the change in buoyency in an aluminum and steel tank of the same size is exactly the same... air weighs the same whether it is inside of steel or aluminum. the only thing is that steel tanks do not become positive
Pros and cons - steel tank vs aluminum?
- Thread starter NJMike
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OP
NJMike
Contributor
Rick, you said:
"If it takes 20 pounds of lead to sink a 7mm wetsuit at the surface (mine takes 22) then at 100 feet you'll need nearly 20 pounds of lift from the BC to counter the lift you've lost from compressing the wetsuit."
Sorry, don't know how to do that quote thing yet...
So, at 100', using your numbers, I would be neutrally bouyant, right?
Then you say:
"In addition to compensating for the wetsuit, you must also be able to compensate for the change in weight of your tank - about five pounds of gas used on a dive with an AL80, for example. In order to be able to control your buoyancy at the end of a dive near the surface, you must carry enough weight at the beginning of the dive to compensate for the wetsuit and the change in weight of the tank... "
I understand this. And quite honestly, I discovered (learned?) this on my last dive, which was my first in a long time...
Here's where it starts to get a little murky (dont' excuse the pun).
You say:
"Let's run some numbers, ok...
Item................Buoyancy
Me.......................+3
Wetsuit...............+20
Steel 72 (500 psi)...-3 (with regulator)
Adding those up algebraically, the weight I need to carry so I can be neutral at the end of the dive, near the surface with only 500 psi left in my tank and with no air at all in my BC = +3+20-3 = 20 pounds."
I'm okay with this...but
Then, you say
"But, at the beginning of the dive, my Steel 72, full, is -8 pounds, so to be neutral at the surface I'll have to carry 5 pounds of positive buoyancy (air) in my BC."
If your tank is -8 lbs bouyant, wouldn't you need 8 lbs of air to be neutrally bouyant?
You continue,
"As I descend, my wetsuit will compress and get less buoyant, for all practical purposes becoming neutral itself by 100' or so, where the negatively buoyant rubber is just balanced by the compressed nitrogen bubbles in it, and I'll need to be able to add another 20 pounds of positive buoyancy to the BC to stay neutral, a total of 25 pounds."
If the suit is compressed enough at 100' to offset the bouyancy of the suit, why would you need to add another 20 lbs of air to stay neutral?
"If it takes 20 pounds of lead to sink a 7mm wetsuit at the surface (mine takes 22) then at 100 feet you'll need nearly 20 pounds of lift from the BC to counter the lift you've lost from compressing the wetsuit."
Sorry, don't know how to do that quote thing yet...
So, at 100', using your numbers, I would be neutrally bouyant, right?
Then you say:
"In addition to compensating for the wetsuit, you must also be able to compensate for the change in weight of your tank - about five pounds of gas used on a dive with an AL80, for example. In order to be able to control your buoyancy at the end of a dive near the surface, you must carry enough weight at the beginning of the dive to compensate for the wetsuit and the change in weight of the tank... "
I understand this. And quite honestly, I discovered (learned?) this on my last dive, which was my first in a long time...
Here's where it starts to get a little murky (dont' excuse the pun).
You say:
"Let's run some numbers, ok...
Item................Buoyancy
Me.......................+3
Wetsuit...............+20
Steel 72 (500 psi)...-3 (with regulator)
Adding those up algebraically, the weight I need to carry so I can be neutral at the end of the dive, near the surface with only 500 psi left in my tank and with no air at all in my BC = +3+20-3 = 20 pounds."
I'm okay with this...but
Then, you say
"But, at the beginning of the dive, my Steel 72, full, is -8 pounds, so to be neutral at the surface I'll have to carry 5 pounds of positive buoyancy (air) in my BC."
If your tank is -8 lbs bouyant, wouldn't you need 8 lbs of air to be neutrally bouyant?
You continue,
"As I descend, my wetsuit will compress and get less buoyant, for all practical purposes becoming neutral itself by 100' or so, where the negatively buoyant rubber is just balanced by the compressed nitrogen bubbles in it, and I'll need to be able to add another 20 pounds of positive buoyancy to the BC to stay neutral, a total of 25 pounds."
If the suit is compressed enough at 100' to offset the bouyancy of the suit, why would you need to add another 20 lbs of air to stay neutral?
OP
NJMike
Contributor
Oh, and as far as the wing/ SS plate...I know less about these than I do about the other stuff...
For right now, I'd like to get some equipment to get started...safely, then learn more about the other stuff while I'm having some fun...
My original question was ... if I need 16 lbs of weight to make me neutral at the surface in a 5mm suit...then in a 7mm suit I'm going to need some more weight...LDS says 2-4 lbs.
Which would then max out the weight carrying capability of the BC. And now that Rick has opened up my eyes about the increased bouyancy due to the used gas...if that's 5 lbs, then ... in the circumstances that I described, I would need a belt with another 5 lbs. so that I can be neutral at the safety stop.
Now, here's another question: I was using a AL tank. If I used a steel tank, UNDER THE SAME CONDITIONS, would I be able to carry less weight?
I thought the answer was yes but now I'm not so sure...
For right now, I'd like to get some equipment to get started...safely, then learn more about the other stuff while I'm having some fun...
My original question was ... if I need 16 lbs of weight to make me neutral at the surface in a 5mm suit...then in a 7mm suit I'm going to need some more weight...LDS says 2-4 lbs.
Which would then max out the weight carrying capability of the BC. And now that Rick has opened up my eyes about the increased bouyancy due to the used gas...if that's 5 lbs, then ... in the circumstances that I described, I would need a belt with another 5 lbs. so that I can be neutral at the safety stop.
Now, here's another question: I was using a AL tank. If I used a steel tank, UNDER THE SAME CONDITIONS, would I be able to carry less weight?
I thought the answer was yes but now I'm not so sure...
Yes, with a steel tank you carry less weight. For example, the typical AL80 is about 3 pounds positive at 500 psi, while the typical steel 72 is about 3 pounds negative - so you can carry about six pounds less lead with the steel 72 than with the AL 80.NJMike:
That does not change the lift requirement for your BC though
Rick
Because you're still carrying the 20 pounds of lead you needed to sink the suit in the first place. The suit's no longer holding that lead up, so the BC has to do itNJMike:
Rick
What we're concerned with is the change in buoyancy from the beginning to the end of the dive. When I figured weighting for the end of the dive I already compensated for the tank's -3 pounds of buoyancy, so at the beginning, with the tank having -8 pounds buoyancy, the difference of 5 pounds must be supplied by the BC.NJMike:
Rick
NJMike:
Gads, someone hand me the Kool-aid because I can't believe I'm making this statement:
NJMike - the difference in going to the wing/12# stainless back plate vs. the setup your talking about is that with the wing/ss you are eliminating the need for 12# of lead on you from the start, and with a few pockets tossed on, you've eliminated the weight belt to. The whole set up is incredibly simple to use, intuitive in fact, and with the cold water gear your talking about donning, it will be considerably easier to 'adjust to fit' and fine tune.
The steel tank further eliminates the need for lead as it doesn't have a tendency to float.
I am an example of this. In my profile is a pic of me 2 years ago in a jacket BC, Mares. I'm diving a standard AL tank and have approximately 12# of lead on.
Currently I dive an Aluminum backplate/wing and steel tanks. In warm water, even a single steel 72, I use NO weight. With an AL 80 I wear a maximum of 6#, depending on the tank... average of 4#. That 4-6# is to offset the buoyancy the AL tank has at the end of the dive (I'm talking 500-700#). That is because the AL tank floats displaces water at approximately +4-5# and wants to go up, so I compensate by having that weight integrated into my rig somewhere.
I've always been a bit of a minimalist diver, this just made it MUCH easier and immensely more comfortable.
As a matter of fact in the European Union alu tanks are legal only for 10 years. Older ones cannot be used. This rule though does not apply to steel tanksWalter:
Mania
The 20 lbs mentioned is the capacity of the BCD weight integration, not the lift capacity..
I would think that a jacket BC would have more than 20 lbs of lift... Most jackets lift capacity vary with the size of the BCD...
If you switch to a BP/W you not only add the weight of the plate, etc. you can also subtract the buoyancy of the material of the jacket BC.
For example, when I switched from a jacket to a 6 lb SS plate, I was able to remove 10 lbs from my weight belt. The additional 4 lbs was from all the material / padding on the jacket, which is not present on a BP/W.
I would think that a jacket BC would have more than 20 lbs of lift... Most jackets lift capacity vary with the size of the BCD...
If you switch to a BP/W you not only add the weight of the plate, etc. you can also subtract the buoyancy of the material of the jacket BC.
For example, when I switched from a jacket to a 6 lb SS plate, I was able to remove 10 lbs from my weight belt. The additional 4 lbs was from all the material / padding on the jacket, which is not present on a BP/W.
Here are a couple of references on tank specs. Note the weight of a steel 100 is pretty close to an aluminum 80.. without being positive at the end of the dive.. (will carry more gas, so the weight full will be a little different - air weighs approx .083 lbs per cu. ft.)
All things considered, different sites will have different specs for the same cylinders.. I will list a few that I used for comparison.
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html
http://www.diveremporium.com/tankspecifications.html
http://www.techdivinglimited.com/pub/tanks.html
Great reading on what to consider when buying a tank..
http://www.diveriteexpress.com/library/tanks.shtml
Even with all the information in hand, it will be good to "test dive" the tanks you are thinking about **before** you spend your hard earned $$$...
I ended up buying 2 low pressure steel 108's... Love them!
-Dennis
All things considered, different sites will have different specs for the same cylinders.. I will list a few that I used for comparison.
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html
http://www.diveremporium.com/tankspecifications.html
http://www.techdivinglimited.com/pub/tanks.html
Great reading on what to consider when buying a tank..
http://www.diveriteexpress.com/library/tanks.shtml
Even with all the information in hand, it will be good to "test dive" the tanks you are thinking about **before** you spend your hard earned $$$...
I ended up buying 2 low pressure steel 108's... Love them!
-Dennis
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