Rapid Descent Rate

[Edward3c]

When I get in the water, I tend to dump all my wing gas and exhale and descend pretty fast. When I get to depth I add gas to the wing and level off. We always worry about rate of ascent for obvious reasons but is there an unsafe descent rate? The only thing I can think of is maybe higher chance of narcosis?

The max descent rate for the BSAC 88 Tables is 30m/min. Going quicker means the bottom time calculations are no longer valid. That said 30n/min is pretty fast.

 

[GF99/99]

first of all- love the username.
second, this is kinda off topic, but yeah. Having a lot of weight by choice or being overweighted makes it harder to stop quick. Like why semis, vans and SUVs take longer- they’re heavier vehicles and usually have more stuff in them than compact cars.

Recently most of my deep diving is volunteer work for my friend in the 60-160m range with the main focus being sub 120m. So on top of all the gear/BO needed for 6hr run time sub 100m dives we end up carting around, video lights, multiple cameras, transects, dissolved O2 loggers, temperature loggers, salinity loggers, sediment traps, etc. etc. and bags and bags of sample.

So I am basically a freight train...

 

[kaylee_ann]

Recently most of my deep diving is volunteer work for my friend in the 60-160m range with the main focus being sub 120m. So on top of all the gear/BO needed for 6hr run time sub 100m dives we end up carting around, video lights, multiple cameras, transects, dissolved O2 loggers, temperature loggers, salinity loggers, sediment traps, etc. etc. and bags and bags of sample.

So I am basically a freight train...

Who needs a gym when they’re carrying all that?

 

[clownfishsydney]

In all 4500+ dives, I have descended as fast as I can. Never had a problem, even on 70 metre dives.

 

[tursiops]

When I get in the water, I tend to dump all my wing gas and exhale and descend pretty fast. When I get to depth I add gas to the wing and level off. We always worry about rate of ascent for obvious reasons but is there an unsafe descent rate? The only thing I can think of is maybe higher chance of narcosis?

Ears

 

[inquis]

if you’re plummeting towards the bottom vs swimming down

There's a difference between overweight and negatively buoyant -- timing is important.

When ideally weighted, you're neutral at the end of the dive near the surface with empty wing. At the beginning of the dive with the common AL80 tank, you have another 5 lbs of air with you. That is 5 lb negatively buoyant. Once below the safety stop, wetsuit compression removes lift, making you even more negatively buoyant (another 4 to 7 lbs at 100 ft depending on wetsuit thickness).

Not to mention some will point their head down, descend even faster, and flare at the end while mashing the inflator. (Suggest you initially practice this by stopping well above the real bottom. Mistakes can be bad!)

TLDR: it's mandatory to be negatively buoyant at the beginning of the dive (with empty BCD). It's bad to be that way at the end (with reserve tank pressure, near the surface) -- that's overweight.

 

[Rilelen]

Pardon me if I’m not experienced enough, but if you’re plummeting towards the bottom vs swimming down once you let the air out of your wing, and requiring adding air to the BCD at depth, are you over weighted?

Sure you shouldn’t have to actively swim with effort down, but you shouldn’t drop like a peice of blue ice from an aircraft in flight either.

Ah, but you become MORE negatively buoyant as you descend. You know how you have to add air at depth to keep from sinking? If you just...don't do that...you sink faster and faster.

So, yes, you can be properly weighted at the surface, and when you let out air to descend, you sink slowly for the first 10 to 15 feet. And a little faster for the next 10 to 15 feet. And a little faster for the NEXT 10 to 15 feet...until you are dropping QUITE rapidly if you're down at 120' and still haven't hit the inflator button.

 

[kinoons]

Ah, but you become MORE negatively buoyant as you descend. You know how you have to add air at depth to keep from sinking? If you just...don't do that...you sink faster and faster.

So, yes, you can be properly weighted at the surface, and when you let out air to descend, you sink slowly for the first 10 to 15 feet. And a little faster for the next 10 to 15 feet. And a little faster for the NEXT 10 to 15 feet...until you are dropping QUITE rapidly if you're down at 120' and still haven't hit the inflator button.

I get that, and I understand the principal behind it.

My understanding was that while some air would need to be added at depth to maintain buoyancy, you should never be so over weight that the air is required to arrest the descent vs using kicks to arrest the descent and then air in the BCD to reduce the work of staying at the desired depth.

As Rob and others pointed out there is a middle ground between being positively buoyant at the surface, negatively buoyant at the start of a dive (which is not only desirable but required) and a nearly uncontrolled descent towards the bottom requiring air in the BCD to arrest it due to overweighting.

 

[JoeTPhilly]

With 500psi at a safety stop, I have to reach back with my hands and flatten my wing to be neutral. And the only way I sink at that point is to completely exhale or swim down. I'm not overweighted.

What exposure protection are you wearing? Steel or Aluminum tank?

 

[Vicko]

Swimming down hard can cause problems with co2 narcosis, as well as good old fashioned overexertion. With a scooter i found that the largest problem is the other divers getting disturbed with me screaming :"WEEEEEEEEEEEEEEEEEEEEEEEEEEEEE" and interpreting it as distress.