Put a full AL tank with a reg attached in the pool without a BC. They almost always sit with the valve on the bottom and the butt up. It's not much of a lift, but it's there. If it seems to be all flat, lift up on either end. Which takes more effort? No, it's not intuitive, but it's still there. What's funny is people trying to use the tank weight to get their head down and moving the tank so high it's almost over their head. It can be pretty comical. Yes, the difference becomes more pronounced as the tank empties
Put a steel tank with a reg attached sans BC in the pool and it lies absolutely flat on the bottom. Lift up on both ends to see which is heavier. You can't really tell on smaller tanks, like an LP72, but get to the water heaters, and you'll find the butt is actually a bit heavier. Again, put a reg on it to better replicate what's happening on your back.
@pauldw, I think that
@JackD342 explained it pretty well. Let me know if I need to try again. You're not stupid for not "getting it" right away, you're just thinking like you're still on land. You have to consider Archimedes principle on a vessel that's not a simple geometric shape nor equally dense along it's length. It's a bit esoteric and not intuitive at all. I have had many disagree with me... until they do the tank in the pool exercise. That's an "aha" moment for many.
Weight is usually added to compensate for the buoyancy of your exposure protection. Most people are pretty close to neutral without it... except me, cuz I'm a cork. So if you're weighted correctly, with your head just awash (not eye level) with no kicking and no air in your BC, then it doesn't matter which tank you have on your back. You will need more lead to begin with on an AL tank, but your buoyancy changes equally as you breathe down either tank.
Best way to figure out your starting weight is at the beginning of your dive and not the end so you can enjoy your dive.
- Full tank
- No weights
- Remove all air from BC
- Cross your legs
- Fold your arms
- Reg in your mouth
- While floating relaxed, add a pound for every inch your head is out of the water
- Recheck
- Stop when your head is just awash
- Water just barely going over
- You should sink if you exhale and hold it
- Dive! Have fun!
- Empty all your air from your BC at your safety stop, cross legs & fold arms
- If you sink, take off a pound or two on the next dive
- If you're floaty, add a pound or two on the next dive
- Pause your breathing on the exhale rather than the inhale to compensate
Most after dive checks use "eye level" which means the top of your head is four or five inches above the water, or four or five pounds. That happens to be about the weight you use in air during the dive. If you use my weighting technique, you'll find that all you have to do to begin your descent is let out the air in your BC and then to exhale. Taking a breath will stop that descent. Continue with this until you get to the bottom, then add just enough air to compensate for wetsuit compression.
I hope this helps.