What would be a safer weight next dive?

[Quero]

Scuba_chicck, most of the points highlighted in the various responses to your question are related. Let me see if I can pull them together for you.

First of all, as you dive deeper you use more air from your tank on each breath. When your tank gets to about 700 psi it starts becoming rather buoyant, and this is particularly noticeable when the tank is made of aluminum. It's quite possible that on your training dives you surfaced with far more air than you did on the deep dive in which you experienced an uncontrolled ascent, so it could be that the tank was dragging you towards the surface.

The second point is that in order to compensate for the buoyancy of a mostly empty tank, we add weight. Knowing how much weight to add is the purpose of the buoyancy check. You should know how to do this check from your open water course, and whether you do it with a full tank or a nearly empty one doesn't much matter as long as you end up with the appropriate amount of weight according to the size of the tank. We know from experience that for the standard 80 cubic foot tank, the extra weight needed is about 5 pounds, so if you add 4 to 6 pounds (depending on how the weights are configured) when doing a weight check with a full tank you should be close to neutral at the end of the dive in order to maintain your safety stop.

Third, in terms of the question on tidal volume, or buoyancy swing between full and empty lungs, as with all air spaces, the swing is greatest at shallow depths. So if you fail to breathe out completely at a deeper depth, you won't generally go flying to the surface, but if you hold air in your lungs at a shallow depth, that can add to your buoyancy swing. In my experience teaching many new divers, once stress sets in people have a very hard time expelling all of the air from their lungs. The rib cage muscles become tense and it's impossible to breathe out completely. This remaining air in the lungs is all it takes sometimes to provoke an uncontrolled ascent, and of course the more uncontrolled it is, the more stress it induces, and the less able the diver is to expel all of the air from the lungs.

Finally, any little bit of air that is trapped in the BCD expands as you surface. If you added air to your BCD when you were deep, you needed to dump that air on ascent. One mistake I see students make is that they forget to continue to dump the air periodically while they are ascending. They think that if they dump it "all" at depth the BCD is empty, but there are nooks and crannies that trap a few bubbles of air; these expand and need to be released now and then as you are on your way to the surface.

In other words, if a diver's tank gets light and s/he is even a little bit underweighted, forgets to dump air from the BCD, starts drifting up, gets stressed and can't exhale fully, an uncontrolled ascent is pretty predictable.

The solutions: do a weight check whenever anything about your gear changes, get your weighting right taking into account the likelihood of a buoyant tank at the end of the dive, dump air periodically as you surface, and make sure that you exhale fully when you begin to experience stress, especially at safety stop depth.

 

[Scuba_chicck]

Thank you Quero...now that I am understanding what likely happened...I don't feel as lost at what I did wrong.

 

[Quero]

My pleasure, Scuba_chicck.

 

[highdesert]

I second the concern brought up by g1138 ...

You are a new diver, 0-24 dives, OW, no mention of AOW or Deep cert, but your dive was to 102' ... why is that?

 

[g1138]

I second the concern brought up by g1138 ...

You are a new diver, 0-24 dives, OW, no mention of AOW or Deep cert, but your dive was to 102' ... why is that?

It's been answered already. The dive boat didn't brief their divers until they were geared and on site.
Just a mistake of not knowing what to do in a situation like that.
We've all been there.

 

[CuracaoJ]

From the post you made as I was typing I'm understanding that you didn't deflate your BC while ascending, am I correct?

I've noticed a lot of times that newly certified divers have this same issue (seen in happen twice in as many weeks). They think that because their "BC" was empty at depth, that it will be empty all the way up. It takes a while to get used to Scuba chicck, but when you go up you have to constantly bleed air out of your BC. No matter how empty you think it is there is always that little puff in there that is just waiting to launch you like a Polaris rocket out of a sub as you start to ascend.

If you were able to descend when your 5MM suit was more bouyant (they get air trapped in the neoprene until it gets all compressed out at depth) you shouldn't have been able to go straight up with your buddy trying to hold you down.

All that being said, 14# seems a little light for a full 5MM suit. What you can do is a bouyancy check before your next dive in your 5MM suit. Once you get good bouyancy throw in another 2 pounds or so to make up for the air you are going to loose out of your tank, that way you won't have to fight to keep down at your safety stop, but shouldn't be to overly weighted.

Keep diving, and hopefully, keep enjoying it!

 

[chrpai]

Put another way, the BC isn't an elevator. It's exactly what it's named: a Compensator. It seemed counter intuitive to me at first to put air *IN* while I was going down and to let air *OUT* while I was going up.

My goal during a dive is to be ever so slightly negative bouyant during the descent so that I can go down slow and easy while making sure to equalize. During the dive and the ascent I want to be neutral at all times.

During the ascent I still want to be neutral. The concept is I'm slowly and effortlessly ascending no faster then my smallest bubbles. Say 30' per minute.

As I rise any air left in my BC and my wetsuit, if I have one, is going to expand making more bouyancy. So I let a little more air out of my BC to keep compansating for this. Finally I get to my safety stop and I'm able to effortlessly hover because my weighting is correct.

 

[NightForce]

This was a worth while topic with some quality answers. Thanks for posting your experience Scuba_chicck.

 

[Jim Lapenta]

There has been a great deal of good useful info here. I am sorry if some took exception to my post questioning training. If I could throw my two cents in from a practical standpoint I would add that determining proper weighting is a process that if taken in steps results in a pretty good idea of what you need. The process I use is this:
1. Determine your weighting requirements with just a bathing suit. Some may not need any but once this is set it remains constant barring changes in overall physique.

2. Determine your weight in the exposure suit you will be using if possible. Rental suits may make this difficult so having your own is a good idea if it;s within your budget. Rental suits may be older and the neoprene compressed permanently and the difference between a new one and a rental of the same thickness, say a 5mm, could be a few lbs. Get in water too deep to stand with only snorkel gear and start checking so that when you exhale you slowly descend into the water.

3. Starting with this weight add your scuba unit and recheck. Make note of the BC type you are using and record it. My students do this check at the beginning of every pool session and are required to note how much weight they need. Again you should have just enough that when you exhale with an empty BC you should SLOWLY sink. If you go down like a stone remove some lead.

4. Repeat the descent check in a horizontal position. If the first three steps were done correctly you should look like a snowflake settling down into the water. If there is some difficulty add maybe 2lbs and try again. Once down on the bottom of the pool we will just lay on our stomachs and breath. Try to get into a nice relaxed rhythm and just do that for a few minutes.

5. Work on getting neutral in a horizontal position between the shallow end and the deep end of the pool if possible. If in open water (pool is preferable for practice though) and using a line I like for students to get to about 10 feet and get in a horizontal position for the rest of the descent. This is easier to manage and control and safer as you can see what you are coming down on.

6. In the pool work on swimming and hovering in trim as much as you can. I like to give students 20 minutes or so at the end of each session (there are 6-8 of them) to just work on buoyancy and trim while doing skills. Mask R&R, Reg R&R, weight system R&R, OOA drills, and buddy breathing.

7. At the end of the last 3 sessions I plan on them having around 500-700 psi in their tanks. I know their air consumption rates by then and will plan for practice to take enough air to get to that. We will then repeat the weight checks and make sure they can maintain a hover at 5-7 feet or so. Again making sure that they are in the suits they will be using for checkouts.

This is all basic OW stuff.

Once we get to open water I do not assign weights. That is not my job. The students decide how much weight they need and are expected to conduct the buoyancy and bubble checks themselves under my supervision. I watch and am within reach if they mess up somehow.

Between dives 2 and 3 they are expected to know if they need to adjust their weights and will do so. I have had students get comfortable on checkouts and tell me they think they should drop 4 lbs. My response is "go for it". If that is too much we soon find out and I always have a couple 2 lb weights handy to give them if they need it.

It sounds like with the OP there was some unexpected task loading -depth for one, and she was not ready for that yet. Coupled with not being fully familar with the gear a situation developed that was not good. No injuries resulted which is good but there could have been and that is why I go back to initial training. Properly weighted and being completely familiar with the gear would have gone a long way towards perhaps preventing this.

More so would have been the realization that this dive was beyond your recommended limits as an OW diver. That is a training issue as well. Another item to consider that no one has yet mentioned or I missed it is that if you need to add more weight do so. But becareful. Overweighting is the cause of more runaway ascents than underweighting among new divers. Too much requires more air to compensate for it at depth. Start to ascend and the new diver may find themselves in a situation where the air expands faster than you can vent. This is why you need to stay on top of your skills, know your weighting requirements, and be intimately familiar with your gear. If you don't own gear before using rental gear ask about it. Research different types, know what the markings on tank mean and especially know what your gauges do and how to read them.

Know your limits and know when to say no to a dive that is beyond them. I have never done a dive a I felt uncomfortable about and will not. There is no reason to. Op changes site midstream? Sit out the dive. THis sounds like a trust me dive. No regular buddy, poor site info, site changed knowing there were OW divers on the boat (but it's ok we do this all the time!), and seemingly no consideration for that. Those are the dives that hurt and kill people. Those are the ones that many instructors like me tell their students to avoid like the plague. You are responsible for the dives you do and more importantly and perhaps better for you- the ones you say no to.

And again all of this is in my basic OW class. There is nothing advanced or beyond the ability of any reasonably intelligent person.

 

[Bubbletrubble]

Third, in terms of the question on tidal volume, or buoyancy swing between full and empty lungs, as with all air spaces, the swing is greatest at shallow depths. So if you fail to breathe out completely at a deeper depth, you won't generally go flying to the surface, but if you hold air in your lungs at a shallow depth, that can add to your buoyancy swing.

@Quero: It might be helpful to clarify your statement above. Otherwise, it might confuse a newer diver learning about buoyancy and Boyle's Law.

The buoyancy swing between full and empty lungs is due to the weight of the water being displaced. It's depth independent. For example, if a diver maximally inhales at a depth of 5 fsw, let's say that he is 10 lbs. more positively buoyant than with a forced exhale. Place that same diver at 100 fsw and have him maximally inhale at that greater depth. He should still be 10 lbs. more positively buoyant than with a forced exhale. The only difference in the two scenarios is the ambient pressure, which affects the density of the gas inside the lungs. Displacement, and consequently buoyancy swing, should remain the same.

To make this point clearer, let's consider a 30 lb. lift wing or BCD. Maximally inflated at a depth of 5 fsw, it will contribute 30 lbs. of lift. Maximally inflated at a depth of 100 fsw, it will still contribute 30 lbs. of lift.

That being said, divers will usually have greater difficulty managing a large air space in the lungs or BCD at shallower depths vs. greater depths. This is due to Boyle's Law (P₁V₁ = P₂V₂). Let's look at an example of this with relatively easy numbers to compute. If a 1 L pocket of air is inside a BCD at 132 fsw (5 ata), that same air pocket taken to a depth of 99 fsw (4 ata) will have a volume of 1.25 L. If a 1 L pocket of air is inside a BCD at 66 fsw (3 ata), that same air pocket taken to a depth of 33 fsw (2 ata) will have a volume of 1.5 L. In both examples the nominal depth change was 33 fsw, but the relative pressure changes were different (example 1: 5 ata to 4 ata vs. example 2: 3 ata to 2 ata).