Buoyancy weight

[Outbound]

This is actually true only when the tank is full. Below 1200 psi or so, the lower end of the tank becomes buoyant so the effect reverses.

Good point. Although I thought this was only true for certain brands of tanks?

 

[Tripp]

Been working on getting my weight down as much as possible to help with my dive time before running out of air. My last 2 dives I've beaten my previous record for time down, the last one at 58 minutes (probably closer to 56 as we surfaced for a minute to better talk about what we want to explore in the quarry). Anyway, in fresh water with a full 3mm wetsuit, an AL80, and my normal gear, I was carrying 2 x 3lbs in my side pockets, and 2 x 2lbs in my trim pockets. I felt good the whole dive, using only 3 or 4 bumps on my inflator to keep me off the bottom. They problem came up at 900PSI left in my tank. On both dives I'm becoming positive at 900PSI left in the tank. My normal plan is to be on the surface at 700PSI, so I'm loosing a few minutes of time, and even after dumping all my air, once I hit 900 I'm going up, ready or not. Should I move up to 4 x 3lbs, or would it be ok to swap out one of the 2lbs for a 3lbs, or will that make me "lop sided" underwater?

Kaneda13: What are you doing to lose weight? Asking for a friend...

 

[elan]

Here what I would do. Without donning your gear

Have the lowest PSIs in you tank - say 500psi. Put the BCD on the tank, attach the regulators etc, get in the shallows where you can stand up if needed. Make sure the BCD has no air - locate the exhaust valve on the top, make sure the air is not trapped anywhere and purge it.
Add the baseline amount of weight. Put all the gear on except do not put the BCD on you. Put the reg in the mouth , hug the BCd with the tank and try to submerge (fully exhale and wait) in the shallows staying horizontally- that will make sure you do not fin. You should be confortable sinking. If you sink too easy remove weight. If you cannot sink add more.

This will give you the baseline. You can tweak it a bit later. Using this approach you will be slightly overweight as normally wet suit will lose some buoyancy during the dive. Make sure you do not have airpockets in the wetsuit.

 

[lowwall]

Here what I would do. Without donning your gear

Have the lowest PSIs in you tank - say 500psi. Put the BCD on the tank, attach the regulators etc, get in the shallows where you can stand up if needed. Make sure the BCD has no air - locate the exhaust valve on the top, make sure the air is not trapped anywhere and purge it.
Add the baseline amount of weight. Put all the gear on except do not put the BCD on you. Put the reg in the mouth , hug the BCd with the tank and try to submerge (fully exhale and wait) in the shallows staying horizontally- that will make sure you do not fin. You should be confortable sinking. If you sink too easy remove weight. If you cannot sink add more.

This will give you the baseline. You can tweak it a bit later. Using this approach you will be slightly overweight as normally wet suit will lose some buoyancy during the dive. Make sure you do not have airpockets in the wetsuit.

This will result in being greatly overweighted. You simply can't get all the air out of your wetsuit at the start of a dive. That's why people have trouble getting down at the start of a dive even with a full tank.

OP- Do your weight check at the safety stop. Add the extra 2 pounds and see if you are neutral to slightly heavy at the end of the dive with a completely empty BC

 

[inquis]

Good point. Although I thought this was only true for certain brands of tanks?

True for basically anything that goes more than about 2 lbs positive when empty (since a first stage weighs about 2 lbs). For example, the common Luxfer AL80 is 4.4 lbs positive. Minus 2 lbs for a first stage is 2.4 lbs positive when empty. Air weighs one pound per 13 cuft, so is neutral with 31.2 cuft in it. That tank is 77.4 cuft at 3000 psi, so will therefore be neutral around 1209 psi as @tursiops mentioned. (That pressure is specific to that tank.) As for other tanks, a Catalina S80 will do it (4.1 lbs empty), but a Catalina C80 won't (0 lbs empty). Luxfer AL100 will (3.2 lbs) but a Worthington AL100 won't (0.3 lbs). No steel tank will to my knowledge.

 

[inquis]

will that make me "lop sided" underwater?

FWIW, I seem to compensate for being lopsided without thinking about it. You may confuse the dive shops/boats a little though -- I've asked for 13 lbs on two separate occasions (different shops) who replied, "are you sure?" They looked so relieved when I told them I have an AL19 pony.

I also don't think being 1 lbs over "ideal" to "even out" is going to be noticeable either. Make sure you can halt an ascent at any depth and any pressure. Planning on surfacing at 700 psi is great, but planning for the worst case if SHTF is better in my view.

 

[SailNaked]

many people just add weight, if they are light at the end of a dive, I think a better solution if you are only 1-2# off is to practice breathing out fully, and to make certain there is zero air in your bc at the end of the dive. the only air you need in your bc is to compensate for the weight of the air you breath during your dive. (5-6# for an 80cf) as far as going up weather you want or not, this should not happen, you should be able to swim down a pound or two negative buoyancy. as a last resort you can pick up a rock that is not being used by any sea life.

 

[elan]

This will result in being greatly overweighted. You simply can't get all the air out of your wetsuit at the start of a dive. That's why people have trouble getting down at the start of a dive even with a full tank.

Good point about air pockets.
Never had that issue may be because my suit is sized properly.
To get the air out of the suit is easy. You need to take a vertical orientation and pull the neck and flood it also you can grab and pull neoprene along the torso so the air moves up.

 

[lowwall]

Good point about air pockets.
Never had that issue may be because my suit is sized properly.
To get the air out of the suit is easy. You need to take a vertical orientation and pull the neck and flood it also you can grab and pull neoprene along the torso so the air moves up.

That gets the big pockets out, but there's going to be little bubbles trapped along seams, at joints, in the irregularities of the suit surface and in cells in the neoprene that are partially open to the surfaces. It adds up over all the square feet of suit surface area.

 

[rick00001967]

Good point about air pockets.
Never had that issue may be because my suit is sized properly.
To get the air out of the suit is easy. You need to take a vertical orientation and pull the neck and flood it also you can grab and pull neoprene along the torso so the air moves up.

when diving wet i always try to remember to flood the suit before gearing up.