Controlling ascent rates and weighting

[tlehman]

I'm getting ready to do a couple of lake dives tomorrow, mostly about just getting wet since it's been quite a few months and doing some skills practice. I have about 20 dives under my belt, the majority of which came a year ago on a dive trip to Cayman Brac.

I was going over my dive profiles from Cayman Brac. On nearly every dive, I have a few intervals where my ascent rate is too fast. These are not sustained, i.e., not more than one in a row, but it is an area where I need to improve. I also note that while a few are at depth, they are more frequently in the shallower part of the dive (where I think this actually matters more).

I try to monitor my dive computer that shows ascent rate, but obviously I'm losing track of how fast I'm coming up. The obviously solution is don't do that, but I'm wondering if more experienced divers here have some tips to help me keep this under control. It is hard to gauge, at least for me.

On a related note, I found in my early dives last summer that I had a very difficult time staying at 15 feet for my safety stop. With the low tank, I was too buoyant and literally had to work to stay down. I added a couple of pounds and that seemed to solve the problem, but by general guidelines I think I would have been considered over-weighted. One person suggested that perhaps I'm not getting all of the air out of my BC. I'm going to spend some time tomorrow looking into that.

I am 5'10", about 190#, and was at that time wearing just a full suit lavacore. In a rookie mistake, I forgot to record my weights for these dives, but I think I started at 14 and then moved to 16. I'll be in a 7mm suit tomorrow in fresh water. Same BCD.

Thoughts?

Thanks

Tom

 

[tbone1004]

so the shallower you are, the easier it is to blow like a cork because the pressure gradient is not linear. I.e. the ascent from 100ft to 30ft has the same 2:1 pressure gradient that 30 ft to the surface has, but with 100ft you get to go twice as far.

It is very difficult to make blue water ascents and many if not most experienced divers struggle with it because there is no visual reference other than a gauge in front of you of how fast you are moving through the water column.

For weighting, if you have a scale at home, weigh yourself in underwear, then hold the 7mm and weigh yourself again. This is likely easier than just trying to get an accurate weight of the suit. Make sure to include booties, gloves, hoods, any neoprene you are going to put on. Multiple by 3, and that should be a good starting point for you. You will have to add another 2-4lbs for the bcd if it is a normal jacket bc and if you are using an aluminum tank. Alternatively if you have a luggage scale, take it to the lake and weigh the rig with a full tank. Subtract the amount of gas in there *6lbs for an al80* and if the value is negative, then add that amount of weight to your belt, if positive, subtract from the amount of weight on your belt. This should get you into a real bloody close range for your weighting and anything else that feels weird on a safety stop is a function of your technique.

Remember that in a thick wetsuit, the buoyancy change is exaggerated because you are not only increasing the air volume in your wing with the pressure delta, but you are also increasing the buoyancy of your wetsuit so you have to really stay on top of dumping to stay neutral. Make sure you are not using the inflator valve for dumping air. You only use it to dump when you are vertical which is your initial descent, and the final 10ft of your ascent to make sure you don't hit your head on anything. Everything else should be done by the hip pull dumps.

After that, it's just experience.

good luck and report back!

 

[tlehman]

Thanks for that substantive and helpful response.

I'm interested in the scale approach and could just bring my home scale along (I'm just driving to the lake). But I'm confused about the calculation. The 3x calculation uses the total weight of the gear? What about my weight?

I just ran numbers on an online calculator - 195#, fresh water, AL tank, and 7mm. The estimate is 20-22lbs, which is just about at the 10% body weight rule of thumb I've heard.

I am renting the wetsuit tomorrow as I'm not really all that sure I want to do a lot of diving water this cold. I have dived a 5mm so I have some experience with the expansion.

I do wonder if I'm retaining air in the bcd. Another area to focus on tomorrow.

Thanks again. I'll post an update.

Tom

 

[tbone1004]

don't use that rule of thumb. The 3x calculation is just a rough approximation of the ballast required to sink the neoprene. It may end up being a few pounds too much depending on how well used the suit it, but unlikely to be more. Your body weight is irrelevant in ballast calculations though your body composition can change it a pound or two in either direction unless you are very overweight or have a very low body fat concentration.

 

[boulderjohn]

I would love to be with you working on this, because the weight you described for the first dive sure seems like a lot to me. Have you done a standard weight check?

 

[tlehman]

That makes sense, tbone. I hadn't really thought about it, but obviously when I just swim or snorkel, I don't need weights. The blinding light of "oh, i get it" comes shining through,

And boulderjohn, I also wish you were around to work on this with me! My dive buddy for this is much more experienced than I am, so I'm hoping he'll be able to offer some input as well.

If by standard check, you mean the floating in the water test, I've not done this with the heavy wetsuit nor in fresh water. My main determination in the Cayman islands was that in my first couple of dives I was having problems staying down at the end of the dive. Those were ocean dives with a lavacore that I'm guessing is equivalent to a 1mm or so. But those weights are from memory, but sound about right. I know I added 2 pounds and that wasn't a huge proportion of the original weighting, i.e., not like going from 6 to 8. Maybe my less than slim and trim figure is a bit buoyant.

Sorry to be dense on the 3x. A 7mm suit is something like 8 lbs and with the hood, gloves and boots, I'm sure it is probably closer to 11, right? So the starting point is 33lbs? If I'm understanding correctly, the regs and tank (less most of the air) count towards the needed weighting. I'm not sure what to add for regs and the tank. If they weigh 20lbs, then I'm now at 13. My BCD is about 7 lbs and I don't know how neutrally buoyant it is. Am I tracking on this correctly? And maybe add a little for salt water and substract a bit for fresh water.

This will be a starting point for tomorrow. Since we're not doing a formal and led dive, I can take time to do weight checks and make adjustments.

And thanks again for the input. It's really helpful.

 

[sadn3sss]

So I'm not really qualified to give you advice but I did have to work through those same issues you mentioned. Maybe the lessons I learned might be able to help you.

The combination of diving in a 7mm and behing overweighted is going to make it a lot rore challeging than diving in the Cayman's, especially if you're wearing a two-piece farmer john. The buoyancy changes between the wetsuit and the bcd, especially in the first 33 feet, is what makes it tough.

1. You'll have to dial in the proper weighting over several dives. Every extra pound hurts you.

2. Learn to work the valves. I remember just yanking on all my valves and thinking my bcd was empty. It wasn't. Depending on your position in the water, certain valves work better than others. Sometimes you'll have to manuever to get them to dump. Vertical position, use the inflator hose, or back should valve if you have one. To use the lower hip valve, you might have to put your head down a bit, or roll to the opposite side. It'll take time and practice to see what works for you. You can also use your hands and reach back to feel the bcd to see how much, if any, air is left if you think it's empty. That should give you a feel. Again, it takes time to get used to.

3. Control your breathing. I used to take very deep breathes because that was what I was comfortable with. Don't do it. Deep breathing or emptying of your lungs should only be used to control your buoyancy, not as normal breathing.

4. Be aware of your depth and its changes. I used to only adjust my bcd when I noticed a major change in my depth. Sometimes it might be too late when you do, you'll already be shooting up or will have to swim down and readjust. If you're swimming deeper and deeper, you'll have to remember to add in small burst of air after every few feet. If you're swmming shallower, you'll have to remember to dump a small amount of air every few feet. Just be conscious of it instead of waiting until you notice a change. It'll take some diving to get used to.

Any one of those factors wouldn't be a big deal but when combined, I had a few uncontrolled ascents and several near misses.

Hope it helps.

 

[tbone1004]

sorry, brain fart forgot some key info for you. So the 3:1 is what it should displace, so it will displace roughly 33lbs of water, but it does have mass, and that will depend on the particular density of the suit in terms of how much lift it will actually provide. Closer to 20-25lbs is more inline with experience for a 7mm. Your boots may have particularly heavy soles and what not which may be artificially inflating that weight.

Jacket BCD will roughly be offset by the weight of the regulator, and an AL80 needs about 4lbs to sink, so I'd start with 25lbs and go from there, if you have to add some more, well that's why you're doing confined water diving to figure that out. Make sure at the end of the day when the tanks are basically dry, you do a proper buoyancy check and you'll be good to go. also of note is that wetsuits will see permanent compression over time so you will be able to drop some weight over time which is good

 

[outofofficebrb]

Hi!

For your ascents, try not to swim vertically because you ascend a lot faster than you think that way. When I first got a dive computer, I would look at the ascent rate graph fill up and visualizing what that actually was in my movement vs. what it was in reality based on the computer linked the two together for me. When I ascend, I try to swim upwards slowly while moving forward. Often times, I don't even have to look at my graph to know that I'm going at a safe rate.

As mentioned above, the air in the bladder may be in a different spot than you think; utilize all your dump valves in the different positions to see if there's any air remaining near them. Breathing also has a bit to do with it so if you're taking large inhalations and large exhalations, you're going to see large fluctuations in your depth profile simply from that alone.

 

[tlehman]

Thanks for everyone's guidance and input. Yesterday was a bit of a bust, but I learned a lot in the process. I'd never dived a 7mm suit and was amazed by how much buoyancy it had at the surface. I found I had a lot of problems doing an initial descent. I just kept bobbing back up. So I added weight a couple of times. Someone here referenced the challenge of the first 20' or something like that. It didn't fully register until someone else at the quarry noted that he had been having the same problem and found if he swam down about 10', the suit compression fixed the issue. That worked for me. And to someone else's point, if I'd weighted enough to easily descend from the surface, I think I would have been seriously overweighted at depth. Anyway, when said and done, about 20lbs seemed to be a good compromise. For various reasons, I wasn't able to fully test out the situation when I had an empty tank. Next time.

This was my first real experience diving in colder lake water. Visibility was perhaps 10' or so and after the thermocline at 15' the temp was 63. I'll try this again, but so far I'm not sure I'm going to enjoy this type of diving. I felt pretty encumbered by the thick suit, hood and gloves. Still at a minimum, I would like to be able to get wet more often and work on my comfort and skills. After a year, I was a little rusty. And who knows, once I get used to the extra gear, I may come to like it.