Going down

[Deefstes]

Hi all,

I have a couple of questions but they all deal with the descent part of the dive so I thought I'd put them in one thread.

1. Do the terms "positive descent" and "negative descent" mean anything to you guys? Are they widely recognised terms or are they just being used amongst the limited number of divers I've had exposure to? What does it mean to you?

2. I understand that the rate of descent does not affect your health, at least not as far as gasses are concerned but, apart from your ears not getting enough chance to equalise, are there any dangers to descending too fast?

3. What would be concidered a normal rate of descent and what would be considered a fast (but achievable) rate of descent?

And just out of curiosity (a noob trying to develop a feel for what other people do), describe for me your typical and preferred descent procedure. Personally I like to fall over backwards and start finning immediately (positive descent?). Some DM's prefer that the group bob on the surface and then descend together (negative descent?) but I hate bobbing on the surface as that is where I often get nausious. If the DM ask the party to group on the surface and then descend together I usually ask him if he would mind if I start ascending immediately and wait for them at 3m or 5m.

 

[seaducer]

Hi all,

I have a couple of questions but they all deal with the descent part of the dive so I thought I'd put them in one thread.

1. Do the terms "positive descent" and "negative descent" mean anything to you guys? Are they widely recognised terms or are they just being used amongst the limited number of divers I've had exposure to? What does it mean to you?

Some good questions. I am not sure about "descent", but there is positive and negative entries. In a positive entry you just drop in with air in the bc, pop to the surface before descending. In a negative entry, you remove all the air in your bc so when you splash you just keep sinking. This is usefull when trying to hit a small target in a current on a drift dive, ie no down line to guide you. Obviously you want to be sure you are squared away before doing this, as you can be 10-15 feet down adn dropping when you realize your air is shut off!

2. I understand that the rate of descent does not affect your health, at least not as far as gasses are concerned but, apart from your ears not getting enough chance to equalise, are there any dangers to descending too fast?

Squeeze hits more than just the ears, your sinus passages for example, and your mask. I have seen people come up with black eyes before from mask squeeze. But on deeper dives it may be that narcosis can hit you faster and harder with a fast descent.

3. What would be concidered a normal rate of descent and what would be considered a fast (but achievable) rate of descent?

Two things you need to consider, as opposed to ft/meter per minute.
1- don't drop faster than your slowest buddy is comfortable with.
2- don't drop faster than you can control. You don't want to crash into the bottom when you get there.

And just out of curiosity (a noob trying to develop a feel for what other people do), describe for me your typical and preferred descent procedure. Personally I like to fall over backwards and start finning immediately (positive descent?). Some DM's prefer that the group bob on the surface and then descend together (negative descent?) but I hate bobbing on the surface as that is where I often get nausious. If the DM ask the party to group on the surface and then descend together I usually ask him if he would mind if I start ascending immediately and wait for them at 3m or 5m.

It is going to depend on the type of dive it is. Generally we group on the surface and descend together, I prefer generally to fall spread eagle (think skydiver), but I can sometimes swim it down, leveling off when I get there. I almost never descend feet first anymore. I have lost the need/desire to be feet down in order to be comfortable on dives. Feet down is for when my head is in air.

 

[Deefstes]

Great, thanks. Those were prompt and good answers, mentally filed for the next diving trip.

I am not sure about "descent", but there is positive and negative entries.

Aaah, that would be it yes.

 

[BGF]

Hi all,

2. I understand that the rate of descent does not affect your health, at least not as far as gasses are concerned but, apart from your ears not getting enough chance to equalise, are there any dangers to descending too fast?

3. What would be concidered a normal rate of descent and what would be considered a fast (but achievable) rate of descent?

And just out of curiosity (a noob trying to develop a feel for what other people do), describe for me your typical and preferred descent procedure. Personally I like to fall over backwards and start finning immediately (positive descent?). Some DM's prefer that the group bob on the surface and then descend together (negative descent?) but I hate bobbing on the surface as that is where I often get nausious. If the DM ask the party to group on the surface and then descend together I usually ask him if he would mind if I start ascending immediately and wait for them at 3m or 5m.

I've been taught that the rate of descent does matter, and should be controlled. I won't go into the detailed lecture I got (GUE instructor) but basically our body is used to a "regular" 1 ata, if we jump in and drop like a rock the immediate squeeze lets our body know something isn't right and it goes into defensive mode. With regard to DCS risk we want our body to be relaxed and calm, NOT on the defensive. Normal rate of descent should be about the same as ascent, ~30ft/min.

the preferred descent should be horizontal (in trim) and relaxed. Finning your way down works your muscles, increases metabolism and starts the build up of CO2 before you even reach your desired depth. For most beginning Rec divers (yup, that's me) our SAC rate is already not optimal. If your working hard from the very start you're gonna be a hoover and have limited BT. Second, and most importanly, if you're working hard and building CO2, you greatly increase the risk of DCS (and/or O2 tox). Stay relaxed, in trim, and let your negative buoyancy do the work for you.

 

[TSandM]

As far as I know, there are no deleterious effects of rapid descents, as long as all air spaces can clear. Narcosis will be more obvious, as you go from your normal consciousness at the surface to the state of narcosis at depth far faster, so the change isn't gradual and you do notice it more.

The biggest thing about descents is that they are one of the places where buddy separation most easily occurs. Especially in current or limited visibility, controlling the descent so that you can stay in visual contact with your buddy is very important. Because things are changing, descents are also where people get into trouble -- Discover their BC hose has popped off, or they can't clear their ears, or that they have some equipment malfunction. It's a time when people often need assistance, so staying together is key. If you do negative entries, you need to make sure your buddy is comfortable with and capable of the same, and that you have the skills to stay together. Of course, in a place like Cozumel, with 100 feet of viz, it's pretty easy to keep track of the team on descent, but still pretty easy to get separated enough that you couldn't really be of any great help to someone who is behind you in the current.

 

[don Francisco]

I'd imagine that within reason, there wouldn't be any consequences to rapid descents, of course assuming that equalization was OK. I often have ears which are slow to equalize and my descent speed varies from about 10fpm or less on bad days to about 90fpm when there are no problems and I want to get down quickly, as I might in a current.

Usually after the first few feet I continue descending in horizontal trim, which allows me to stay with others or stay on target in a current. When new divers ask, I always suggest the feet down position which makes it easier to modulate descent speed or stop and go back up when clearing is problematic. I've seen too many new divers swim down trying to keep up with the group & come back to the boat with earaches.

In short I'd say that with descending, anything goes, if it works for you in the given situation.

 

[ronbeau]

Great, thanks. Those were prompt and good answers, mentally filed for the next diving trip.


Aaah, that would be it yes.

The NAUI open water course I took 4 years ago had a mximum suggested decent rate of 75 feet per minute.