HP 100 Steel Tanks - What am I missing?

[jebmobile]

Worthington HP 100 steel tanks have a full buoyancy of -10.0 lbs and an empty of -2.5 lbs (a difference of 7.5 lbs). Catalina 80 aluminum tanks have a full buoyancy of -1.6 lbs and an empty of +4.1 lbs (a difference of 5.7 lbs). You still have to add weight to account for the difference between the start and end of a dive change in the tank buoyancy with either tank. Since the Catalina S80 has a smaller buoyancy change, why is it that everyone talks about steel tanks being better in terms of buoyancy? It looks like to me that you'd need 1.8 lbs more of lead with the steel tank (7.5 lbs - 5.7 lbs) than the aluminum. I've been thinking about getting a couple of more tanks (steel) instead of more aluminum tanks but I'm having second thoughts after looking at this. I've got to be missing something, what is it?

 

[mala]

the steels are -2.5 empty
the ali,s are + 4.1.

so you need 6.6 extra with an ali .
more lead and less gas.

the weight difference you are looking at is the gas weight which will be less for the ali because its smaller.

 

[decompression]

Don't think of the buoyancy change, consider that the lead weight should account for the use of air. Simply, at the end of your dive on the steel 100 the tank is still 2.5 lbs neg, while the AL 80 is 4 lbs pos. if you've balanced your rig (f you're unsure of this term do a quick search), the 2.5 lbs is lead that you do not require to add on your belt or elsewhere, with the AL80 the 4.5 lbs must be added in lead to account for the loss(used) of gas. Make sense?

 

[pickens_46929]

Because they start out negative and remain negative, you are able to remove weight from your weight belt or weight pockets.

A steel 80 has the same swing in weight (roughly 6#) as an aluminum 80 does due to the weight of the air inside of it.

A good chart can be found at Tech Diving Limited: Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400

You are comparing an 80 cu.ft. tank with a 100 cu.ft. tank. 20 cu.ft. of compressed air weighs about 2# which explains the difference.

You are correct though in pointing out that the downside to larger capacity tanks is their wide swing in weight difference from full to empty.

Hope this clears things up.

Dave

 

[jebmobile]

Thanks all. That clears things up. Get it now.

 

[Splitlip]

Don't think of the buoyancy change, consider that the lead weight should account for the use of air. Simply, at the end of your dive on the steel 100 the tank is still 2.5 lbs neg, while the AL 80 is 4 lbs pos. if you've balanced your rig (f you're unsure of this term do a quick search), the 2.5 lbs is lead that you do not require to add on your belt or elsewhere, with the AL80 the 4.5 lbs must be added in lead to account for the loss(used) of gas. Make sense?

He is essentially saving or dropping 7 # of lead by moving from an al 80 to a 100 steel..

 

[socaljohn]

I found a chart that shows you how to adjust your weight based on what tank you are using here

http://knows-dive.com/scuba-diving-tips/safety/weights-estimation/

It is base lined to a ST HP 120 but knowing that I add 4 pounds if I rent a AL80 from my normal ST LP 72, I was able to convert the table to baseline ST LP 72.
BTW I am trying a ST LP 85 Fri and Sat and looking forward to that.

Happy Diving!

 

[diveprof]

He is saving more than weighting. The Worthington steel 100s are shorter than AL80s (I like that) and a steel 100 provides more gas allowing one to take better advantage of Nitrox.

 

[redacted]

13.5 cu ft of is about 1 pound regardless of what tank you put it in. So the larger the tank, the greater the swing in buoyancy between full and empty. If you are using a fair amount of weight on your belt, a less buoyant tank helps take some of that weight off while a more buoyant tank will require more added weight. In some conditions, any steel tank may be too heavy for some divers. But in most conditions with most divers, using an Aluminum tank means adding more lead.

 

[jebmobile]

With an AL80, I use 4lbs of lead in saltwater and 8lbs if I have a fully 3mm suit on. That's probably below average compared to many. Would a steel HP100 be too heavy?