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

[Dan]

@Dan_T & @mac64 , Luxfer's data for their AL80 is -0.64 kg full.

View attachment 453361

I guess my rough calculation is close enough (16.81 - 17.98 = - 1.1 kg) with 200 bar. For 207 bar, it'll be closer (-1.0 kg)

I also add the valve weight of 2.5 lbs (1.14 kg) from this: DGX Premium Pro Valve, Right (Typical Side) Luxfer may use lighter valve (0.78 kg?)

When the air in the tank is down to 500 psi, which means the tank lose 2500 psi of air or 2.23 kg (2500 / 3000 x 2.68 kg), then the tank becomes positively buoyant by 2.23 - 0.64 = 1.59 kg. The table shows 1.54 kg positively buoyant, pretty darn close with the rough estimation.

 

[Dan]

Surface area is irrelevant. The definition of density is (mass per volume).

Exactly! Surface area (shape) has nothing to do with buoyancy. It has a lot to do with density of the object relative to the fluid density. The greater the density difference between the object & the fluid, the greater the degree of buoyancy. An object with density less than water will be more positively buoyant in sea water than in freshwater. We would need to put on more lead when diving in the sea than in the lake.

Surface area (shape) will affect the drag coefficient (how fast the negatively buoyant object will sink). A metal disk would have higher drag coefficient & sink slower relative to the same metal rod.

The tricky part is to estimate the average density of you + wetsuit + tank + fins + cats & dogs you are carrying on a dive. That's why we do a checkout dive to estimate how much lead we need to add to get neutrally buoyant + 2 kg to counter the weight loss by the end of the dive after sucking up the air out of AL80 from 3000 to 500 psig if you want to stay neutrally buoyant during safety stop.

 

[Celt]

Since you know density, let’s talk about it, in relation to AL80 cylinder. You mentioned in earlier post that it would be positively buoyant (floating) even if it is full of air (at 200 bar). From physics, if an object has density greater than water, it will sink. If its density is less than water, it would float. So, let’s calculate the density of AL80 full with air at 200 bar.

Say we pick the spec of Luxfer 80 from Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan
Outside diameter (d) = 7.25” = 18.41 cm
Cylinder height (H) = 26.06” = 66.19 cm
Dome height = half of outside diameter (r) = 1/2 d = 18.41/2 = 9.21 cm
Cylinder section height (h) = H - r = 66.19 - 9.21 = 56.98 cm
Weight empty w/o valve = 31.38 lbs = 14.25 kg
Valve weight = 2.5 lbs = 1.14 kg

Let’s calculate the weight of 200 bar air in 11.1L AL80 cavity. From ideal gas law, the air density (D) is calculated from the following equation:
D = MP/RT,
where:
D = air density in g/mL
M = molecular weight of air = 28.96 g/mol
P = air pressure = 200 bar
R = gas constant = 83.14 (mL.bar)/(K.mol)
T = ambient temperature = 25 C = 298 K
Plugging in the number into the equation,
D = 28.96 x 200 / 83.14 / 298 = 0.233 g/mL = 0.233 kg/L
W air = 0.233 kg/L x 11.1 L = 2.59 kg

Total AL80 full = empty AL80 + valve + air = 14.25 + 1.14 + 2.59 = 17.98 kg

Total outside volume of the AL80 = cylinder part + dome part.
Cylinder part = (Pi) (r) (r) (h) = 3.14 x 9.21 x 9.21 x 56.98 = 15176 mL = 15.18 L
Done part = 4/6 (Pi) (r) (r) (r) = 4/6 x 3.14 x 9.21 x 9.21 x 9.21 = 1635 mL = 1.63 L
Total AL80 outside volume = 15.18 + 1.63 = 16.81 L

Density of AL80 full = 17.98 kg / 16.81 L = 1.07 kg/L, heavier than water (1 kg/L).

Therefore, it will sink.

Thanks Dan for your work and detailed reply. Im not qualified to debate your figures or formula, from my diving experience the use of aluminium cylinder over steel has always required the use of more lead. I will do some study on the work you have produced here as i find it very interesting.

 

[boulderjohn]

rom my diving experience the use of aluminium cylinder over steel has always required the use of more lead.

All other things being equal, that will generally be true. All other things are not always equal.

And, in contradiction to the post that started it all, being required to use enough lead does not further require you to use too much lead.

 

[Umuntu]

I also add the valve weight of 2.5 lbs (1.14 kg) from this: DGX Premium Pro Valve, Right (Typical Side) Luxfer may use lighter valve (0.78 kg?)

Yes, it think their calculations do not account for valves or manifolds, as that will vary.

 

[Coztick]

If it wasn't for bouyancy, our wings would need 300 lbs. of lift!
(Assuming air still had lift)

 

[rsingler]

For what it's worth, I just posted these spreadsheets in another forum, but they may have value in this discussion about "too much lead" and not enough buoyancy. It's not as sophisticated as the Ultimate Planner posted in the BCD forum, but is a bit easier to use.
Text from that post follows:

This subject seems to come up with regularity.
So I put together this little spreadsheet that will calculate lift requirements based upon a number of factors that you can change.
It is formatted in the current Excel .xlsx, and in the older Excel format .xls
It can be used with both wetsuit and drysuit diving, with some assumptions as follows:
I arbitrarily assumed that your wetsuit would lose 60% of its buoyancy at depth.
I arbitrarily left the weight of 15% of your stated air supply on board in calculating lift requirements at the end of a dive.

To use the spreadsheet, just fill in the grey areas in Column F.
MAKE SURE THE SIGN IS CORRECT.
Positive numbers are buoyant, Negative numbers sink.
For example, carried lead is -10, not 10.

The spreadsheet is protected with the password "scuba", so if you find mistakes, feel free to jump in and edit the formulas.

There are a number of "Calculated Wing Requirements" given at the bottom of the spreadsheet, depending upon your risk tolerance.
For example, if you want to assume that you'll be able to trap 5# of buoyancy in the leg of your flooded drysuit, go ahead and carry a smaller wing.

The spreadsheet has been set up for doubles, so if you want to calculate single tank wing requirements, just enter "0" for manifold weight and cut the air carried in half. Conversely, if you're calculating for doubles, and carrying twin AL80's, make sure to enter 160 for air.

There should be a table of suggested values for the various fields, but I haven't gone to that effort yet. You'll need to decide how much native buoyancy your suit carries, and you'll need to look up the 500psi buoyancy of your tank, but that data is available on SB in one thread or another.

In the calculations section, there are groups of "things you can control" and "things you can't control easily". This too, was arbitrary. Obviously you can change your backplate, but not easily. Conversely, maybe you can't change the weight of your air easily, but I grouped air weight there to show more graphically what happens when you switch from big to small tanks, or steel to aluminum.

Finally, there's a little section at the bottom for "redundant lift required". In the event of a torn wing, how do you want to handle it? A second bladder? A lift bag? Your drysuit? Just swimming that weight up on your own? This tells you how much add'l lift you need to carry, or how much you'll be finning up to the surface without redundant lift, and is obviously the opposite of your empty wing buoyancy at that point, which is duplicated in several cells a few rows up.

Have fun!
Please let me know if you find mistakes. I just did this off the top of my head.

Cheers!
Rob

 

[TMHeimer]

This is apparently a very difficult concept.

Your buoyancy is based on your entire package--your body, your thermal protection, your BCD, your fins, your mask, your light, your tank, your regulators, your weights--everything. That whole package has a certain total weight. It also has a total volume. If the total weight of your total package is equal to the weight of that volume of water, then you are neutrally buoyant. If it weighs more, you are negatively buoyant.

In your total package, there are individual items that are negatively buoyant. There are certain items that are positively buoyant. The negatively buoyant items include your thermal protection, the fat in your body, some BCDs (especially with air). The negatively buoyant items include all tanks at the beginning of the dive, some BCDs, and the weights you carry. To be properly weighted, you need to consider all the factors over which you have control. Ideally, you want to be slightly negatively buoyant with no air in the BCD at the end of the dive so that you only need a minimum amount of air in the BCD to remain at depth.

• Let's say that a diver's total package needs to weigh 250 pounds in order to be be properly weighted at the end of the dive. If the diver uses a PST 80 steel tank, the tank will be 9.3 pounds negatively buoyant at the beginning of the dive. If the diver uses 4 pounds of gas during the dive, then that diver must make sure he or she weighs 254 pounds at the beginning of the dive to compensate for air loss. That weight can be achieved by adding lead or by using a different BCD.
• Let's say that a diver needs to weigh 250 pounds in order to be be properly weighted at the end of the dive. If the diver uses a Luxfer 80 aluminum tank, the tank will be 1.4 pounds negatively buoyant at the beginning of the dive. If the diver uses 4 pounds of gas during the dive, then that diver must make sure he or she weighs 254 pounds at the beginning of the dive to compensate for air loss. That weight can be achieved by adding lead or by using a different BCD.

If we are talking about the same diver in each case, the diver with the aluminum tank will need to have about 7 more pounds of added weight in the form of lead (or a different BCD) to achieve the needed weight. If the diver adds that extra weight, he or she will be in exactly the same situation throughout the dive as the diver with the steel tank.

I mentioned that I don't use any additional weight when using one of my steel 85s with a 5mm wetsuit in salt water. That is because I use my steel back plate. If I used my aluminum back plate, I would have to add some lead to achieve the same buoyancy. If I used an aluminum tank, I would have to use more weight to achieve the same buoyancy. But it all comes out the same.

Yes. Thank you. I was going to say the same thing without the details. Regardless of type of tank, you need the same amount of total (body and kit) weight to dive properly (be "properly weighted). With a steel tank there is simply more of that weight in the tank as opposed to lead being worn.
It has been pointed out that some switch to larger steel tanks than the standard AL80 to get more gas. And that those tanks are quite heavy for long walks (I once owned a steel 120). I also owned a steel 72, which of course meant carrying less lead than with my AL80s, but it also had less air.

 

[USMC CPL.]

the problem here is simple people are carrying to much weight and trying to compensate with a wing or bcd. get rid of aluminium cylinders and you won't have to carry lead to compensate for their bouyancy when empty. If you use a neoprene drysuit and trim properly you'll have no use for a bcd or wing. The scuba industry have everyone conned into buying to much gear and convinced them that they need it.

I have to agree with the aluminum tank comment. Went out diving with a new HP100 and 4 pounds and was way overweight. Looking forward to no weight at all. Gonna sell my two 80's and go with steel only.

 

[Bob DBF]

It has been pointed out that some switch to larger steel tanks than the standard AL80 to get more gas. And that those tanks are quite heavy for long walks (I once owned a steel 120). I also owned a steel 72, which of course meant carrying less lead than with my AL80s, but it also had less air.

A 120 is a beast to be sure, but you are packing a lot more air and tank to hold it. To be fair a steel 80 weights about the same as an AL 80 and since it is more negatively buoyant you need less lead than the AL and will be lighter walking to the beach with the same air.

The old steel 72's are within a pound or so of the buoyancy of the AL 80, but only weighs about 26# empty.



Bob