Buoyancy changes between 12L and HP100

[Tigerpaw]

I had a very humbling and embarrassing experience while diving on vacation and I was wanting to get thoughts and opinions.

Whenever I go on vacation I do a "shakedown" dive, in the configuration I'm going to be in before heading out. In Europe and Canada, steel tanks are generally used (HP100, LP80, etc) So for my shakedown dive, I dove HP100's. And even though it was 100 degrees in Texas, I still dove in my drysuit with under garments.

So I make it to Silfra and the guide breaks out a 12L tank. I had requested a HP100 but that's another story. I adjusted the tank cam bands and my bcd/wing barely fit. (12L tanks are short and fat, stubby tank vs longer thinner HP100) When we did the weight check I couldn't go down so the guide added 5lbs.

I struggled to maintain my buoyancy and trim. I was severely listing to the left side. While I did touch both continents I didn't fully enjoy the dive because I was struggling.

At the end of the dive as i was breaking down my gear, i discovered the guide had placed a 5lb weight on my left trim tank.

So what I'm wondering is, did the change in tanks and a 5 pound weight cause my buoyancy and trim issues.

(To clarify, 5lbs weight was only placed on left pouch, nothing was placed in right pouch)

Thanks!!!

 

[rongoodman]

That seems like more than enough weight to throw you off balance.

 

[lexvil]

First off, what is a 12L in ‘murican? Did you use your own drysuit? Same undergarments? Did you train in fresh or salt water?
5 lbs can make trim hard to maintain but shouldn’t have you flailing about, unless you have a big bubble in the suit.

 

[Landau]

Was the 12l tank steel or aluminum? I’ve dove with 2 types of HP 80, 3 types of HP100, HP 117 and HP 120 and all needed about the same amount of weight.

My math shows a 12l tank is about an HP100 assuming a 237bar/3450psi fill.

And were the weights in lbs or kg? In Mexico I’ve been handed a “2kg” weight that was clearly stamped “4lbs”. The difference is about 10% and would add up when using enough to sink a dry suit and undergarments.

Dry suit weighting is more art than science and mostly about sinking the undergarments. I have a thick vest I put on in winter that needs 4 extra pounds of weight.

 

[Tigerpaw]

I train similar conditions. In both cases I dove the exact same undergarments and dry suit. Used the same BP/wing configuration and both instances were in fresh water.

The 12L tank had 3000psi and was steel. It is short and stubby. The weights did not have any markings on them, so I trusted the guide. My buoyancy at home was dead on. My BP is 18" long vs the standard 15", I mention this because the 12L is so stubby, the tank bands barely fit, we had to take off the tank boot so that the lower tank band would fit.

And in both cases I had zero air in my drysuit, squeezed it all out. @lexvil I was constantly listing to the left, which is where the weight was placed on the trim pocket on the tank. Staying motionless, I would list to the left until I was upside down. I'm asking the question because I didn't think 5lbs to one side would affect trim like that.

 

[divezonescuba]

I can't see how 5 pounds in a trim pouch on the left side of a tank band wouldn't make you list to the left. Even 2 pounds would probably have an effect.

So, its hard to believe why a competent dive guide would do something like that.

Unbeknowst to me, I once instructed a lady with an artificial femur in one leg. It wasn't until she complained that she was always rolling to one side, did I bother to start asking questions. The weight of that was enough to require a counter weight.

Among other reasons thats why sidemount and independent backmount doubles divers alternate between their tanks to maintain balance.

 

[tursiops]

I didn't think 5lbs to one side would affect trim like that.

5 lbs on one side will affect trim a LOT, as you discovered.

 

[lexvil]

5 lb will make roll, especially if you don’t know it’s there.

 

[lowwall]

The 12L tank had 3000psi and was steel. It is short and stubby.

A standard 12L steel tank is an HP100. I suspect you got a version of what Faber calls the 12.2 Compact. It's 110mm (4.3") shorter and has a 26mm (1") greater diameter than the standard 12L/HP100. It's also 1.5kg more negative.

I'm asking the question because I didn't think 5lbs to one side would affect trim like that.

It certainly will.

 

[thomschoon]

I have gotten the short 12l tanks a couple times this year. I usually dive HP 100’s so I have found I need to add 4-6 pounds additional weight. Trim is also a challenge as the weight is higher so I get floaty legs. The biggest issue is the larger diameter and how it makes me want to turtle a bit so I keep the weight up front versus in trim pockets. Now when I see one coming I just make the changes but it was a learning curve.