Descending Vertically Vs Horizontally

[ScubaTwo]

OK, maybe I can do better this time. Probably not, but I'll try.

Your tank is HEAVIER in the water when it is full, & LIGHTER in the water when it is nearly empty. Heavier full, lighter empty. At the beginning of a dive the tank is full, so the tank is heavier. At the end of a dive the tank is nearly empty, so it is lighter. Now, with the tank being lighter at the end of your dive, if you can hold a really shallow stop with no air in your BC, you would need MORE air in your BC to hold the same stop at the start of the dive (remember, your tank would be HEAVIER).

So how did I do?

just fine *sigh*

 

[ScubaTwo]

Thank you for that lesson. I had forgotton all that stuff.

 

[Firefyter]

Whether you dump all the air out of your BCD or just some to start your descent is irrelevant as long as you start your descent.

Agreed, as long as you don't HAVE to dump it all in order to descend. I still maintain that if you HAVE to dump it all, there are other issues that need to be addressed. If you knowingly choose to do so in order to make a quick descent, be it to get out of a current or just because you like it that way, then I say go for it, it's your dive, have fun!

What is relevant though is maintaining a controlled descent. You don't want to get in the habit of hitting the bottom and then adding air. Add air in short bursts on your way down to remain slightly negative and then get nuetral once you have reached your depth. Yes, not letting all the air out on the surface and still descending does save you some tank air on descent, but if your that obsessed with conserving your air then don't use your power inflator and orally inflate on the way down.

For me, it's not so much about saving air as it is maintaining control. If I haven't let it all out, then I'm able to control a large part of my descent with my breathing, and have to add less to get neutral.

With 500psi in your tank at 15ft with no air in your BC, you should be able to easily stay neutral. If your struggling to stay down or up, than your weighting needs to be adjusted accordingly. One final way to check this is the buoyancy check at the surface as described above.

Jason

Agreed 100%.

 

[ScubaTwo]

For me, it's not so much about saving air as it is maintaining control. If I haven't let it all out, then I'm able to control a large part of my descent with my breathing, and have to add less to get neutral.

I guess Im a freak of nature cause I do let all the air out and I dont add air to get neutral. But as far as how slow I descend I thank you all for your suggestions and will work to improve on it. Happy and safe diving

 

[Randy43068]

I am somewhat disturbed by your indication that weighting and trim were not very important in your pool dives. Proper weighting is stressed and addressed in all of the classes I assist with. It creates a much easier time for the student and instructor. Also although you said you were recently certified you did not indicate how much open water experience you have and in what conditions. I started experimenting with sky-dive ascents at dive number 28 in the devil's eye entrance in Ginnie Springs in my first ow dive in a drysuit to a depth of 22 feet. what a great feeling! clear, cool water,no rocks to impale myself on should my descent go a little too fast, and a nice soft landing area with no one coming down on top of me. I continued to practice the whole time we were down there and at manatee springs and in the devil's den. I also do it now alot when assiting with training dives with new divers as long as I'm out on the edge of the group. If I need to assist some one down the line it's feet first. Also why would you use the dump valve to bleed air on a normal descent unless you're not carrynig enough weight or a boat is coming at your head? You should be adding small bursts with your inflator once you start to descend in order to effect a nice easy controlled descent. That way you don't end up picking up speed and ending up knee deep in silt with zero vis and icepicks in your ears and sinuses. Believe me I'm not picking on you but the seeming lack of instruction concerns me. It's these things that cause problems which can lead to accidents that make diving seem unsafe and give competent instructors and their assistants a bad rap. I woul encourage you to at the least find some experienced divers in your area to dive with and let them know about your experience. Most will be more than willing to help and in fact be more willing to dive with you if you are honest with them. I would hesitate to dive with someone who has not at least gotten control of his descents. I also urge you to take a course in peak performance buoyancy or whatever your system of education calls it. It will not only make diving more fun but alot safer.

Proper buoyancy wasn't stressed much at all in my PADI course either. In fact it was almost ignored in a friend's PADI class. IMHO PADI training should make more of an effort to include it in the basic OW course.

 

[fmw625]

All the math is confusing. I do know that I use 4# in freshwater, and let out only enough air to start my decent. Is it all out? I don't know or care. Usually feet first, been trying horizontal lately. Pertty cool. At the end of the dive no problems with safety stop. Recently in saltwater (limited experiance) with the same gear configuration, I figured 6# would be enough (50% more than fresh water) Smallest increment avaliable. To my amazement I could not descend. Added 2# more (now at 100% more than freshwater), all went well. Horizontal decent used here. The math just doesn't work. Salt water is not twice as dense as fresh water.

 

[markfm]

Salt water vs fresh:
It's on the order of 2.5% difference.
The thing is that that is 2.5% of your total weight -- if you were on land, fully geared, walked onto a scale.
An AL80, full, is about 37 pounds.
BCD, wetsuit, fins,... is about 15 - 20.
Add you fresh water weight.
Add your body weight.
I'm 185 lb, wear 12 lb in a 5 mm. 185 + 12 + 37 + 15 (gear) = 249 lbs.
To wear the same configuration, salt, I would need 2.5% of that 249 lbs. About 6 pounds, so I would change my add-on weight from fresh water 12 to salt water 18.

For a quick rough order -- think of it as 1 lb (tank/gear), 1 lb rounding, plus 1 lb for every 50 lb of body weight. (1 lb per 50 is 2%, and we're trying to cover a 2.5% change)

For me that would translate to 1 lb + 1 lb + 4 lb (185 is close to 200, so that's 4 sets ofi 50 lb) = 6 lb.

A big change can occur depending on suits. Fresh/cold is that 5 mm, 12 lb for me. Switch to a lycra skin and my fresh weight drops to 4 lb (so salt is 10 pounds)

(Edit: corrected math for the 1/2% part)

 

[spectrum]

On the return leg of a shore dive we can see a considerable time span in less than 15 feet. Sometimes there's lots of critters and back in the shallows you can have a lot of fun getting from 1000 to 500 psi. I was with a new buddy last night at a site he didn't know and we spent the whole 60 minutes with a max of 15 feet. IMO you should always be able to make a controlled ascent to the surface.

I use the eye level bobbing with 500 psi and normal lung volume at the end of a dive to set my weighting baseline.

Pete

You are overweighted if you have to descend in that manner.

I dive AL80s on most boat dives, being weighted for the end of the dive you still should not have any air in your BCD when you start your descent.

A properly weighted descend isn't as easy as simply releasing some air and sinking. You have to release all the air in your BCD, and your lungs. You need to exhale and when you think you have no air left, exhale some more, completely relax, and then you'll get down. You'll still be properly weighted at the end of the dive. While you gain some bouyancy from the tank, remember, you are doing your safety stop at 15 feet, not the surface. You need to be weighted for a depth of 15 feet with a bouyant tank.

As you descend, you should only ever have to add a short burst to control your speed, depending on the wetsuit, I sometimes don't add air till I'm at 50 feet. I can simply change descent speed by breath control.

It really does sound like you are just a couple of pounds overweighted.

 

[m3830431]

I can hold it 5' with a near empty tank, havent tried 2' tho. And yes I need to vent all air in order to descend. My instructor also thinks Im holding alot of air in my lungs. I keep practicing Its more excuses to dive..must practice!

Once I get above 5 feet with approximately 700 psi and a 6.5 mil farmer jon, I have to swim and usually down or I will come up.

I can descend and don't usually have to add air until 25 feet with a full tank for control or hovering.

 

[AquaHump]

I'm a bit confused as to your question. You should have all the air out of your BCD at the surface, so there is no need to release more to descend, no matter what orientation you use.

Start feet down, dump all your air, exhale completely, exhale some more, when you get to about 10 feet, roll to horizontal, then add air to your BCD to slow your descent. You shouldn't be taking any breaths in until your are a few feet down, otherwise you'll require too much weight to descend.

Thank you man I thought I was the only one thinking that. You should have no air in you bc for you desent