Fine tuning weight for Ascent Control for last 10 feet

[drewh1]

I am fine tuning weight after switching to a single tank BC/Wing (DSS 40#/). I dive northern California with a Steel 100 with a 7 mil Farmer John (14 mil core) hood, etc. Going from a standard BC to wing and steel tank, I dropped about 16#s. So I dive 10 lbs on my weight belt with stainless plate of 6# and Worthington Steel 100. I actually was using 8 lbs but felt I needed to add a pound or two.

I recently did a Reef Check training that required good buoyancy control at 15 -25 ft. Had to hover close to bottom while counting invertabrates, algae and logging data.

So I had no problem with buoyancy or hovering at 15 ft or more. However, I felt that I could not control my ascent from 10 feet to the surface. Seems to me this is a critical piece of control because of the pressure change and also in case I must change direction to avoid surface obstacles.

Do you think this is an issue of technique or do I just need to add a bit of weight? I want to minimize the amount of lead I carry as I really enjoy the current set up. I am also changing to a DSS 30# wing to optimize things even more.

Thanks in advance for the input!

 

[DaleC]

That is just part of wetsuit diving. Your greatest decrease in pressure (and thus the expansion of neoprene which adds buoyancy) occurs at the shallowest depths.

It is an issue often pondered by vintage equipment divers who do not use BC's. Do you want to be neutral shallow and a little heavy at depth or light shallow but neutral at depth. Added to that question is the fact that you will gain 2-4lb's of buoyancy during the dive from gas consumption so you also have to decide if you want to be neutral at the end (so heavy at the beginning) or light at the end (so neutral at the beginning).

The equation basically has two variables: suit compression and gas consumption, both of which effect buoyancy.

Using a BC (deflated of course), you should take your normal gear and a spent tank (with 300-500psi in it) and weight yourself to be neutral in the water, on the surface. Then add 2-5lb's. Now you will be slightly heavy at the end of your dive but not so much that you cannot swim up (should your BC fail). Some others would argue for being neutral at the end but if retaining the ability to re-submerge due to boat traffic kelp etc... were an issue I would prefer slightly negative - but just slightly.

After doing that it is just a skill issue in regards to controlling ascent speed, venting the air from you BC proactively and relaxing enough so physics can do its job.

What you don't want to be is grossly over weighted, as you are relying on only one buoyancy device (your BC). If your BC fails then your only option is to dump your weightbelt which could pose the problem of an uncontrolled ascent. Better than drowning but easily avoidable with proper weighting.

One benefit of diving "old school", at least for a little while, is that it gives you a very good "feel" for these sorts of issues.

 

[Peter_C]

Make sure your wing is 100% vented. Even a little air in it will affect buoyancy swings in the last few feet. Are you using the butt dump, or the inflator for venting? If using the butt dump, like you should in trim, try to wrap your hand over the top of the tank. This will force the vent to the highest point, letting the air out. The mistake most people make is to pull down on the vent making it the lowest point, and of course nothing comes out.

Of course adding back on a pound or two might just be the answer.

 

[Bob DBF]

Normally I weight for 500# @ 15', but if I am going to dive shallow I will add a couple of pounds, especially if there is a swell. I also keep the air bubble as small as possible and at the dump I am using. All that being said, I learned to dive without a BC and control my depth with lungs so I establish buoyancy at depth with the wing and then use my lungs to stay there. It just takes practice to get the weight, BC, and breathing to work together, at that depth an old steel 72 may help. I had a great dive at 5' to 15' on inshore "reef" at Van Damme state park, Ca one winters day, only breaking the surface to end the dive.

Don't worry about much lead you are carrying, make sure you are carrying enough. Sometimes divers are so fixated on reducing the lead they carry that they wind up light. Weigh your lead, you may find it is not as accurate as the markings indicate, OK if you use the same weights not so much if grab any available.



Bob
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Always use the right tool for the job. A hammer is the right tool for any job. Anything can be used as a hammer.

 

[drewh1]

Good points, all. I appreciate being able to consult with a group knowledgeable divers!

1. Wetsuit buoyancy - Great point about the wetsuit. I don't recall noticing this issue when doing deeper dives. Maybe the shallow dives don't compress the suit as much and I am just a bit more buoyant at the end of the dive.

2. I do vent with the OPV when in trim and try to make sure all air is vented. It is possible I didn't get all the air out. Part of the reason I am going to a 30# is to minimize the possibility of air being trapped. With my rig, I could probably even go to a 25#.

3. I think it makes a lot of sense to add a pound or two for shallow dives. My diving buddy, who had a whole lot more experience than me, adds a bit of weight just to do the surveys. She feels it helps her manage everything when trying to juggle a slate, measuring device, pencil etc. I am pretty conscious about the risk of boats, jet skis, etc so I do like the options of changing course while surfacing!

4. I also like the idea of measuring my weight. I generally dive with my own weights but actually have never checked them on a scale.

All of this is making more me more aware how overweight I used to dive. Scares me now to think about those deep dives . . . Thanks!

---------- Post added July 20th, 2013 at 09:53 AM ----------

Oh, one more thought -

I seem to be fixed on finding the "right" weight and maybe should be thinking about the right weight for conditions, depth, task etc.

 

[elmer fudd]

Use your lungs! Take a full breath at 10' and exhale as you ascend. Works great.

You can also actively swim downward to control your ascent or grab a rock or object for additional weight

 

[drewh1]

Dove the 30# wing with 10# of weight on a belt. seemed like a noticeable difference. I had no problem hovering at 5 ft with 600#s in my tank. I felt balance down to about 15 ft and then needed a shot of air in the Wing. Also went to a weight belt instead of pouches on my harness. I like that better too. I can carry it a bit lower and it trims me out perfectly. Now all I need is more diving! I am going to take this setup to Belize in September and dive without any additional weight at all. Should be fun!