Questionable Advice From A Dive Instructor?

[zoocan]

I am and newish diver and have a question about advice given to me by my dive instructor.
I recently bought some new equipment and so went to a pool dive to get used to everything. I bought the scubapro lite hawk bc and was wearing a 7mm wetsuit (the only one I have and yes I was sweating in the pool). I loaded my weights with the minimum I thought I would need (12lb) and went for a swim. With a full tank and my BC empty of air I hovered around with the top of my head sticking out of the water. I descended by exhaling and seemed to be under control. During the dive I noticed that I was head down a lot and found it hard to stay horizontal. So as a test I added the the remaining weight I had (6lb) and tried again. Now I stayed more horizontal but i definitely felt over weighted.
I asked advice from the instructor and he said that I should move the 6lb weight to the trim pockets of the BC which are higher up my body. Firstly I said that I thought I was over weighted with the extra 6lb and he said it was fine??? and secondly surely if I added the weight higher up my body that it would push my head down even more?
Am I missing something with this advice?? Everything I have read says that most divers are overweighted and this is a problem at depth. I thought that the aim was to not have air in your BC as you start your decent?? so surely the 12lb worth of weight is ample??
Thank you for your time!

 

[Wookie]

Well, first, if he said to move your weights up to be in trim, I would agree with that. Second, you were wearing a 7 mil. I can't get neutral in a 7 mil with 12 lbs of weight in a BCD with an aluminum tank, although I haven't tried in about 20 years either. The last time I wore a 7 mil steamer and john, it took 24 lbs to sink me in the ocean with a steel 95. Now, I'm a big guy, but at that time I had about 4,000 dives, so it wasn't a case of being new, a 7 mil just takes a lot of weight to sink. I do not find your instructor's advice out of line.

But the answer is you're doing the right things, practicing in the pool and asking for advice. You might try the same 12 lbs but set 6 of them higher up and see how you trim out, and how your buoyancy is. Remember the PADI test. If you have all of the air out and the back of your head just breaks the water with a full breath, you're just right. I paraphrased that for folks who don't dive in the vertical position.

 

[zoocan]

Thank you for the response. I am still not sure how the physics of moving weight towards my head would help my backside sink but there are still lots of things I don't understand.
As for adding weight…with the 12lb I was able to sink pretty easily. I didn't have to swim down or use my arms. Do I still need to add weight? Most of my diving is in fresh water and I understand that I would have to load more pounds for the ocean.
Thank you again for the quick response.

 

[Wookie]

Again, you are properly weighted when your head just breaks the water with a full breath of air and BCD deflated. I'm 300+ lbs, so that takes more lead than a normal person uses. For all it's different, and the placement on the body is different. If you're happy, comfortable, and feel good in 12 lbs, than it's just right for you. Don't forget to go back with an empty tank (500 PSI) and double check.

 

[zoocan]

Great. Thanks for the response and advice.

 

[tbone1004]

12lbs is actually not abnormal for most of the newer 7mm steamers, however based on what you said you need to add about 6lbs for the gas that you hadn't used yet... No need to go back with an empty tank, just calculate how much gas you had in the tank, and compensate for it.

If you want really exact numbers, take a luggage scale and weight the bc/reg/tank assembly, and then you know how negative that is. Probably not very much, maybe 1-2lbs when the tank is full. Make sure to subtract the mass of the air in the tank. Roughly 6lbs for a full AL80. Then put the wetsuit in a mesh laundry bag or similar, slosh it around underwater to get all of the air out, and put a weight belt with through the handles and put weight on there until it sinks. Start with 12lbs. At that point you know your rig information, and just need your personal info which can be done with a snorkel and some lead weights doing the neutral buoyancy test. Once you know all of that, it is a sum of all the parts to get 0lbs, and voi la, you are not diving a balanced rig *unless of course the sum of those parts is negative, but that's essentially impossible with aluminum tanks and a 7mm*.

Regarding moving the weight up higher, it all depends on your body position when diving, and if you were actually head down. When you are in a true horizontal position it feels head down, only way to verify this is with a camera of some sort. Ideally with a buddy, but you can set a gopro on the floor of the pool and hover in front of it.

 

[Colliam7]

I loaded my weights with the minimum I thought I would need (12lb) and went for a swim. With a full tank and my BC empty of air I hovered around with the top of my head sticking out of the water. I descended by exhaling and seemed to be under control.

So, if I read this correctly, with a full cylinder, you were nearly optimally weighted, possibly 1/2 lb over (based on the 'top of my head' comment). So, you should add back some weight, in order to be optimally weighted at the end of a dive, with ~500 psi in the cylinder. For an AL80, I tell students that 4 lbs is roughly the amount of weight they should add back to account for air use during the dive.

During the dive I noticed that I was head down a lot and found it hard to stay horizontal. So as a test I added the the remaining weight I had (6lb) and tried again. Now I stayed more horizontal but i definitely felt over weighted.

This one is a little trickier to 'diagnose from afar'. Early on, most divers feel they are perfectly horizontal, when in fact they are in a slightly 'head up' orientation. When they are truly horizontal, they feel like they are 'head low'. The best way to know is have someone else in the water to observe you. Even better is having someone else in the water to observe you AND film you, so YOU can see what you look like. I cannot tell from your post if your orientation in the water was confirmed by another diver to be a 'head low' position, or if that was just your sensation. But, the best way to determine your actual orientation in the water is with the use of another (external to you) set of eyes, or camera lens.

I asked advice from the instructor and he said that I should move the 6lb weight to the trim pockets of the BC which are higher up my body. Firstly I said that I thought I was over weighted with the extra 6lb and he said it was fine??? and secondly surely if I added the weight higher up my body that it would push my head down even more?

Was your instructor in the water with you? That makes a big difference. If he was observing you, and made the suggestion to add weight, and add weight higher on your torso, then it is hard to disagree from afar - we were NOT there. Is 6 lbs too much? Well, it is more than I would usually recommend, based on your description of your buoyancy check with a full cylinder, AND assuming you were wearing an AL80. It might not be egregious overweighting, even if 4 lbs might be better. I am not defending overweighting someone by 2 lbs. Rather, if all you had were 3 lb weights, the instructor might have thought it better for you to go with four 3-lb weights, than with three 3-lb weights, and be slightly positive at the end of the dive.

Everything I have read says that most divers are overweighted and this is a problem at depth. I thought that the aim was to not have air in your BC as you start your decent??

Yes (overweighted), and yes (no air).

so surely the 12lb worth of weight is ample??

Not necessarily, if the 12 lbs was what you needed to achieve neutral buoyancy with a full cylinder. The best way to do a proper buoyancy check is using a cylinder drained down to ~500 PSI. We often do buoyancy checks at the start of a dive (usually the start of OW Dive 1), with OW students, who are wearing full cylinders. In that case we do what I mentioned earlier - find the optimal weight, and add 4 lbs. THEN, we do a proper check at the end of the dive, with 500 psi cylinders.

 

[tbone1004]

jynx to above, however I disagree with 4lbs as that only represents 2000psi in an AL80 or 50cf of gas. While 6lbs represents 2900psi/75cf, it is a bit more accurate imho

 

[zoocan]

Firstly I was using an AL80 tank. I had 2 x 6 pounds in the quick release pockets on the front and 2 x 3 ponds spare. (left on side of pool to try later) so I will add those to the trim pockets of BC and see what happens with trim. I didn't have anyone in the pool watching me. The instructor was keeping dry. It felt as though I was head down. I guess next time I go I will take my go pro with me and film myself and see what is actually happening. I have snorkelled for years and it might be that I was just used to that feeling/positioning and now it is different. I felt as though it was harder to look parallel to the floor then it was in the past. Maybe I was in wrong position before!?!? I will try out the advice from all of you and see what happens. I just wanted to confirm that the information was relatively accurate so I don't waste time underwater trying something that will never work.
Thank you!

 

[claymore]

I am not pointing a finger but I have seen instructors who did not really understand trim. They understand buoyancy but not the concept of trim and moving weight around to achieve horizontal trim. I have seen Peak Performance Buoyancy classes held and no mention of trim in the water just being neutral in the water column.