-9 pounds, but I would imagine 6-7 lbs for a 120. However, I'm not an expert and Dave will have a lot of experience with this!

 

[JBRES1]

Okay, I already know the answer, but I'm asking for a friend...

What exactly are the differences...

There are AL80s and AL100s.... I'm figuring the AL100 has more air... how much more, how much more bottom time?

There are steel 120s.... I'm guessing these are the biggest most capacity available? More capacity then an AL100? How much more?

Let's say on an AL80 our hypothetical diver gets 40 minutes, what would you get with the AL100 and the Steel 120?

Mike,

In rough numbers
80 cu ft cylinder = 40 min dive
100 cu ft cyliner = 51 min dive
120 cu ft cyliner = 62 min dive.

These numbers are all based on your 40 min. / al80 cylinder.
Also these numbers are for the same profile dive and depth.
note: that on some dives you run out of NDL time before you run out of gas.



Jim breslin

 

[Ripple in still water]

I never quite got the "aluminum tanks suck because they are positively buoyant when low" thing. Seems to be me it's all about buoyancy swing, and that should be the same (i.e., the weight of air consumed) whether the tank is aluminum or steel. So I think it pretty much comes down to where you like the weight, on the tank or on your belt (or in your BCD pouches). I've got less than 100 dives, and my taste may evolve, but for now I like the weight close to my body in my BCD pouches.

That said, if the choice is big steel tank or little aluminum tank, I would probably go with the steel. I like to breathe.

Wow. I never knew a 100AL was generally rated for higher pressure. I think my 100s were always right around 3000? As for maintaining trim, I am your 6' and then some and I find the 100AL becomes a SERIOUS floater late in the dive. I have to weight up a bunch to not be fighting at the safety stop. That being said, I finally got the the point that I can't use the 100 AL because they are all air and I keep running them into deco. I started using an 80 nitrox on the second dive and I have got to the point that I can come pretty close to keeping up with the DM on a good day. (Of course two or three times during the dive I sneak up on her and squeeze the purge on her octo for 30 second or so.....:shocked2: )

 

[cvchief]

Ripple,

I haven't dove with steel and I don't have a lot of experience either, but I have used an AL100 for alot of my dives. Pinpointing the correct weight for the 100 AL is the hard part I think for me. Most of the time too when I was using the AL100 I was also using a weight belt. That meant I had a tank and BCD going up and a weight belt going down and me in the middle. Now that I have gone integrated weight, that issue is minimized. I swiped this from Aluminum vs. Steel Scuba Cylinders :

Buoyancy

The type of cylinder you use only has one major effect on your diving: your buoyancy. Aluminum tanks are more buoyant than steel, and thus you will require more weight when diving with them. It is worth knowing what you usually dive with in order to compensate one way or the other when diving with a different cylinder.
The usual recommendation is to add about 5 lbs / 2 kg to your base weighting you get from a weighting guide to compensate for an aluminum cylinder.

Aluminum has a particularly annoying characteristic. A full steel tank is negatively buoyant. An empty steel tank is also negatively buoyant, but less so. This is why during a proper buoyancy check it is recommended to use a near-empty cylinder, or add weight to compensate. Aluminum cylinders also become more buoyant as air is consumed, but they change from being negatively buoyant to positively buoyant. This means a full aluminum cylinder will sink while an empty will float. This makes it harder to pin down a perfect weighting for the entire length of a dive.

 

[jd950]

It will not be an HP tank. The larger tanks they provide are AL 100's.

The ONLY operators on the island (aside from the tech guys) with STEEL HP tanks are Aldora, Liquid Blue and Living Underwater. Anyone else that has larger tanks available are AL100's and will range anywhere from $5 - $10 more per tank.

I still encourage divers to work on gas management vs. relying on larger tanks if that is the reason they like the larger tanks. With a properly profiled dive, most of my divers still get 60 - 70 minutes from an AL 80 - and if they aren't getting that at the beginning of the week, they are by the end of the week from the tips and help from myself and crew to help them improve their gas management!

I largely agree with that, but sometimes the extra gas is nice to have. When I use larger tanks with air I usually have my dive time time governed by my nitrogen uptake rather than my gas consumption, but using a large tank and nitrox can give some nice long dives. On Cozumel I am financially limited to two dives a day and so I usually want as much bottom time on those dives as possible. Some dive ops don't require everyone to go up together and if so, I can often take advantage of the extra gas. I don't think that translates to a gas management issue. I have also had a few Cozumel dives where the dive op felt it was a good idea to swim against the current a lot, or kept having the customers hang out in current for several minutes while the DM killed lionfish, and extra gas can be very worthwhile then, just to get a full dive.

I also like the buoyancy characteristics of steel tanks.