Weighting question

[knfmn]

Hello all,

I'm in the process of buying my first set of gear and just got my BC and my first tank the other night. The tank is a steel 120, which I've never dove with before. I've got a dive lined up Saturday and have to rent my weights, so I was hoping for some help on getting my weighting right. Here's the calculations I've been able to find. Body Weight X 0.5. 4Lbs for an AL80. 1 lb for every mm of thermal protection. Unfortunately, I'm not sure what to do with a steel tank. Do I take off the 4 Lbs since they are still negative when empty?

Thanks so much for your help,

Kristopher

 

[Pauly854]

I'm not sure what formula you are using, but multiplying by .5 is half your weight. So if you are a 200 pound guy, you are starting off with 100 pounds on your weight belt...not to mention the tank and neoprene. Or did I misread this in some way?

 

[knfmn]

I'm not sure what formula you are using, but multiplying by .5 is half your weight. So if you are a 200 pound guy, you are starting off with 100 pounds on your weight belt...not to mention the tank and neoprene. Or did I misread this in some way?

Oh, shoot! I'm not supposed to be diving with 115 pounds on my weight belt? No wonder my back hurts so bad when I get done!

I think that .5 was supposed to be .10. That is much more like it....lol.

Kristopher

 

[ajtoady]

I dive Faber Blue Steels @117 cuft and take 6 lbs off due to the steel vs aluminum. To be sure though I went to a pool and dressed in wetsuit, hood, boots, etc. and did a check before I went and did my Ice Cert. dives.

 

[Teamcasa]

There is no perfect rule but one thing is certain, that 120 is a big, heavy tank. I would start with the weight you used when you did you first proper weight check and given the same other gear configuration, take off a two - four pounds and do another proper weight check.

 

[knfmn]

I dive Faber Blue Steels @117 cuft and take 6 lbs off due to the steel vs aluminum. To be sure though I went to a pool and dressed in wetsuit, hood, boots, etc. and did a check before I went and did my Ice Cert. dives.

That's actually the kind of tank I just bought.

Teamcasa,

I'm doing a really minor dive on Saturday and will do a weight check as you suggest when I go it. I just don't want to be so overweighted that I jump in and sink right away, you know?

Thanks,

Kristopher

 

[Insta-Gator]

This isn't perfect but it will get you started. Enter your own specs on the spreadsheet.

 

[Colliam7]

The tank is a steel 120, which I've never dove with before. I've got a dive lined up Saturday and have to rent my weights, so I was hoping for some help on getting my weighting right. Here's the calculations I've been able to find. Body Weight X 0.5. 4Lbs for an AL80. 1 lb for every mm of thermal protection.

You have already addressed the 0.5 vs 0.1 issue. A not uncommon starting point for many new OW divers is 10% of body weight. While that turns out to be too much for more than a few divers after they get past the initial 'hyperventilation' stage of diving, it doesn't hurt to start there. Better to rent more weight than you need, than to get to the dive site and not have enough.

It is not clear from your post if you already have some feel for your weighting already. If so, it might help to know that, as a basis for comparison.

Unfortunately, I'm not sure what to do with a steel tank.

An AL80 is ~1.4 lb negative when full, and ~4.4 lb positive when empty. At 500 psi, therefore, it is closer to 4 lb positive (actually probably closer to 3.5), probably the basis for the recommendation you cited for adding 4 lbs for an AL80. On the other hand, a HP120 is ~10.5 lb negative when full, and close to neutral when empty. Therefore it is probably slightly negative at 500 psi. But, for the sake of discussion, consider the tank to be neutral when at 500 psi.

Do I take off the 4 Lbs since they are still negative when empty?

That would be a reasonable starting point, based on the relative buoyancy of an AL80 when empty vs a HP 120 when empty. Not precise, but functional as a beginning.

Since your question addresses how much weight to rent, at least that is my understanding of your post, you might even rent weight to cover an AL80, then adjust your weighting to fit actual conditions.

 

[Brendon]

Oh, shoot! I'm not supposed to be diving with 115 pounds on my weight belt? No wonder my back hurts so bad when I get done!

I think that .5 was supposed to be .10. That is much more like it....lol.

Kristopher

I am 235lbs, I dive with 3mm + boots and gloves, al80, weight at 16lbs that is more then enough. Even In a crushed neoprene drysuit I only wear 26lbs.

So where are you getting that formula...? You know there is a bunch of info on this site to help you calculate the weight you might need. I definitely would not go with 10% + 4lbs. Unless the boat captain plans on using you for an anchor...

 

[MauiScubaSteve]

An AL80 is ~1.4 lb negative when full, and ~4.4 lb positive when empty. At 500 psi, therefore, it is closer to 4 lb positive (actually probably closer to 3.5), probably the basis for the recommendation you cited for adding 4 lbs for an AL80. On the other hand, a HP120 is ~10.5 lb negative when full, and close to neutral when empty. Therefore it is probably slightly negative at 500 psi. But, for the sake of discussion, consider the tank to be neutral when at 500 psi.That would be a reasonable starting point, based on the relative buoyancy of an AL80 when empty vs a HP 120 when empty. Not precise, but functional as a beginning.

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

Luxfer 80 .......... 3000 .... 77.4 .... 7.25 ....... 26.06 ....31.38 ..... 4.40 .... -1.4
Luxfer S80 ........ 3000 .... 78.2 .... 8.00 ....... 22.93 ....35.12 ..... 2.26 .... -3.6
Catalina S80 ...... 3000 .... 77.4 .... 7.25 ....... 25.80 ....31.60 .... 4.00 .... -1.8
Catalina C80 ...... 3300 .... 77.4 .... 7.25 ....... 25.10 ....34.40 ... -0.20 .... -5.9

According to the linked specification page, the two most popular AL80's are not exactly the same, and some AL80's are way different.

Rodale's Scuba Diving Tank Chart

Looking at the 2 linked charts, there is also a wide variety of buoyancy characteristics of HP120's as well. I own Asahi 120's, which are ~3 lbs negative empty. Faber's non-LP steel 120 is 7.7 lbs negative empty, but that is not really an HP tank (3180 psi). Worthington's (119 & 120 cft) are 2 lbs negative empty.

What tank(s) are we talking about?