Weight Swing When Going From AL80 to a Steel 119

[lowtech]

FWIW, I did the drop 4 lbs and then another 2 lbs.
With only dropping 4lbs...
I felt a little heavy on safety stops,
but I was still able to empty my wing all the way with an empty tank.
If I had more than say 700 psi then I would need to have the wing slightly inflated.
Got rid of the other 2 lbs. and everything checks out.

It took a bit for me to start my descents in an AL80 and jacket BC,
but I was properly weighted and I wasn't hitting the sea bed
stirring up silt like some.

 

[Bubbletrubble]

Granted not all gear has the same buoyancy in salt water I would go with 24# and test my weighting at that level first. Again I am much bigger than OP and therefore have more buoyant neoprene on regardless of type of gear he should probably have less weight on then I do. Again this is all speculative and just my opinion.

@MrPfeffer: That's interesting. Given the changes I specified (HP100 --> AL80 and fresh water --> salt water) and all other gear remaining the same, I'd recommend that you add approx. 15 lbs. to whatever you had on previously (16 lbs. lead). 15 + 16 = 31 lbs. lead. FYI, I calculated the 15 lbs. from: 6 lb. difference for the tanks and 9 lb. difference for the greater density of salt water vs. fresh water (2.5% of the entire weight of you + your gear + your lead).

I have no idea whether the OP is optimally weighted. I wasn't there when he was conducting his weight check. He may have been kicking up. He may have been breathing off the "top" of his lungs. I don't know what the inherent buoyancy is of his body or his wetsuit or his BCD. I'd be willing to bet, though, that he's within 5 or 6 lbs. of optimal weighting. That's pretty good for someone who just started diving.

 

[Bopper]

Well thanks for everybody's input. It's funny - yes, I wanted a straight bottom line answer as to what would be a reasonable and safe weight adjustment, and I got that. But in the course of the conversation it also triggered ideas that I've not really paid enough thought to. Specifically, comments about the importance of making sure you have enough weight being as important as too much weight. I've never done a buoyancy test other than the one I did in my OW course last summer. It's probably time I become more aware of fine-tuning this aspect of diving and become more aware of the optimal weight rather than "Hey we approximated this amt in the OW course and it's worked so far so why change it" approach.

Also, Banyan asked about whether or not I had difficulty w. buoyancy on my safety stops. Well it made me realize that I've really only done two safety stops in all of my 20 or so dives. You don't realize these things until you're forced to think about them. The two safety stops came on the deep dives I did for Adv Open Water Cert. And for those Safety Stops I was holding on to a mooring line. For all my other dives, they were shore dives so I'd kick out, drop down, then come in on a slope. So this is another thing to think about. Maybe I should practice Safety Stops in my 20 ft of water so I'll be better prepared for a Safety Stop coming off a 100ft dive. (And if you think it's ridiculous that someone w. less than 20 dives is Adv Open Water and Nitrox Cert well I completely agree. But I think I'm a pretty conservative diver.)

And Bubbletrubble and Banyan also eluded to kicking and breathing when descending. I never thought about crossing my legs as was suggested but I will give that a try. And take deep breaths and fully exhale and not breathe "at the top of my lungs" so I can use my own intrinsic "variable buoyancy" device.

All things to think about. Thanks for the insight!

 

[Bubbletrubble]

Also, Banyan asked about whether or not I had difficulty w. buoyancy on my safety stops. Well it made me realize that I've really only done two safety stops in all of my 20 or so dives. You don't realize these things until you're forced to think about them. The two safety stops came on the deep dives I did for Adv Open Water Cert. And for those Safety Stops I was holding on to a mooring line. For all my other dives, they were shore dives so I'd kick out, drop down, then come in on a slope. So this is another thing to think about. Maybe I should practice Safety Stops in my 20 ft of water so I'll be better prepared for a Safety Stop coming off a 100ft dive. (And if you think it's ridiculous that someone w. less than 20 dives is Adv Open Water and Nitrox Cert well I completely agree. But I think I'm a pretty conservative diver.)

You described finishing all of your dives by swimming back to shore up the slope. If you were doing these dives at La Jolla Shores, I can guarantee that you were doing safety stops (3 minutes in the 10-20 fsw range) on all of those dives. In fact, from a deco theory standpoint, your dive profile was probably much "better" than a simple square profile with a mid-water safety stop (you know, the kind of profile you might do on a dive at the Yukon).

 

[DivemasterDennis]

Once you read bubble trouble's post, you can end your reading o this thread. He has it exactly right.
DivemasterDennis

 

[Bopper]

You described finishing all of your dives by swimming back to shore up the slope. If you were doing these dives at La Jolla Shores, I can guarantee that you were doing safety stops (3 minutes in the 10-20 fsw range) on all of those dives. In fact, from a deco theory standpoint, your dive profile was probably much "better" than a simple square profile with a mid-water safety stop (you know, the kind of profile you might do on a dive at the Yukon).

Well I guess technically they were safety stops that never stopped.

 

[keyshunter]

Simple math.
Look at the empty buoyancy specs for each tank.
Using the Tech Diving Limited chart...
For a Catalina S80, an empty tank is +4 lbs. buoyant.
For a Worthington X8-119, an empty tank is -2.0 buoyant.
The difference is +4 - (-2) = 6 lbs.

This means that, when you move to the X119, you should be able to remove 6 lbs. of ballast so that you're carrying only 22 lbs. of lead. Of course, this assumes that you were properly weighted when using the AL80. I think it's reasonable to carry 24 lbs. of lead to ensure that you aren't under-weighted. If, for whatever reason, you cannot conduct a weight check prior to diving, it would be prudent do a conservative dive with respect to max depth, nitrogen-loading, and gas management. Then, prior to exiting the water, purge your tank down to 500 psi and conduct a weight check in the shallows.

That being said, you should make every effort to conduct a proper weight check prior to the dive. Since you know that the buoyancy swing (full vs. empty) of your X119 is 8.9 lbs. according to the Tech Diving Limited chart, you could easily do an in-water weight check with a full X8-119 and compensate for the gas in your tank. If you're diving La Jolla Shores, it's easy enough to do a weight check just beyond the surf zone. Arrange for your buddy to help you.

I can't emphasize enough the importance of being properly weighted and knowing how to conduct a proper weight check. If you have a difficult time sinking with a certain amount of weight (and you've ruled out the possibility of air in the BCD, air bubble in your wetsuit, you doing weird things with your breathing, unconsciously finning up, etc.), that might be an indication that you are under-weighted. Something to think about...

I have never done this calculation. By experimentation, I have learned that dropping 6 pounds when going from one of my Luxfer or Catalina 80's to a PST 119 gives me approximately the same buoyancy characteristics with the same gear.