Buoyancy issues after ~75 minutes

[BabyLitigator]

Ran into an odd buoyancy issue. Was doing a Tec50 drills dive (bottom time 140 minutes for one dive), and found that my buoyancy went to absolute crap after ~75 minutes (as did my frog kicking). Managed to get through the remaining drills but it was rough. Not sure there is any fix other than "dive more" but maintaining after that long (my prior record was 81 minutes), esp. with two stage tanks, was rough.

 

[inquis]

I'd say that gas use caused a misalignment of the centers of mass & buoyancy, becoming head-heavy. I know if I'm at all head-heavy, my back kick goes to crap. When balanced, it's great.

If so, one quick fix is to raise your arms a bit so gas in your drysuit helps counter the imbalance.

Ideally, shift your ballast/lead to be stable/horizontal with nearly empty main tanks and minimal gas in the wing. Then position the wing so you're stable with full tanks (and fuller wing obviously). The deco tanks add a complication, but hopefully their gas mass is aligned well enough with the wing.

 

[Imla]

What tanks were you using, and what was the pressure on your tanks?
If Double Aluminiums, they tend to be bottom light when passing 100Bars. (IE, a bit less than half left) It is an oddly distinct, "oh, we're at half gas" moment where balance shifts.

 

[BabyLitigator]

What tanks were you using, and what was the pressure on your tanks?
If Double Aluminiums, they tend to be bottom light when passing 100Bars. (IE, a bit less than half left) It is an oddly distinct, "oh, we're at half gas" moment where balance shifts.

Double Steel HP120s, and pretty much emptied them by the end, plus an Aluminum 80 and Aluminum 40 (both still pretty full at the end) as stage tanks. To some extent I may just have been tired, though the cramping didn't start until about minute 130.

 

[billgraham]

If you use more conservative gas planning then you won't have such a big bouyancy swing. HP 120's may have more of a buoyancy shift than LP tanks, someone more knowledgeable than me can answer that one.

 

[Vicko]

Water in the wing is also a possibility, was there a unusual amount of it when you got out of the water?

 

[BabyLitigator]

Water in the wing is also a possibility, was there a unusual amount of it when you got out of the water?

No - actually slightly less than usual.

 

[BabyLitigator]

If you use more conservative gas planning then you won't have such a big bouyancy swing. HP 120's may have more of a buoyancy shift than LP tanks, someone more knowledgeable than me can answer that one.

The gas plan was "drills drills drills until one of us ran low on gas," which in this case was 140 minutes. It was Tec50 drills and we had stages of 50% and 80%, with sand at 65 feet. So could switch to deco gas once done to clear any deco obligation. It was about a 2 minute deco obligation on the end, but the dive plan required a two deco gas simulated deco of ~15 minutes, so no problem.

I honestly think it was just exhaustion. Was also a much tougher than usual pre-dive due to having to wheel 200 pounds of tanks on and off the Catalina Express.

 

[billgraham]

The gas plan was "drills drills drills until one of us ran low on gas," which in this case was 140 minutes. It was Tec50 drills and we had stages of 50% and 80%, with sand at 65 feet. So could switch to deco gas once done to clear any deco obligation. It was about a 2 minute deco obligation on the end, but the dive plan required a two deco gas simulated deco of ~15 minutes, so no problem.

I honestly think it was just exhaustion. Was also a much tougher than usual pre-dive due to having to wheel 200 pounds of tanks on and off the Catalina Express.

If anyone had a GoPro, you can take a look at that and see what it looked like. Sounds like you’ve added some new gear in the process so check out what your trim looked like at the end if you can.

 

[ginti]

I assume you were too light, right?

Have you ever emptied your tanks as much as in this dive before? If yes, were you using the same amount of weight with the same suit and undergarment, and in the same type of water?