Minimum Diving Weight

[Akimbo]

Here is my view on this often debated subject. It is always dangerous to make absolute statements, especially on Scubaboard, but I am in my flack jacket so fire away.

A diver's submerged weight should be no less than neutrally buoyant

• At the shallowest decompression or safety stop
• With a fully deflated BC (if you use one)
• With drysuit deflated to minimum without discomfort (if you use one)
• With nearly empty Tank(s), like 200-300 Lbs or 14-20 Bar
• With lungs comfortably inflated to your normal respiratory inhalation peak

Here comes the disclaimer
You may never plan on sucking your tanks down to 200 PSI/14 Bar, but completing decompression or safety stops matters more than getting back on deck with a lot more air. Besides, rules of thumb like not running your tank below 500 PSI are not motivated by somebody hanging 10' below the boat on a rope. I am not suggesting anyone plan on breathing cylinders below your comfort level, just that your stops are more important. Miscalculates happen — so this value represents the minimum weight contributed by the gas in your tanks.

Here's the Logic
Being too buoyant at a decompression or safety stop is really annoying, especially if you don’t have the luxury of a weighted line to hang on off — like drifting on a sausage buoy. A nearly empty tank is your most buoyant condition, unless you have picked up some treasure or maybe scallops.

Most divers find that neutrally buoyancy is the most comfortable and requires the least amount of effort. Too negative on the bottom has drawbacks. First, it is harder to swim. Those ecologically minded divers will find it more damaging to marine life and habitat because you are crawling across the bottom. Photographers, wreck, and cave divers react with varying degrees of hostility to buddies who stir up the bottom. Spear-fishermen are less agile and attract more attention from their prey. Bottom-line: Shoot for neutral unless you plan on running a jack hammer.

Depth Matters: Materials, like foam Neoprene used on wetsuits, padding, and some accessories are buoyant, but compresses as depth increases. There may also be some trapped air in your BC or in a rolled up safety sausage.

You can carry significantly more weigh than minimum by adjusting your BC. Aside from lugging more lead around, above and below the water, you also have to fuss with adjusting your BC more. This is because the gas compresses and expands as you change depth — especially in shallow water. More weight = more air in BC = more change with depth, = more annoying.

OK, so how do I figure it out

Here is the best method I have seen thus far: make up a rope about 15'/5 M longer than your shallowest decompression or safety stop. Tie a Bowline at one end, measure up about 3'/1M and make a mark for the diver's stop. Measure up the length of your minimum stop and make a mark representing the surface.

Tie 20-30 Lbs/10-15 Kg of lead, an extra anchor, or chain to the Bowline and lower it over the side of a boat, dock, pier or in a deep swimming pool — you need calm conditions. Tie the surface-end to a fish scale with the "surface mark" just at the air-water interface. Ideally the fish scale is digital because they tend to be more accurate, water resistant for obvious reasons, and can handle the weight you tie off.

Record the weight hanging on the scale. A single diver then hangs off at the "stop mark" empties their BC, drysuit, and is low on air. Record the weight again. If the weight is less than before, it indicates how much more weight they need. If the recorded weight is more, then that is the amount of lead the diver can shed. Add 2½% if the test is in fresh water and you plan to dive in sea water.

[ZKY]

The best medicine I found on how to cure abusing a BC is to learn to dive without one. If you can do that then a BC is just icing on the cake.
:popcorn:

[stairman]

Or just dive 2 steel tanks and no pesky weights are neede.

[peterbj7]

I'll accept what the OP said, so long as it's understood that this is the situation at the end of a dive when any trapped air has been eliminated.

That's for OC dives. For CCR it's quite different, and at the start of the dive it's typically very hard to get down.

[Pearldiver07]

Or set up per the OP, but put no weight on your rig.

Get a LONG 2" web belt and place weights at approx 6" lengths along the belt. If you're very sure that you'll need at least a minimum number of pounds, you can place that weight on your rig, only putting the balance on the belt.

Now, slightly above the bottom at the depth of your shallowest safety stop, let the end of the belt touch down. As you let weight settle onto the bottom the amount that is pulling you down is decreasing.

When you get slightly positive you will not be able to place more weight on the bottom, as you'll now be slightly positive and drift upward and pull that weight back up. At this point, the weight that is suspended is your minimum requirement.

It's going to require a bit of patience, but you should already have what you need on hand to do it.

[tflaris]

With Aluminum tanks you have buoyancy swing from negative to positive. IMHO I would suggest at least 6 lbs.

Everything else being equal.


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[knotical]

Akimbo,

Your post is well thought out.

Another issue you might want to consider is the ability to make a slow ascent from your last stop to the surface. With this in mind, a simpler variation on your test could be to have just enough weight to descend from the surface, with the other caveats you listed. At least that's my preference. ymmv

k

[Walter]

I agree with knotical. Weighting for the safety stop can make for a very rapid ascent from the stop to the surface.

[Gilldiver]

Dress your tanks when full with all the do dads that you normally carry – regs, back plate, BC, light, ………

Now with a 25 pound fish scale from the sporting goods section of Wally World, weight the rig in the pool/lake/ocean. Record weight a A

When the tanks are at 300-500 PSI re-weigh the rig. Record Weight as B.

A-B = C the swing weight of your rig

Do this for every tank set you have.

Now dress in your normal thermal protection (Shorts & Tshirt, Bikini, Skin, Wet Suit, Dry Suit, naked,) with any items that are attached to you and not the tank/BC rig

Get in the pool/lake/ocean and add weight to yourself till you are neutral. Record weight as D

The weight you need to add to be neutral when dressed is D-B, your swing weight is A-B

[GrumpyOldGuy]

I have to say I am rather amused by all the methods suggested to objectively measure your weight. Some are rather ingenious and have merit for a new diver, but I have wonder why all the effort when it is fairly easy to sense you buoyancy at the end of the dive at 10' (full stop or just pausing on the final leg of your ascent).

Like Peterbj mentioned, trapped air messes up a lot of methods done at the start of a dive. I would add with many thick wetsuits that they un-compress very slowly on ascent. This results in a thick wetsuit having more buoyancy at 10' on the way down than 10' on the way up. On top of that minor gear changes and the simple aging process of (cheaper) wetsuits continually changes ones buoyancy over the years so it is an ongoing process.

I remain with the low tech process, if I can't hover at 10' on my ascent with an empty BC I adjust my weight for the next dive. So simple, even a caveman can do it.