Negative Weights

[Skippums]

Hmmm... my guess is that the steel tank I am referring to is the -13 lb one (which I beleive is actually an HP80), because I typically add 6 lbs with the AL80 on the same exposure suit and am significantly closer to neutral. Perhaps I could try one of the "PST E8-119" tanks, which wouldn't be as bad at -10 lbs. Thanks for the link,

-Jeff

 

[vixtor]

Am I missing something? Ignoring the tank - empty, at the end of the dive it is maybe -1kg - -2kg, a 7mm wetsuit is 8-10kg buoyant (17-22lbs). What is that heavy in your body to sink it? Because if you can sink with an empty BC and tank, this is almost like being able to sink without a weight belt when wearing only the wetsuit (and I never heard of that).

The only explanation that I can imagine now.. have you ever had some kind of surgery that put large chunks of metal in your body?

Otherwise, you have a very negative tank - just change it.

 

[Bubbletrubble]

I agree with vixtor's assessment.
@Skippums: If you could provide us with the specific tank markings, we would find it very helpful.
I guess it's possible that someone slipped some extra weight inside the tank boot (or something sneaky like that).

 

[Skippums]

I didn't think to check the boot for weights... the steel tank is a rental, and I won't be going to the quarry that gives them out again this year. However, I will be sure to investigate it next year when I go back.

As for the explanation of how this is happening, I have nothing. I agree, the physics makes it appear impossible (assuming your math is correct, I didn't really check it). All I know is, with the particular tanks the guy at this quarry is handing out, I sink with no additional weight, and I sink fast. I will be sure to look into alternatives when I go to purchase my own tank in the coming months. Thanks to everyone who provided help and advice, and the general conclusion that I read out of this is to "use a different tank". I truly appreciate your help, and will let you all know what I end up with when I finally purchase my own tank later this year.

-Jeff

 

[ptyx]

The second problem is actually due to torque that occurs when I use the steel tank. With aluminum, I can add some weights to ensure that my weight is symmetrical along my longitudinal axis, which allows me to dive comfortably in any position. However, once I don the steel tank, I am very back-heavy (since I have to remove the weights in the front, and no longer have something positively buoyant on my back). Because of this uneven weight distribution, the slightest twist underwater, and I start to capsize (and yes, it is a little funny to watch).

This might be a BC issue on top of your ballast issue - and the obligatory plug for BP/W - I'm surprised it arrives so late

Consider:

• strapping the BC tighter - or getting a smaller one: having the tank loose on the back will amplify torque effects as you need to move more to compensate
• get a back inflation BC: having the air bladder around the tank will work much better than in front of you (traditional BC) with a heavy tank

A backplate and wing setup solves those issues nicely, but any back inflation BC of the right size (i.e. feels slightly too small) might do. (It would be an AL or plastic plate in your case, you certainly don't need 6 extra pounds of steel in your back!)

 

[The Kraken]

Get a dry suit.

the K

 

[Web Monkey]

I didn't think to check the boot for weights... the steel tank is a rental, and I won't be going to the quarry that gives them out again this year. However, I will be sure to investigate it next year when I go back.

There are also some really negative steel tanks. He may just have some of the weird ones. For example, Faber makes an 80 that's -14 lbs when full.

Terry

 

[spectrum]

Skippums

If you happen to be a small/trim person such that your wetsuit is not a very large size then you may have less neoprene than some are visualizing. Also I gather this is a 7mm full suit and 5mm hood such that you don't have a second layer on the core. Put that together in fresh water and with one of the more negative cylinders out there I can see you ending up heavy.

As it's a rental situation the answer is simply to rent something else. If you're the kind that just has to make this work for the heck of it then carry some buoyancy. A pint of air works out pretty close to a pound of buoyancy. Containers want to be rigid and cylindrical or spherical. A dive light without batteries would be an example, also an empty dry box. You could also take a length or 2 of PVC pipe and cap the ends then attach them to the back of your BC like little booster rockets!

When I was diving a HP80 wearing a shorty in fresh water I was negative. Since these were shallow dives I carved up some Great Stuff foam to form blocks that fit onto a soft weight belt. This was for a dive with a max depth of 20 feet. At some depth the closed cell structure of the foam would fail. I has some fun tossing the "loaded: weight belt at people. Since then I snagged a few AL80's for times when the steel just isn't the right cylinder.

Pete