To Weight, Or Not Weight?

[debajo agua]

Hi

Here's my question. My body is very negative buoyant; in fact, it's so negatively buoyant that I don't need any weight to descend (Aluminum 80 w/ shorty wetsuit) albeit negative buoyant in freshwater. Taking into consideration that Aluminum 80's become positive buoyant when most of the air is depleted, I was entertaining the idea of using a steel tank without weights. Is there any danger to having no weights at all, as long as you can maintain negative buoyancy?

 

[Karibelle]

Think about why we use lead weights in the first place... how are your safety stops when wearing no weights?

 

[sam1]

In the event that your BCD or wing fails, you need to be able get to the surface. If you can do that without ditchable weight (e.g., the steel tank), fine. If not, then AL and ditchable weight.

 

[TSandM]

Let's look at the physics. To hold a safety stop with a near-empty tank, you have to be able to be neutral at safety stop depth. This means that, at that depth at the BEGINNING of the dive, you had to be negative by the amount of gas you intended to use from your tank, since all that weight will get exhausted into the water.

[Editorial insert here: Yes, I know that an adult male can compensate a great deal for this changes with his lungs. I'm assuming that the diver doesn't WANT to spend his safety stop trying to breathe off the bottom third of his vital capacity.]

Okay. So we have established that, at the beginning of the dive, you need to be negative by the weight of your gas. In an Al80, that's about five pounds. If you are five pounds negative at the beginning of the dive in an aluminum tank, you will be about five to six pounds MORE negative in a steel tank, depending on the tank (and this is assuming the same capacity tank). So you are now starting your dive as much as 11 pounds negative. And if you go to a higher capacity tank, as many people do when they switch to steels, you can add another two or three pounds for gas weight.

Assuming you have a BC failure at the beginning of the dive, can you swim up 15 pounds, and can you manage to remain at the surface when you are 10 or more pounds negative? If the answer to this is no, then I think it's clear that it's not a good strategy to use a steel tank with no exposure protection - or if you do, you need some kind of redundant buoyancy.

 

[debajo agua]

Think about why we use lead weights in the first place... how are your safety stops when wearing no weights?

I've only experiment with no weights in a swimming pool, so I don't have any prior experience with the safety stop. I was thinking of trying it on my next "experimental" dive (less than 40')

 

[debajo agua]

Assuming you have a BC failure at the beginning of the dive, can you swim up 15 pounds, and can you manage to remain at the surface when you are 10 or more pounds negative? If the answer to this is no, then I think it's clear that it's not a good strategy to use a steel tank with no exposure protection - or if you do, you need some kind of redundant buoyancy.

15 lbs. is a lot but I'm sure I could make it to the surface ;however, I definitely don't want to be treading water for any length of time at the surface. This raises another question in my mind: I know saltwater makes a person more buoyant than in fresh water but I've never actual seen a figure. What's the average buoyancy weight difference between the two?

 

[fisheater]

Weigh yourself and your gear. Then multiply that by 2.5%. The result is the extra buoyancy in salt water.


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[TSandM]

For an average person, you would be about five or six pounds less negative in salt water.

 

[debajo agua]

For an average person, you would be about five or six pounds less negative in salt water.

I multiplied my total weight with gear on by 2.5% and came up with 5.5 lbs... I average as hell!

 

[Hatul]

Yes it's better to have some weight you can ditch in an emergency, especially for a beginning diver.