15 litre versus 10 litre cylinder

[Skydiver1]

Hi,

Was alternating between these two tank sizes over the past week. Probably pretty standard if doing deeper dive in the morning followed by shallow one.

Is there much of an impact on weights needed when using the bigger tank compared to when using the smaller one.

Also read before about an empty tank having effect on buoyancy when most of the air has been used up. Is this more a theoretical concern, or does it have a real impact on weighting.

And finally, why dooesn't everyone just use a bigger tank when available when diving to at least 25 meters? My air consumption is alright, but we had to cut a number of dives short due to some of the group using up their air too quickly. If you have an easy entry, dont have to carry the equipment far, arent planning on any penetration, why not spend an extra couple of dollars to get more air and extra bottom time every time.

Thanks

 

[muddiver]

I don't know the standards and common practices in Europe, but....

The buoyancy of a cylinder depends on the volume, material and wall thickness for that brand and size of cylinder. Example: A typical American aluminum 80 cubic foot cylinder (American cylinders are sized by capacity of compressed air volume) will be 1 pounds (lbs) of lead negatively buoyant when full, but 5 lbs positively buoyant when close to empty. (Luxfer: SCUBA Cylinders: Specifications) Yet a high pressure steel 100 cubic foot cylinder will be 10 lbs negitive when full and 2.5 lbs negitive when empty. (see XS Scuba Worthington Steel Cylinder Specifications)

In the U.S. the aluminum 80 cubic foot cylinder is the most common, the least expensive and lightest in weight for the volume and is therefore the recreational diving industry standard. Most scuba divers don't receive adiquate gas management training so for every dive they just go get a typical scuba cylinder, typically an aluminum 80, and do what ever dive with only 77 cubic feet of usable air. Yes a large cylinder, or two meduim cylinders, is the correct way to do scuba diving deeper than about 60 feet of depth (18 meters).

You need to check the specification data for the cylinder you are looking at. Otherwise your question is too general in nature for anyone to answer.

 

[Skydiver1]

Oh okay.

I think 80 cubic feet is about similiar to 10 litre at 200 bar.

Interesting it has that much effect. I presume the positive to negative buoyancy effect is even greater for bigger tanks.

Hard to find a compromise which means not having to pump my bc with so much air because too heavy versus having difficulty with safety stop because too light.

Supposed will just have to be more meticulous with my dive log, and keep a good record of whats working and whats not.

 

[cloudflint]

i haven't dived on a 10 litre but i do alternate between diving on a 15 litre and diving on a 12 litre. I found with a full 15 is heavier than a full 12 and an empty 15 is lighter than a empty 12 and there is quite a big swing in buoyancy with the 15 compared with the 12.

When I dive on the 12 litre I use 12 lbs of lead, when i dive on the 15 litre I use 16 lbs of lead. on the 15 that makes me quite heavy to start with but by the time my tank gets to 50 bar im slightly buoyant with an empty BC on the surface. So as a guestimate i would say maybe add about 6 lbs when going from a 10 litre to a 15 litre? If you wanted to be accurate you could put on a 15 litre at 50bar and keep adding weight until you are neutral at the surface with an empty BC and do the same with a 10 litre cylinder.

The main reason alot of people i know don't use large tanks for every dive is that they are heavy to get in the water so if we are shore diving we will generally use 12's but if we are diving of the RHIB we will use 15's. Also the bigger your tank the more lead you are going to need to wear and the bigger the swing in bouyancy is going to be as you breathe your tank down. The final reason is that once you get below 20 meters we find that on a 12 we can generally reach our no stop time quite easily doing a sawtooth profile so we only really use a 15 if we are going to be on a fairly square profile.

 

[muddiver]

I agree that there is a swing in buoyancy as you use the air in the cylinder and you "dump it overboard". But as cloudflint stated, weight yourself for your safety stop depth (4 or 5 meters) with an almost empty cylinder.

 

[String]

you might need 1.5kg maximum difference between 15l and 12l but a lot depends on construction - same are thicker than others etc so do a proper weight check with both.

As to why people dont use bigger tanks all the time, if its not needed why take it? They're heavier to move around, more uncomfortable to dive with so if not needed dont take it.
Some people also dont want to spend more money on bigger tanks if they deem their dive is already long enough as multiplying 5 euro by 10 dives adds 50 euros onto the trip etc. One thing though - if you're all having to end dives early due to a few in the group low on gas find a different dive centre. A guide should NOT be doing one-up-all-up.

If its steel tanks are least they aren't as ridiculously heavier as american based steel tanks by about 3-4kg a tank but you dont say exactly what tanks there are or where. Im assuming you're using faber or heiser if in europe as opposed to the idiotic "calling a 100cu ft monster a 15l" they do here in Egypt.

As far as american aluminium tank sizes an "aluminium 80" is their standard size. Its equivalent to an 11l steel tank filled to only 200 bar. It also weighs 3kg more on land so is harder to move around than a 12l real steel, its buoyant in the water so needs 1-2kg extra lead to sink it and physically bigger.

In short a real steel 12l filled to its working pressure holds 2784l of gas whereas an AL-80 filled to its WP holds 2240l of gas. The steel is lighter, less buoyant and physically smaller too.


The 15ls which are really 100 cu ft are actually 13 litre tanks and even more crazily proportioned.

In short, look up the manufacturer of the tanks on the internet (its stamped on the neck) and from there you can see its weight,work out displacement etc to see difference in required weight.

This site:-
http://www.subaqua.co.uk/cgi-bin/cylinder-buoyancy.cgi

Is very useful.

 

[Skydiver1]

you might need 1.5kg maximum difference between 15l and 12l but a lot depends on construction - same are thicker than others etc so do a proper weight check with both.

As to why people dont use bigger tanks all the time, if its not needed why take it? They're heavier to move around, more uncomfortable to dive with so if not needed dont take it.
Some people also dont want to spend more money on bigger tanks if they deem their dive is already long enough as multiplying 5 euro by 10 dives adds 50 euros onto the trip etc. One thing though - if you're all having to end dives early due to a few in the group low on gas find a different dive centre. A guide should NOT be doing one-up-all-up.

If its steel tanks are least they aren't as ridiculously heavier as american based steel tanks by about 3-4kg a tank but you dont say exactly what tanks there are or where. Im assuming you're using faber or heiser if in europe as opposed to the idiotic "calling a 100cu ft monster a 15l" they do here in Egypt.

As far as american aluminium tank sizes an "aluminium 80" is their standard size. Its equivalent to an 11l steel tank filled to only 200 bar. It also weighs 3kg more on land so is harder to move around than a 12l real steel, its buoyant in the water so needs 1-2kg extra lead to sink it and physically bigger.

In short a real steel 12l filled to its working pressure holds 2784l of gas whereas an AL-80 filled to its WP holds 2240l of gas. The steel is lighter, less buoyant and physically smaller too.


The 15ls which are really 100 cu ft are actually 13 litre tanks and even more crazily proportioned.

In short, look up the manufacturer of the tanks on the internet (its stamped on the neck) and from there you can see its weight,work out displacement etc to see difference in required weight.

This site:-
UK Scuba - Cylinder Buoyancy Calculation

Is very useful.

Thats detailed information, thanks.

I was using the tanks in Malta, unfortunately I didn't get the manufacturer.

Interesting the difference in how cylinders capacity are described between the US and Europe.

I thought a difference of units was bad enough, but also difference in how to measure capacity. I kinda messed up my one and only dive on the Vandenburg last summer. It was my second dive in the States using PSI instead of bar and I was too conservative with whatever crappy conversion I was using (slight narcosis at depth didn't help either), and ended up ascending way too early with loads of air :shocked2: - in future am going to bring my own SPG !

 

[asj]

In the U.S. the aluminum 80 cubic foot cylinder is the most common, the least expensive and lightest in weight for the volume and is therefore the recreational diving industry standard. .

Al lighter per volume? Just in the US?

Here steel cylinders are lighter per volume. A 12l is about 13kg vs 14kg al80, and better buoyancy. The XS scuba worthington site you list has incredibly heavy tanks...is that all you guys can get?

 

[kittles]

The difference isn't huge with Fabers (the tanks I use). Only a 1.7lb swing difference.

12L
Empty: -0.75 kg (-1.7 lbs)
Full: -4.25 kg (-9.4 lbs)
Swing: 3.50 kg (7.7 lbs)

15L
Empty: -1.0 kg (-2.2 lbs)
Full: -5.25 kg (-11.6 lbs)
Swing: 4.25kg (9.4lb)

 

[ronbeau]

I don't know the standards and common practices in Europe, but....

The buoyancy of a cylinder depends on the volume, material and wall thickness for that brand and size of cylinder. Example: A typical American aluminum 80 cubic foot cylinder (American cylinders are sized by capacity of compressed air volume) will be 1 pounds (lbs) of lead negatively buoyant when full, but 5 lbs positively buoyant when close to empty. (Luxfer: SCUBA Cylinders: Specifications) Yet a high pressure steel 100 cubic foot cylinder will be 10 lbs negative when full and 2.5 lbs negative when empty. (see XS Scuba Worthington Steel Cylinder Specifications)

In the U.S. the aluminum 80 cubic foot cylinder is the most common, the least expensive and lightest in weight for the volume and is therefore the recreational diving industry standard.

This might have been a typo in your post.

I agree that the aluminum is is the most common cylinder and the least expensive in the USA but I would disagree with your statement that it is the lightest weight for the volume. Using the Luxfer & Worthington specifications an aluminum 80 weighs more than a comparable HP 80.

The Worthington HP 80 weighs 27.7 pounds when empty and the Luxfer aluminum 80 weighs 31.4 pounds when empty. The HP 80 is also a lot more compact than an aluminum 80 19.7 inches long vs. 26.1) and has better buoyancy characteristics than the aluminum 80.

In fact, the HP 100 only weighs about 1.5 pounds more than an aluminum 80, is more compact than an aluminum 80 (24 inches in length vs. 26.1 for the aluminum 80) and has a 25% higher capacity (ignoring the fact that an aluminum 80 only contains 77.4 cf).