Add extra weight for air in tank?

[Matt Beckwith]

I'm taking the on-line Advanced Diver training, the section on buoyancy control.

It says that, as your air is used, the tank becomes lighter, which requires extra weight...? So if I'm neutrally buoyant at the surface, with an empty BCD and lungs full, with water at eye level, at the beginning of the dive-- then I'll need extra weight at the end of the dive to keep from surfacing...?

So at the beginning of my dive I've been weighted just right with 16 pounds. Does this mean that I need to have 18 pounds instead?

It says that the best way to figure out your weights is to check your buoyancy with a near-empty tank, then switch to a full tank for the dive. Yeah, right...

I have to admit that I've been doing my dives mostly with an empty BCD, and adding just a puff or two of air down around 55 feet.

I'm not clear on this principle of needing extra weight for an empty tank, appreciate any clarification offered!

Thanks. --Matt

[spectrum]

Hi Matt,

You are correct is your suspicions.

Air weighs about .08 LB / CU foot. in other words your 80 CF cylinder holds about 6.4 pounds of air. Of that you can count on dropping about 5 pounds.

What often happens in a quick empty tank check is that gear is not saturated and has extra buoyancy that has weight assigned to it. When you make a real dive that air gets purged and the weight you added can roughly make up for the weight loss of your depleted cylinder. You can also attribute some offset to neoprene not rebounding 100% instantly. These are all small puts and takes that add up.

It sounds like you're into something that is working for you. I suggest doing your precision tests at the end of the dive while at safety stop of your final depth that you need to remain down at. See here

So for your class question the adding the weight of the consumed air would be appropriate.

In practice there are other options like managing lung volume and so forth but to be theoretically neutral you need to account for the released air.

Pete

[jar546]

If you dive the same equipment all of the time, you will have to go through this at least once to figure out where you need to be. If you aren't having problems up to this point and have good buoyancy control then you should not have to worry about it unless you are going to be using different equipment.

I know where I am with my equipment but recently dove someone else's tanks and checked the charts to see the difference in weight full and empty to know how much I had to adjust. I then added some weight to compensate for the difference shown on the chart before I got in the water. The charts are specific to brand and size of the tank and of course steel or aluminum. The reason for your post is one of the reasons I stay away from aluminum 100s.

[CamG]

Greetings Matt what is being explained is that once your tank reaches 500-700 psi it is has become more positively buoyant.
We all need to compensate for that shift in buoyancy.
It usually amounts to 3-5 lbs depending on the diver.

This is a guideline though and not a constant.
In fresh water 7mm wet suit with 5mm / 3mm hooded vest you are closer to the 3-5lbs.
In salt water in a 3mm full or shorty it will be less or pretty close to what it takes in Fresh water with the heavier exposure protection.

In my case I dove fresh water in a 7mm wet suit and 5/3mm hooded vest with 16lbs.
In MX with a 3mm full I dove 16lbs as well all gear was the same except the exposer protection.

What the course is teaching is proper weighting technique which it sounds you learned pretty well in OW.
As you continue to dive and gain experience you will learn how just very small things will affect your buoyancy.
The holidays are horrific to me an extra 10lbs of body weight really suck!

Have fun and enjoy your AOW class.
Be sure to practice your skills in the pool and out in the OW!

CamG Keep Diving....Keep Training....Keep Learning!

[TSandM]

Well, you have to realize that the compressed air in the tank has weight. That weight is part of the buoyancy of the tank when it's full. When it's empty, the tank will have different buoyancy characteristics. A full Al80, for example, will sink; an empty one will float (fairly aggressively, in fact!).

So, when you begin the dive, you have things that are negative and things that are positive. If the two cancel out to make you neutral at the beginning of the dive, then once you have breathed the contents of the tank and exhausted five pounds of gas out into the water, you will be five pounds positive at the end of the dive. That means you won't be able to hold a safety stop, especially in shallow water, where your wetsuit has rebounded to all of its "floatiness".

So you have to carry extra weight in the amount of the weight of the gas you intend to use. With an Al80, that's about five pounds. (Every 13 cf of compressed air weighs one pound.) That way, once the air is gone, you are still neutral, and not five pounds positive.

Make sense?

[DevonDiver]

I'm taking the on-line Advanced Diver training, the section on buoyancy control.

I wonder if PADI et al factored in the benefits of active community forums when they introduced their on-line training programs to replace interaction with real-life instructors. LOL

It says that, as your air is used, the tank becomes lighter, which requires extra weight...?

Yes. Here is an extract from my article series of buoyancy control:
Scuba Buoyancy Masterclass 3of9 - Achieving Great Buoyancy Control -Scuba Tech Philippines


The weight of air is (depending on pressure, temperature and humidity) approximately 1.2 grams per litre. To calculate your approximate weight of air, simply multiply the tank capacity (i.e. 11 litres for an AL80 cylinder) by its working pressure in BAR (i.e. 200 Bar).


11litres (AL80) x 200 bar = 2200litres.
2200litres x 1.2 grams per litre = 2.64 kg (total weight of air in the cylinder)


To calculate the air that you use, simply subtract the reserve (i.e. 50bar) from the initial calculation.


11lites (AL80) x (200-50) bar = 1650litres used.
1650litres x 1.2 grams per litre = 1.98 kg (total weight of air consumed)


As we use this air, we become more buoyant. We have to add weight to offset for losing this weight of air by the end of the dive.

So if I'm neutrally buoyant at the surface, with an empty BCD and lungs full, with water at eye level, at the beginning of the dive-- then I'll need extra weight at the end of the dive to keep from surfacing...?

Correct.

Have a browse through my resources; specifically the 'Tank Buoyancy Calculator' (5th down on the left column):

Scuba Resources | Diving Articles | Links and Blog | Scuba Tech Philippines

So at the beginning of my dive I've been weighted just right with 16 pounds. Does this mean that I need to have 18 pounds instead?

Approximately, yes. It'd depend upon the exact specifications of the tank you used... ali/steel and the capacity.

It says that the best way to figure out your weights is to check your buoyancy with a near-empty tank, then switch to a full tank for the dive. Yeah, right...

I have to admit that I've been doing my dives mostly with an empty BCD, and adding just a puff or two of air down around 55 feet.

I'm not clear on this principle of needing extra weight for an empty tank, appreciate any clarification offered!

[richkeller]

I'm taking the on-line Advanced Diver training, the section on buoyancy control.

It says that, as your air is used, the tank becomes lighter, which requires extra weight...? So if I'm neutrally buoyant at the surface, with an empty BCD and lungs full, with water at eye level, at the beginning of the dive-- then I'll need extra weight at the end of the dive to keep from surfacing...?

So at the beginning of my dive I've been weighted just right with 16 pounds. Does this mean that I need to have 18 pounds instead?

It says that the best way to figure out your weights is to check your buoyancy with a near-empty tank, then switch to a full tank for the dive. Yeah, right...

I have to admit that I've been doing my dives mostly with an empty BCD, and adding just a puff or two of air down around 55 feet.

I'm not clear on this principle of needing extra weight for an empty tank, appreciate any clarification offered!

Thanks. --Matt

I use a BCD the same way you do if I am using one at all. If you want to be neutral at the surface with a near empty tank at the end of a dive then you must take into consideration the weight difference between full and empty into account. The link below will give you that spread for the most common tanks on the market today.

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

[ozziworld]

You check out your weights at the end of a dive with near empty tank. It does not matter what kind of tank. It is all you have to do. Regardless of tank size or material. What counts is you are weightes properly at the end of the dive.

Aluminum tanks have a greater buoyancy difference than steel tanks. Do buoyancy check with near empty tank each time you change gear configuration.

[oly5050user]

remember also that due to suit compression as soon as you descend as little as 2' you loose buoyancy and it increases as you get deeper. I find no need to add weight after a buoyancy check ..many bcd's have padding that compresses at depth also. Make a dive,stop at 15/20' and if you can hover there with a bcd that is pratically empty you are usually good to go.This way at the very end of the dive you are just a bit positive on surface and it should require next to no effort to stay on surface even with a empty/malfunctioning bcd.

[TSandM]

Aluminum tanks have a greater buoyancy difference than steel tanks.

This is a common misconception. In fact, the buoyancy swing for tanks of the same capacity is the same. What happens with steel tanks is that they swing from very negative to a little negative, whereas the Al80 swings from a little negative to fairly positive. The swing in both cases is 5 lbs, though, and that 5 lbs is the weight you need to carry to compensate for the gas you intend to use.