Weighting for Neutral Buoyancy

[SmashMonkey]

What is a good quick practical way to make sure I am perfectly weighted? I've struggled a bit with this particularly because we sometimes rent BCs or wetsuits on vacation. I've done the bob in the ocean level with eyes/top of head, and found it lacking. Calculators don't work because I don't have access to inherent buoyancy specs for rental gear. I try to be within 1 lbs over weighted because it's easier to mitigate than being slightly under weighted in an emergency.

In theory I aim to be neutrally buoyant with 0.00 psi in the tank at 15-20 ft, BC empty. In practice it's kind of difficult to achieve because, up to this point, I always end dives with at least 500 psi, usually closer to 1,000 psi. What is the typical inherent buoyancy of an AL80 at 0.00 psi and 1,000 psi?

 

[Outbound]

Air weighs roughly 1 lb per 500 psi in an AL80. At least that's a quick and dirty way to figure it out. So if you start with a full tank and ascend with 500 psi left, that's a 5 lb. positive buoyancy shift.

As for everything else, as you've noted it's difficult to know your exact weighting if you are always renting BCDs. Most jacket style ones are a few pounds positive. If you rent wetsuits, that's also difficult because an old rental that has been to depth many times will be less buoyant than a new suit.

The weight calculators just don't work for me. Not even close. I'm lean with very little body fat. Those calculators generally estimate my lead to be about twice what I actually use. But, they can at least be a start, especially when vacation boat diving and everyone else is eager to get going. Best thing to do is to gear up and jump in first so that you can do a proper weight check and have one of the DMs or a buddy on the boat assist in getting you more lead or taking some away if necessary.

 

[jadairiii]

An empty AL80 is about 4 pounds positive. So regardless of what you need, you will need about 4 pounds to neutralize the buoyancy of the 80 at the end of the dive.

 

[boulderjohn]

An empty AL80 is about 4 pounds positive. So regardless of what you need, you will need about 4 pounds to neutralize the buoyancy of the 80 at the end of the dive.

In my continuing quest to end this myth....

Buoyancy depends upon your total weight in the water (you, your tank, your weights, your fins, your other gear) and the volume it occupies. If weight of that total volume equals the weight of the water, you will be neutrally buoyant.

During the dive, you will lose weight and become more buoyant as air leaves your scuba tank. At the end of the dive, you will be lighter by the amount of air you use. It does not matter if you were breathing that air from a steel tank or an aluminum tank; you will be lighter by the amount of air you use. Period.

An aluminum tank weighs less than a steel tank with the same external volume, so you will need a few pounds more weight with an aluminum tank than a steel tank because of that, but if you need that weight, you will need it from the start.

 

[jadairiii]

In my continuing quest to end this myth........

Not sure I understand the point? A diver is the sum of his/her "parts". Exposure suit, tank(s), regulators, ect.. A diver must account for all of that.

Example, in the summer, on a recreational dive, I dive an AL 80, and just a rash guard. I know that at the end of the dive (if I empty that 80) my 80 will be about 4 pounds positive. Now, I know my regulator 1st stage has a bit of weight, and I know I am slightly positive with a normal inhalation. With that, I dive a 6 lb SS back plate. That addresses the positive buoyancy of the tank at the end of the dive along with my normal inhalation. I also know that at the beginning of the dive I will be roughly 4 pounds negative with everything.

In the end, some of your gear will float, some will sink and some of the gear will vary depending on depth. All must be balanced.

 

[boulderjohn]

In the end, some of your gear will float, some will sink and some of the gear will vary depending on depth. All must be balanced.

In the end, it does not matter whether the tank floats or still sinks. What matters is the total package.

Here is a link to scuba cylinder specifications. If you check, you will see that an aluminum cylinder with about 80 cubic feet of air in it will be about 6 pounds lighter when empty than when full. A steel cylinder with about 80 cubic feet of air in it will be about 6 pounds lighter when empty than when full, even though by itself it is still positively buoyant.

 

[NorCalDM]

Not sure I understand the point? A diver is the sum of his/her "parts". Exposure suit, tank(s), regulators, ect.. A diver must account for all of that.

Example, in the summer, on a recreational dive, I dive an AL 80, and just a rash guard. I know that at the end of the dive (if I empty that 80) my 80 will be about 4 pounds positive. Now, I know my regulator 1st stage has a bit of weight, and I know I am slightly positive with a normal inhalation. With that, I dive a 6 lb SS back plate. That addresses the positive buoyancy of the tank at the end of the dive along with my normal inhalation. I also know that at the beginning of the dive I will be roughly 4 pounds negative with everything.

In the end, some of your gear will float, some will sink and some of the gear will vary depending on depth. All must be balanced.

I think what boulderjohn means is something I always said is weight is weight. If you weighed your gear including tanks, bc or plate with regulators and added lead and it 60lbs just say and that is what you need to offset you and your exposure suit that is what you need doesn't matter how you come to that weight.

 

[boulderjohn]

Not sure I understand the point? A diver is the sum of his/her "parts". Exposure suit, tank(s), regulators, ect.. A diver must account for all of that.

Example, in the summer, on a recreational dive, I dive an AL 80, and just a rash guard. I know that at the end of the dive (if I empty that 80) my 80 will be about 4 pounds positive. Now, I know my regulator 1st stage has a bit of weight, and I know I am slightly positive with a normal inhalation. With that, I dive a 6 lb SS back plate. That addresses the positive buoyancy of the tank at the end of the dive along with my normal inhalation. I also know that at the beginning of the dive I will be roughly 4 pounds negative with everything.

In the end, some of your gear will float, some will sink and some of the gear will vary depending on depth. All must be balanced.

Your comment specified a buoyancy swing for aluminum tanks. I was just pointing out that steel tanks have the same buoyancy swing, and it does not matter if they are positive of negative at the end of the dive.

 

[inquis]

So regardless of what you need, you will need about 4 pounds to neutralize the buoyancy of the 80 at the end of the dive.

To clarify, that does NOT mean you need at least 4 lbs *of lead*. Much depends on the rest of your gear and personal build. Using a stainless steel backplate may completely account for the tank buoyancy (and then some, depending on thickness). My fins (Hollis F1) are 1.5 lbs negative in water, for example. As @boulderjohn said, it's the net of the entire package that matters.

With rental gear, you'll often struggle to find optimal weighting. Use of a travel luggage scale may allow easy measurement if you can get the rental gear a little in advance of the actual dive.

 

[TMHeimer]

I agree with the OP in that being a tiny bit overweight is fine. Let's not forget that other than for a split second when you've consumed a specific amount of air, you can never be "perfectly" weighted/neutrally buoyant since the amount of air in your tank is constantly changing.