Descending Vertically Vs Horizontally

[TeddyDiver]

I'm not sure, but have beeen seeing comments that a 7mm compresses about 1.5 - 2mm at 100' depth, call it 30%.

It's not quite right. I'm size M, and my wet suit 7+5shorty. Their total volume (with cloves and booths) is about 16litres =>16kg/35lbs, weight about 2,5kg/5,5lbs, so the net boyancy 13,5kg/30lbs. This boyancy is affected by the pressure accordingly- 10m/33' half, 20m/66' third, 30m/100' quarter (3,3kg/7,3lbs) and that's where I become negatively boyant IF I'm neutral at the surface and IF I'm using low volume BC. With overweighting this point of no return comes sooner...

 

[markfm]

Newbie time. I thought one of the goals was to get your required weight down to the minimum. I've dropped enough weight now (185lbs, 3 mil, 8 lbs lead) that I can't descend by emptying my bcd and exhaling. I now have to fin down (assuming I don't have a line to descend with). ...

The only goal is safety; minimizing weight comes with time/experience, form factor. If you have to fin down at the start of a dive, you are underweighted. You use gas during a dive. A full 3000 psi AL80 holds about 6lb total of gas. The tank will have lost about 5 lbs if you are on the surface at 500psi, 6 lb if you are at 0 psi (breathed it dry, bad to do but it can happen).

You should be able to hold a clean end-of-dive safety stop, off gas, at 15' without having to fin down or rely on something external. If you are already slightly buoyant to start off with, then lose 5 lb of gas, you will certainly be positively buoyant at the end.

I'm the same weight as you, run 8 lb in a 5mm core warmer only, but it took some dives to get to that point. Do a few dives at a given weight, make sure you can do a clean safety stop at that weight with a near-empty tank, a SLOW ascent from 15' to surface at that weight when the safety stop is done. Only then try knocking off another 2 lb.

If you have to fin down at the start, it sounds like you're actually maybe 5 - 6 pounds underweighted right now. Give it time -- rushing to drop weight too fast is an invitation to DCS.

 

[ScubaTwo]

You add no air to get neutral...so what do you do when you go deeper? Do you add air then? How about when you come up to a shallower depth? I'm sorry but if you are nuetral at 60 ft with no air in your BC then there is no way in the word that you can be nuetral at 15 ft. Not happening in this world.

Ok

 

[markfm]

I'm not sure, but have beeen seeing comments that a 7mm compresses about 1.5 - 2mm at 100' depth, call it 30%.

It's not quite right. I'm size M, and my wet suit 7+5shorty. Their total volume (with cloves and booths) is about 16litres =>16kg/35lbs, weight about 2,5kg/5,5lbs, so the net boyancy 13,5kg/30lbs. This boyancy is affected by the pressure accordingly- 10m/33' half, 20m/66' third, 30m/100' quarter (3,3kg/7,3lbs) and that's where I become negatively boyant IF I'm neutral at the surface and IF I'm using low volume BC. With overweighting this point of no return comes sooner...

Ahh, thanks. I must have interpretted the numbers I was seeing wrong. What you say is consistent with a DAN writeup.
Might it not actually be slightly worse, in that case? If your surface displacement is 16l/16 kg, then at 30m you have a displacement of 4l/4kg. Now, the physical weight of the suit hasn't changed -- it's still 2.5kg. 4kg of displacement minus the 2.5kg actual weight means you would be at +1.5kg buoyancy. You would have swung from that +13.5 initial buoyancy to +1.5 -- a full 12kg difference.

Just trying to understand, to learn the correct way to do this. Thanks again.

 

[Charlie99]

Newbie time. I thought one of the goals was to get your required weight down to the minimum. I've dropped enough weight now (185lbs, 3 mil, 8 lbs lead) that I can't descend by emptying my bcd and exhaling. I now have to fin down (assuming I don't have a line to descend with). At depth I give my bcd one quick burst and I'm good until I want to ascend. Then I dump the remaining air from my bcd and slowly fin up.

I've read on this thread about uncontrolled descents using the finning down technique. Would this be because of overweighting? I'm I looking for trouble descending in this manner? I do like the head first feeling.

The goal is to have the RIGHT weight. IMO, 3 or 4 pounds too little lead is worse than 3 or 4 pounds too much, since if you are too light, then you will have trouble doing a safety stop with a near empty tank.

At the beginning of the dive and no air in your BCD, you should be negative by the weight of air in your tank --- normally about 6 pounds or so. OTOH, your wetsuit will be a little more buoyant than normal because it hasn't yet saturated with water.

IF you are able to properly hold a safety stop, and then do a controlled ascent to the surface with a near empty tank, then you have the right weighting, otherwise you should add back on a few pounds.

 

[Randy43068]

Originally Posted by Xanthro
The starting and ending bouyancy characteristics will be determined by tank composition, but the swing between the starting weight and ending weight is affected only by the amount of air used.

Ding! Ding! Ding! Ding! We have a winner!

but what if I have a bag full of bugs I'm bringing up? Huh? Then what? < kidding of course >

 

[markfm]

...OTOH, your wetsuit will be a little more buoyant than normal because it hasn't yet saturated with water...

Curious -- how much water does a wetsuit normally pick up? I've never tried to weigh before vs after. Thanks.

 

[Sasquatch]

That's 27lbs for a 7mil. That's usually going to be too much weight. I dive 21lbs in a full 7 with a 3/5 hooded vest and that's still likely on the high side.

When we start talking about specific pounds/weight we have to start talking about ALL factors. Freshwater or saltwater? How much do you weigh? What's your lung volume? Is the rest of your gear neutral, pos or neg? It's different for every diver. Just a small point but hey...

 

[Charlie99]

Curious -- how much water does a wetsuit normally pick up? I've never tried to weigh before vs after. Thanks.

Just guessing from carrying wetsuits when dry and wet, it's at least 3 or 4 pounds for my 5mm. I just measured my XL size 3/2 full suit. Crude measurements on a bathroom scale, but it looks like about 2 pounds dry, 4 pounds soaked.

In addition to the wetsuit, the fabric of my Seaquest ProQD BCD will be picking up water.

I definitely know that over the first few minutes of the the 1st dive of the day, my buoyancy is decrease. I don't know for sure the exact reason, but my suspicion is gear getting saturated. I initially thought it might be air trapped in gear, but doing various contortions to make sure all air comes out doesn't seem to speed up the process.

 

[markfm]

Thanks! I was thinking about 2.

Really beats me on the 7mm loss of buoyancy. I rechecked my wetsuit, it's a full 5mm. I know the change from diveskin to 5mm is 8 lb buoyancy, so would expect a 7mm to be no more than 12 lb suit contribution, since 7 is 1.5 x 5. That would indicate I cannot get more than a 12 lb buoyancy swing on a 7mm, at 100' depth.

I guess that's why it's important for each person to check their own weighting, with their own gear. If it's doable, a baseline in a pool with just trunks or a diveskin is a nice starting point, so that you can then tell the delta going to different neoprene configurations.

Good stuff!