Lead weight with steel vs Al tanks concerns
- Thread starter jfw432
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A few people have commented on this already. I agree that there is roughly 6 lbs of air in an 80 cubic ft tank regardless of what the tank is made of. I'll give him the benefit of the doubt in maybe he was trying to say that steel tanks remain negatively buoyant regardless of the air capacity while aluminum can go positive. Either way, I understand what's going on. Just need another trip to experiment with tanks a little more before making decisions on another steel tank.
Its a very common misconception. People tend to think of bouyancy as only a postive force, and not necessarily the degree to which something is negatively bouyant. Probably because that huge sack of air on their back can make up for an absurd amount of overweightedness in their initial training, but if you're too "light" you can't hold your safety stop.
A full AL80 is negative, and an empty AL80 is positive. A full HP100 is very negative, and an empty HP100 is slightly negative. People read that into "aluminum cylinders change bouyancy more than steel", not realizing that the change in bouyancy for any amount of gas consumed is identical, its just that one crosses over the arbitrary density of water and one does not, but both still affect the diver identically for any given change in gas volume.
I don't blame anyone for missing that nuance at first, its kind of an esoteric concept as you start to build some knowledge on diving physics. I think it really starts to click once someone realizes that an AL80 + 5lbs of lead on the cam bands dives identically to most common steel tanks and/or diving a steel tank just lets you take some lead off the weight belt (provided you have some to spare).
So, I’m local to the Florida springs, and I would say *most* of my buddies are women diving single steel tanks in shorties or 3mm wetsuits. They are not overweighted. But. They also aren’t wearing any additional ditchable weight. I can swim up double HP100s from 30’ deep in a wetsuit in freshwater, swimming up a single HP80 is not a problem. I’m also a petite woman.
A balanced rig is just one you can swim up from depth with no air in your BC without ditching weight. That means, something that is a balanced rig for me might not be balanced for you. A rig that is balanced for me in a drysuit may not be balanced for me in a wetsuit. A rig that’s balanced in the ocean may not be balanced in freshwater. But it’s all subjective. If you personally can swim it up in the configuration and environment where you’d be diving it, it’s balanced for you, regardless of whether it would be balanced for someone else. What that means is your wife really needs to try it for herself and see if she’s able to swim it up herself.
You said that you use 11 pounds of lead when diving an AL80, and were using 3 pounds of lead in the HP80, and were still overweighted. I suspect you are using too much lead in both configurations.
An HP80 is about 6 pounds more negatively buoyant than an AL80. An HP80 is -8 pounds when full, and -1.74 pounds when empty. An AL80 is -1.8 pounds when full and a positive 3.2 pounds when empty. That means you need six pounds less weight when diving the HP80, all other factors equal.
In other words, if you’re properly weighted in the AL80 at 11 pounds, you should need about 5 pounds in the HP80. That that is still too heavy suggests that your 11 pounds in the AL80 is also probably too heavy. If you’re actually properly weighted in the HP80 with no additional weight, then you should need about 6 pounds in the AL80 in freshwater in a 5mm, and that sounds roughly accurate to me for experienced divers. It is not possible for you to need *both* no weight in the HP80 and 11 pounds in the AL80 if everything else is identical.
At 1800psi in the tank, you still had about 40cf of air in your tank. That means your tank was still almost five pounds negative when you let the air out of your BC and felt like you were dropping. That’s twice as negative as a full AL80 for reference; the discussion above about steel tank’s buoyancy is correct, but if you’re used to diving aluminum you’re probably used to your tank being neutral when it’s about half empty. A steel tank will still be quite negative, and that can be a surprising feeling if you aren’t used to it. Especially if you’re overweighted.
Tl;dr: Yes, it’s safe to dive single HP80 tanks in a 5mm wetsuit for the vast majority of people, but you need to be sure you’re properly weighted. And for a lot of people, that means no additional weight and ensuring you’re diving a balanced rig.
A balanced rig is just one you can swim up from depth with no air in your BC without ditching weight. That means, something that is a balanced rig for me might not be balanced for you. A rig that is balanced for me in a drysuit may not be balanced for me in a wetsuit. A rig that’s balanced in the ocean may not be balanced in freshwater. But it’s all subjective. If you personally can swim it up in the configuration and environment where you’d be diving it, it’s balanced for you, regardless of whether it would be balanced for someone else. What that means is your wife really needs to try it for herself and see if she’s able to swim it up herself.
You said that you use 11 pounds of lead when diving an AL80, and were using 3 pounds of lead in the HP80, and were still overweighted. I suspect you are using too much lead in both configurations.
An HP80 is about 6 pounds more negatively buoyant than an AL80. An HP80 is -8 pounds when full, and -1.74 pounds when empty. An AL80 is -1.8 pounds when full and a positive 3.2 pounds when empty. That means you need six pounds less weight when diving the HP80, all other factors equal.
In other words, if you’re properly weighted in the AL80 at 11 pounds, you should need about 5 pounds in the HP80. That that is still too heavy suggests that your 11 pounds in the AL80 is also probably too heavy. If you’re actually properly weighted in the HP80 with no additional weight, then you should need about 6 pounds in the AL80 in freshwater in a 5mm, and that sounds roughly accurate to me for experienced divers. It is not possible for you to need *both* no weight in the HP80 and 11 pounds in the AL80 if everything else is identical.
At 1800psi in the tank, you still had about 40cf of air in your tank. That means your tank was still almost five pounds negative when you let the air out of your BC and felt like you were dropping. That’s twice as negative as a full AL80 for reference; the discussion above about steel tank’s buoyancy is correct, but if you’re used to diving aluminum you’re probably used to your tank being neutral when it’s about half empty. A steel tank will still be quite negative, and that can be a surprising feeling if you aren’t used to it. Especially if you’re overweighted.
Tl;dr: Yes, it’s safe to dive single HP80 tanks in a 5mm wetsuit for the vast majority of people, but you need to be sure you’re properly weighted. And for a lot of people, that means no additional weight and ensuring you’re diving a balanced rig.
passeparici
Contributor
Not always. If you have a 8 mm wetsuit plus a 2 mm vest when it is cold and you go quite deep, you might not be able to swim up from depth with no air in your BCD but still have a balanced rig (quite neutral at the safety stop with no air in your BCD). It can also happen with less thickness with your suit...
LI-er (apparently pronounced Liar) just admitted he was a much banned troll and got banned again.
This is an interesting discussion. I usually dive HP100 and recently started using Sharkskin instead of 3mm in the Springs. With 500psi I am about 2-3 pounds heavy, which I find an acceptable amount to swim up if needed. I also have AL80&100’s but the steel give me nice flat trim.
Personally I have not had a bladder failure since 1976 when my atpac blew out, I feel the risk is mitigated but there is still a risk.
Personally I have not had a bladder failure since 1976 when my atpac blew out, I feel the risk is mitigated but there is still a risk.
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