Question about experimenting with weighting

[dmaziuk]

Pick a better boat. Also tell the DM beforehand it's you first dive in new gear and you need the weighting sorted out -- but on a better boat they won't go down and start swimming until they're sure everyone's good to go. Especially when they know you put a single-digit dive count on your check-in form. Which, on a better boat, the person at the check-in counter would have warned the DM about.

 

[DBPacific]

I've never actually understood advice like this. You say, float with your head half out of the water. Since at the beginning of a dive, you are 5 pounds heavy (to account for the air you haven't used yet), this implies the top half of your head supplies at least 5 pounds buoyancy. Now, a quart of air instead of a quart of salt water gives about 2 pounds buoyancy....so you are saying the volume of the top half of your head is at least 2+ quarts and is completely empty?

It definitely feels like my head is empty sometimes...

That's the same method that I've always used and been instructed to use for pre-dive weight checks though, and it seems to work well enough

 

[cmrangel]

It definitely feels like my head is empty sometimes...

That's the same method that I've always used and been instructed to use for pre-dive weight checks though, and it seems to work well enough

He He, me too, mine seems to be filled with helium!

I was told this advice, tried it, seemed to get close. Once I was dialed in based on the 15ft safety stop, I tried to again on the next dive just to see. I was pretty close to eye level actually. I have not tried to figure out the math but know you have to also account for whatever change in wetsuit compression from 15ft to surface so that likely makes up some of the 5 lbs.

 

[inquis]

@tursiops , I've wondered the same thing and rationalized the common approach to myself as follows: the "half breath" will ideally compensate for the 5 lb of air you will lose (assuming an AL80 tank). Floating "eye level" puts about 2 lbs out of the 5-6 lbs your head weighs out of the water. Being +2ish at the surface at the end of the dive will be quite controllable and hopefully be neutral(ish) at the safety stop due to suit compression.

The amount of breath is obviously the largest source of error. If one can jump in first and conditions allow (i.e., negligible current), you can test with 5 lbs less than you think you need and see if you are eye level with a normal exhale. If so, get the remaining 5 lbs from someone still on the boat and go dive. I've done this and added more weight a couple times before the entire group was in the water. Coordinating with the DM or crew member or buddy beforehand to have a few bricks ready for you (and the additional 5 lbs) makes this very quick. Easily accessible weight pockets makes this easier as well.

 

[inquis]

For scaling from fresh to salt, you should add 2.4% of your entire dry weight (you + rig + tank). AL80 tanks weight about 32 lbs. So a 200 lb diver (240 lb fully kitted) will need to add about 6 lbs.

That particular percentage comes from the water densities, so very salty water might need 3%. As with most things, calculations can only get you so close.

 

[boulderjohn]

I've never actually understood advice like this. You say, float with your head half out of the water. Since at the beginning of a dive, you are 5 pounds heavy (to account for the air you haven't used yet), this implies the top half of your head supplies at least 5 pounds buoyancy. Now, a quart of air instead of a quart of salt water gives about 2 pounds buoyancy....so you are saying the volume of the top half of your head is at least 2+ quarts and is completely empty?

In many a ScubaBoard debate about the common procedure about floating at eye level while holding a normal breath with an empty BCD, there have been two schools of thought on what to do next.

1. Add the weight of the air in the cylinder to your weighting.
2. Don't add any weight because when you use that process, you have at least that much added buoyancy through trapped air that will leave during the dive.