New guy looking for weighting advice

[eelnoraa]

Are you sure it is negative buoyancy AL80? It is extremely rare tank if exist at all. colder water divers mostly use steel tanks for a good reason.

 

[Jason]

You should also be aware that the more overweighted you are, the harder it will be to control your buoyancy.

Imagine that you are at a certain depth and you are neutral. Now imagine that you inhale and start to rise up and you end up 1' more shallow than where you were. The air in your BCD bladder will expand a little bit when you raise up that 1' and that will increase your lift, making you tend to float up even higher.

Now imagine that you only need 12# of weight to be neutral at that depth. To get yourself neutral, you have a small amount of air in your BCD. When you raise up 1', that small amount of air expands and makes you a small amount positive in your buoyancy. You exhale and that more than counteracts the expansion in your BCD and you sink back down. You probably drop a little bit below where you started, but you start to inhale and that brings you back up. It's a cycle where you are constantly bobbing up and down around your target depth. The better your breathing and boyancy control gets, the smaller that bobbing motion will become until eventually you can hope to appear to be completely motionless.

Now imagine that you only need 12# of weight, but you actually have 26# of weight. That means you have enough air in your BCD to keep you neutral plus enough more to compensate for that extra 14# of weight that you are carrying but don't need. You have a LOT more air in your BCD than if you only had 12# of weight. Now, when you breathe in and raise up 1' of depth, the air in your BCD expands by the same percentage as it would if you only had 12# of weight. But, since the actual volume of air in your BCD is larger, the percent increase means that actual amount of increase in lift is correspondingly bigger. In other words, when you raise up 1', you have much more lift in your BCD to compensate for when you exhale.

That is why being overweighted makes buoyancy control harder.

Here is a specific example:

Assume you are using an aluminum 80 and diving in board shorts (so no additional complication of wetsuit compression to affect your buoyancy).

An AL80 carries 6# of gas. So, if you are properly weight, and the tank is full, you will be 6# negative on your bouyancy. That way, if you get to the end of your dive and the tank is nearly empty (so that 6# of gas is almost gone), you will still be neutral.

Now imagine you're in the pool hovering at 10' depth. Your are 6# negative, so you have enough air in your BCD to give you 6# of lift. When you inhale, you raise up, when you exhale all the way, you drop down.

If you inhale and rise up to 9' of depth, the ambient pressure of the water around you drops from 1.30 ATA to 1.27 ATA. Which means your 6# of lift in your BCD goes from 6# to 6.14 #.

If you had an extra 14# of lead strapped to you that you didn't need, then that would mean, at 10' depth, instead of 6# of lift, you would have enough air in your BCD to give 20# of lift. When you inhale and rise up to 9' of depth, the lift in your wing would change from 20# to 20.5#.

By having that extra 14# of lead, when you inhale and rise 1' (to 9'), instead of having an increase of 2 ounces of lift to compensate for, you have half a pound (8 ounces) of extra lift to compensate for. If you actually rise up more than just 1', it gets even worse.

And if you are wearing a wetsuit, it compresses as you go down and decompresses as you come up, adding even more to the "extra" lift you have to compensate for (for example), when you bob up to 9' and exhale to drop back down to 10'.

So, like everyone has said, it sounds like you have way too much weight. If you get that sorted out, your buoyancy control should be a lot easier.

One of the best explanations that I've read on this topic. Thanks for putting all of that work in to explaining it.

 

[Darcimus]

Are you sure it is negative buoyancy AL80? It is extremely rare tank if exist at all. colder water divers mostly use steel tanks for a good reason.

• -1.4 lbs ``negative`` (standard AL 80 is ``positive +4 lbs) buoyant at the end of your dive
• 3300 PSI
• 7.25 Inches in Diameter
• 25.1 Inches in Length
• BUOYANCY FULL: = -5.9lbs
• BUOYANCY EMPTY: = -1.4lbs
• "Real Weight" = 34.4lbs
• COLORS: Black, Electric Blue or Yellow
• Manufactured by Catalina Cylinders

Here's what's on scuba.com. Compact AL80 cylinder. Little shorter (0.7") than a standard AL80.

 

[USMC CPL.]

I'm 35 dives into my vast diving career but I got myself down from 18 lb to 10 lb diving a 5mm full suit by spending an hour in a pool with a private instructor (actually a weekly group pool session that I was the only one to show up to) tweaking my weight and trim, then after a few more dives taking the PADI peak buoyancy course and then on several more dives asking the DM for advice about weighting and trim. The dive ops that I have gone with recently seem to have taken it on as a challenge to get my weight down, which is pretty cool. And to Stuartv's point, control of buoyancy is way easier with less weight. At depth I now have very little air in my bcd and can control buoyancy with breath instead.

I have started to do the same. Was diving 16# in 80 feet but down to 10 and I am hoping the new wing system will let me drop another 4.

 

[eelnoraa]

• -1.4 lbs ``negative`` (standard AL 80 is ``positive +4 lbs) buoyant at the end of your dive
• 3300 PSI
• 7.25 Inches in Diameter
• 25.1 Inches in Length
• BUOYANCY FULL: = -5.9lbs
• BUOYANCY EMPTY: = -1.4lbs
• "Real Weight" = 34.4lbs
• COLORS: Black, Electric Blue or Yellow
• Manufactured by Catalina Cylinders

Here's what's on scuba.com. Compact AL80 cylinder. Little shorter (0.7") than a standard AL80.

Still sounds a bit off. The full vs empty weight difference should be about 6lb for 80CF of air or nitrox. But from the rest of the spec, it sounds like a Catalina C80 with empty buoyance of about 0lb. I wonder why this tank isn't more popular

 

[Darcimus]

Still sounds a bit off. The full vs empty weight difference should be about 6lb for 80CF of air or nitrox. But from the rest of the spec, it sounds like a Catalina C80 with empty buoyance of about 0lb. I wonder why this tank isn't more popular

Has it been available for a while?

 

[eelnoraa]

I have seen the spec since I started diving, about 8 years ago, but I have never seen anyone use it in person. I am interested to know why too. Maybe after you get them, let us know how you like them

 

[stuartv]

One reason that I have read, but I can't remember if it was these specific tanks, is that they get really butt light (or butt heavy?) as they get closer to empty, so they really throw off your trim.

 

[Marie13]

I actually tried one of those compact AL80s in a pool a couple of months ago just for the heck of it. Felt kind of weird.

 

[Darcimus]

Yeah I've been doing some reading and it's supposed to be butt-heavy. Meh...I'll make it work, I'm a taller guy.