Questionable Advice From A Dive Instructor?

[Colliam7]

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

A very reasonable position. BUT, just so we are on the same page with regard to reasoning, the actual - optimal - amount of weight to account for 2500 psi of gas (in an AL80) is 4.8 lbs, not 6, nor 4 - even if 4.8 is mathematically closer to 4 than is 6. But, I choose 4, as opposed to 6, for two reasons: 1) it is easier to equally distribute, across both sides of the diver, 4 pounds than 5 pounds (which would be my preference) unless I have some really small weights at hand. AND, I count on the contribution of the 'first dive hyperventilation' factor, which adds to weight requirements at the beginning of the first dive, more than the end. Nonetheless, as I implied - add 4, add 6, whatever. The best choice is a proper buoyancy check at the end of a dive, with 500 psi.

 

[Colliam7]

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.

You may want to see if you can find/use some 1 lb weights, to fine tune your weighting. Whether Tom (tbone) and I agree or disagree on the exact weight you might need is irrelevant - we are both commenting from our respective electronic armchairs. The best thing for you to do is conduct a competent weight check, in your gear.

I didn't have anyone in the pool watching me.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.

Excellent. That is the best way to assess your trim. Your 'sensations' may be misleading, and they often are for more than a few divers I have worked with, who vehemently assert that they are horizontal when, in fact, they are slightly 'head up' and 'legs down'. As I mentioned, it is more likely that you will feel - at least at first - 'head down' when, in fact, you are actually horizontal. That is not a guarantee, just an observation based, perhaps presumptively, on a few years of teaching experience.

 

[tbone1004]

fair, though .08*77.4*2500/3000=5.16, and since many come up shorter than 500, I like to give 300 so they can hold a safety stop easier and that puts it at 5.6lbs, so the half pound isn't bad, but yes, 4lbs is certainly easier to distribute.

 

[KevinNM]

I didn't have anyone in the pool watching me. The instructor was keeping dry. It felt as though I was head down.!

Find a smooth part of the pool bottom. Ideally the deep end. Get yourself to what you think is flat and slowly sink until you touch. if you are really flat your face should be at or just above the bottom (like an inch or two) when your body first touches the bottom. If you are like me when you feel flat your knees will touch first and your face will still be a foot or so from the bottom, which means you are head-high.

Someone else with a gopro is really helpful here, but it will work if you set it up yourself and then play it back.

 

[Colliam7]

fair, though .08*77.4*2500/3000=5.16, and since many come up shorter than 500, I like to give 300 so they can hold a safety stop easier and that puts it at 5.6lbs, so the half pound isn't bad, but yes, 4lbs is certainly easier to distribute.

OK, you use the 300 psi (rather than 500 psi) weight, which adds a margin of safety to the process. I can go along with that.

 

[tbone1004]

I normally actually use the full tank volume since backmount bottles aren't ditchable. If you have a valve failure of somesort or for whatever reason can't shut it off and the tank drains, I prefer to not be light, but 6lbs is close enough with wetsuit compression etc. It's easier in thick wetsuits to go a bit light because of the suit compression, but I try not to rely on that if at all possible. either way, a pound here or there is splitting hairs since you can just breathe a bit shallower

 

[Wookie]

Kind of moot when you get down to pounds of lead. I haven't met a lead weight yet (aside from Sea Pearls) that are within 5 or 8 ounces of the number stamped on them.

 

[redacted]

The downside of a little more weight than needed is....minimal.

The downside of a little less weight than needed is possible difficulty in controlling your final ascent.

 

[admikar]

What @KevinNM said. You can make a variation of getting neutral and what feels horizontal,and then start finning. If truly horizontal dept will remain same. Btw, if you make video, would you mind show it here?

 

[The Chairman]

I am not pointing a finger but I have seen instructors who did not really understand trim.

Tru dat.

@zoocan, Without seeing you, it's impossible to know how to help. Seek out some mentoring in your area or find a ScubaBoard event close to you. Aluminum tanks are counter intuitive. In the water, it's usually better to think of the butt of the tank as a bubble rather than the entire tank as a weight. All the way "down", the bubble raises the butt up. The further the tank is up on the BC, the less the bubble pulls your butt up and especially so when close to empty. There is a point where it reverses this trend, so you have to experiment. Get pictures from a buddy. Ask them to make sure that the camera is parallel with the surface.