Cylinder type and proper weighting (SPLIT FROM 'Pull Dumps - Lose them')

[Dan]

When i dive in warm water, i use aluminum tanks.

I pack about 2-2,5 kg of lead.

If i change one AL80 to an Steel 12l i need to drop about 4,5kg of weight.

Where can i get the extra 2-2,5 kg i need to drop??

You just keep some air in your BCD to counter the extra weight & have a backup SMB ready to deploy when your BCD won’t provide enough lift.

 

[Magnus Lundstedt]

You just keep some air in your BCD to counter the extra weight & have a backup SMB ready to deploy when your BCD won’t provide enough lift.

No thank you! I think I will keep trying to dive balanced.

 

[Bob DBF]

Agree. The steel tank is less buoyant than the AL, so you can use less lead. But you're saying your AL and steel tanks are similar in weight (you didn't say exactly the same). They are also similar in size. That would mean you would have to use about the same amount of lead. I could be missing something here regarding the properties of AL vs. steel. But I don't think so if they are similar in size. An AL tank is less buoyant even though the metal is thicker than that of a steel tank. But again, you said they weigh the same.

Weight, size, material and pressure vary between tanks, however the only thing that makes a difference for the amount of lead on your belt us the actual buoyancy in the water. As we were discussing before, carrying less weight across the beach is an added attraction.

As shown below the same weight of steel can hold the same amount of gas at a higher pressure and in a smaller container, this makes it less buoyant and helpful for us that wear thick wetsuits as we can leave some lead at home.

A Catalina C80 weights 31.6# and +4# buoyancy empty. ( Size 26 X 7.25)

A Faber MP 80 weights 32# and -7# buoyancy empty. ( 20 x 7.25 )

The tank weights are almost equal but you can leave almost 11# of lead in your vehicle when you walk to the beach so the load is less. Depending on the tank manfacturer the numbers can change.

In the case of an old steel 72' ( 25 X 6.9 )it's empty buoyancy is about neutral empty so you loose only 4# of lead but at 26# you also loose 5# of tank weight, the total difference is only 9# not counting a bit less air weight. Actually I lust after my buddy's old Faber MP 72's ( 20.5 X 6.75 ) which weights 20# and -7# buoyancy.

Now my old Faber MP 120 ( 28.6 X 7.25 ) weights 48# and is a bit over -7# buoyancy and has around 3# more air. That makes it about 12# heavier over the beach, but it feels even heavier, and more so as I get older.


Bob

 

[Dan]

No thank you! I think I will keep trying to dive balanced.

How?

 

[TMHeimer]

Weight, size, material and pressure vary between tanks, however the only thing that makes a difference for the amount of lead on your belt us the actual buoyancy in the water. As we were discussing before, carrying less weight across the beach is an added attraction.

As shown below the same weight of steel can hold the same amount of gas at a higher pressure and in a smaller container, this makes it less buoyant and helpful for us that wear thick wetsuits as we can leave some lead at home.

A Catalina C80 weights 31.6# and +4# buoyancy empty. ( Size 26 X 7.25)

A Faber MP 80 weights 32# and -7# buoyancy empty. ( 20 x 7.25 )

The tank weights are almost equal but you can leave almost 11# of lead in your vehicle when you walk to the beach so the load is less. Depending on the tank manfacturer the numbers can change.

In the case of an old steel 72' ( 25 X 6.9 )it's empty buoyancy is about neutral empty so you loose only 4# of lead but at 26# you also loose 5# of tank weight, the total difference is only 9# not counting a bit less air weight. Actually I lust after my buddy's old Faber MP 72's ( 20.5 X 6.75 ) which weights 20# and -7# buoyancy.

Now my old Faber MP 120 ( 28.6 X 7.25 ) weights 48# and is a bit over -7# buoyancy and has around 3# more air. That makes it about 12# heavier over the beach, but it feels even heavier, and more so as I get older.


Bob

So you are agreeing with BRT and not boulderjohn, correct? You are saying that someone's total body & kit weight can be less with certain steel tanks than with aluminium ones? And their buoyancy on the dive will be the same, and that weight checks will prove that you can have less of a total weight with some steel tanks and still be properly weighted to dive neutrally (properly)? Based on the buoyancy of the tank, yes?
Of course I am aware of the benefit of carrying less weight with steel, as we are about the same age. Was I wrong in my thinking that it is related at all to the weight of steel vs. aluminium?
I guess that means that if two tanks are the same size and weigh the same, one could still be more or less buoyant than the other, thus altering your total body/kit weight, correct? If they weigh the same and are the same size, would that not mean they displace the same amount of water, thus sink (or float) the same?

Let me word it one more way.
I've never weighed any of my tanks.
An object's buoyancy in water depends on two things--it's weight and the amount of water it displaces. That's why an ocean liner floats and a nail sinks.
Let's say--you have have an aluminium tank and a steel tank that are exactly the same size and shape and are holding exactly the same amount of gas--maybe they're half full (or both are empty). Since they displace the same amount of water and weigh exactly the same, would they not both sink at the exact same rate speed ( or both float, etc.)?
I haven't heard of any steel tanks that are exactly the same size as an AL80, I just used "exact same size/shape" for my example.

 

[Magnus Lundstedt]

How?

By using the right tank for the right dive.

When i dive in a thin wetsuit an AL80 makes me carry 2-2,5 kg of lead, with good balance (empty bcd at saftey stop)
If i would use an 12l steel instead i would need to shave of more lead than i carry.

But when i dive in drysuite, I use steel tank, becouse then it is the right tank for the dive.

If i would try to dive my thin wetsuite at home, in fresh or brackish water, with steel tank (i do not own any aluminum tank) i would be grosly owerweighted without any weights.

 

[BRT]

So you are agreeing with BRT and not boulderjohn, correct? You are saying that someone's total body & kit weight can be less with certain steel tanks than with aluminium ones? And their buoyancy on the dive will be the same, and that weight checks will prove that you can have less of a total weight with some steel tanks and still be properly weighted to dive neutrally (properly)? Based on the buoyancy of the tank, yes?
Of course I am aware of the benefit of carrying less weight with steel, as we are about the same age. Was I wrong in my thinking that it is related at all to the weight of steel vs. aluminium?
I guess that means that if two tanks are the same size and weigh the same, one could still be more or less buoyant than the other, thus altering your total body/kit weight, correct? If they weigh the same and are the same size, would that not mean they displace the same amount of water, thus sink (or float) the same?

Let me word it one more way.
I've never weighed any of my tanks.
An object's buoyancy in water depends on two things--it's weight and the amount of water it displaces. That's why an ocean liner floats and a nail sinks.
Let's say--you have have an aluminium tank and a steel tank that are exactly the same size and shape and are holding exactly the same amount of gas--maybe they're half full (or both are empty). Since they displace the same amount of water and weigh exactly the same, would they not both sink at the exact same rate speed ( or both float, etc.)?
I haven't heard of any steel tanks that are exactly the same size as an AL80, I just used "exact same size/shape" for my example.

You are correct that what the tank is made out of doesn't matter if they displace the same amount of water and weigh the same amount. For the same amount of gas capacity, steel tanks often weigh the same or less and displace less water. This is because the tank walls can be thinner on the steel tanks and they can be made to hold higher pressure giving the same gas capacity in a smaller package. Every pound less of buoyancy in the tank is a pound that you do not have to carry in lead. Therefore if the tanks are the exact same displacement of water the steel tank tends to weigh more AND carry more gas AND because it weighs more and has less buoyancy you can take weight off your belt. Or you can use a physically smaller tank, with a similar weight to have greater gas capacity and take weight off your belt.

Example Catalina S80 holds 77.4 cf, weighs 31.6 lb and is 2.26 lb positive when empty.
PST E7-80 holds 80.0 cf weighs 28 lb and is 2.50 lb negative when empty

You can take 4 3/4 lb of lead off and the tank weighs 3.6 lbs less and you get slightly more air.

Total weight loss of 8.35 lb and 2.6 more cf of air.

PST E7-100 holds 100 cf, weighs 33 lb and is 1 lb negative when empty

You can take 3 lb off your belt and you get 22.6 cf more air

Total weight loss of 1.86 lb and 29% more air

 

[TMHeimer]

You are correct that what the tank is made out of doesn't matter if they displace the same amount of water and weigh the same amount. For the same amount of gas capacity, steel tanks often weigh the same or less and displace less water. This is because the tank walls can be thinner on the steel tanks and they can be made to hold higher pressure giving the same gas capacity in a smaller package. Every pound less of buoyancy in the tank is a pound that you do not have to carry in lead. Therefore if the tanks are the exact same displacement of water the steel tank tends to weigh more AND carry more gas AND because it weighs more and has less buoyancy you can take weight off your belt. Or you can use a physically smaller tank, with a similar weight to have greater gas capacity and take weight off your belt.

Example Catalina S80 holds 77.4 cf, weighs 31.6 lb and is 2.26 lb positive when empty.
PST E7-80 holds 80.0 cf weighs 28 lb and is 2.50 lb negative when empty

You can take 4 3/4 lb of lead off and the tank weighs 3.6 lbs less and you get slightly more air.

Total weight loss of 8.35 lb and 2.6 more cf of air.

PST E7-100 holds 100 cf, weighs 33 lb and is 1 lb negative when empty

You can take 3 lb off your belt and you get 22.6 cf more air

Total weight loss of 1.86 lb and 29% more air

Agree on all points. But as you say, a steel tank of the same size can (safely) hold more gas. That gas has weight, and you are carrying that weight--instead of the extra lead you need for the AL tank, which has less gas. Still comes to the same "total package" weight, no? I don't think you're any lighter walking to the shore entry.

 

[Dan]

Agree on all points. But as you say, a steel tank of the same size can (safely) hold more gas. That gas has weight, and you are carrying that weight--instead of the extra lead you need for the AL tank, which has less gas. Still comes to the same "total package" weight, no? I don't think you're any lighter walking to the shore entry.

I think you are correct. It’s all about relative density of you & your dive gears combined with the media you are in (freshwater or seawater or air). If you want to be neutral, then your total density needs to be more or less equal to the media. So for freshwater, you & your gear combined density needs to equal to 1 kg/L or 62.4 lb/cf.

After a dive with single AL80, you will lose about 4-5lbs of air that you exhale off your tank. So, in order to be neutral during a safety stop, you want to be sure to have 4-5lbs heavier in your gear to start with + a little puff of air added to your BCD. As you empty the tank, you will also need to release some air out of BCD to maintain that density of 1 kg/L. So, it doesn’t matter where you put the weight in your gear, whether in your integrated weight BCD pouch or weight belt or higher density stainless steel tank, instead of AL80. Archimedes Principle, High School Physics.

 

[TMHeimer]

I think you are correct. It’s all about relative density of you & your dive gears combined with the media you are in (freshwater or seawater or air). If you want to be neutral, then your total density needs to be more or less equal to the media. So for freshwater, you & your gear combined density needs to equal to 1 kg/L or 62.4 lb/cf.

After a dive with single AL80, you will lose about 4-5lbs of air that you exhale off your tank. So, in order to be neutral during a safety stop, you want to be sure to have 4-5lbs heavier in your gear to start with + a little puff of air added to your BCD. As you empty the tank, you will also need to release some air out of BCD to maintain that density of 1 kg/L. So, it doesn’t matter where you put the weight in your gear, whether in your integrated weight BCD pouch or weight belt or higher density stainless steel tank, instead of AL80. Archimedes Principle, High School Physics.

Well, you agree with me & John. I understand all of that (obviously, as a retired DM). Archimedes. I think we just got a lot of extra debate regarding exposure suit, salt/fresh water, et. al. I think that all was just about the fact that you need to add more lead weight with an AL than with a steel. Thus it's less cumbersome and more pleasant to dive with a steel. We all agree on that of course. But your total weight is the same either way.