Getting rid of my weight belt?

[jonnythan]

I've read that book and definitely understand the concept and your logic sounds right, but how do you explain the info I'm finding on SCUBA.com. I checked SCUBA.com and here's what they have to say about two different tanks:

Neutral 80

BUOYANCY FULL: = -5.9lbs
BUOYANCY EMPTY: = -1.4lbs
SWING = 4.5LBS

Aluminum 80

Buoyancy Full = -1.9lbs
Buoyancy Empty = 4lbs
SWING = 5.9LBS


http://www.scuba.com/shop/product.asp_category_200

The numbers are wrong. Any tank that holds 80cf will have the same "buoyancy swing."

http://members.aol.com/jsuw3/tankspecs.html

That page will help. You will see both Catalina 80cf tanks have a swing weight of 5.8 lbs.

You will further see that the PST LP-80 and E7-80 both have a swing weight of 6 lbs. The small variance is probably due to measuring differences among manufacturers.. but it should always be approximately 6 lbs for 80cf.

 

[lamont]

I took DIR-F a couple of weeks ago and it was suggested that if you are at any point during your dive more than 10lbs. negative, that you probably won't be able to swim that up from 30ft.
[...]
At the bottom, with both wing and drysuit deflated (100 feet) I could not swim up. With my drysuit inflated, I could. Even then it was hard.

I swam up 2500psi of nitrox in double 130s easily from 30 fsw with wing and drysuit deflated. That's something like 14# negative and it was no sweat. Next time I have a full set of nitrox tanks I'll try to remember to re-do this test with a full 20# swing...

And when it comes to people jumping in with their air off and their wing deflated, can't we fix this problem just by gear-up procedure? At a minimum:

1. I always check my valves immediately before gearing up in the harness
2. I always take deep breaths off of both of my regs
3. I always substantially inflate my wing
4. I always use my elbows to push back on my wing to ensure that it is holding gas and the OPV isn't stuck open

And then:

5. I can reach my valves
6. I can at least swim up and hold position against my rig with the wing deflated

 

[DIR-Atlanta]

I've read that book and definitely understand the concept and your logic sounds right, but how do you explain the info I'm finding on SCUBA.com.

Easy - the data on SCUBA.com is wrong.

Actually, I'm serious - the Sherwood "neutral" cylinders are actually manufactured by Catalina Cylinders, and the buoyancy figures that SCUBA.com is showing for an "empty" tank is actually the buoyancy of a tank with 500 PSI in it. The full specifications for the tank can be found on the Catalina Cylinders website. If you look at the correct figures, you'll see that a truly empty tank has a buoyancy of -0.2 lbs, for a total swing of 5.7 lbs. That's still a little less than what I would expect, and is probably the result of "creative rounding".

The bottom line is that no matter what the manufacturer says, they can't change the laws of Physics - "a pint's a pound the world around", and 80 cf is always 80 cf.

 

[JeffG]

and 80 cf is always 80 cf.

except when its 77

 

[DIR-Atlanta]

and 80 cf is always 80 cf.

except when its 77

Actually, it's 77.4, but somehow saying "77.4 cf is always 77.4 cf" doesn't have quite the same impact

 

[TomR]

DIR-Atlanta,

Thanks for sorting that out. I was wondering what type of magic those neutral tanks offered, I guess it's none. Good to know to check the manufacturer site, not the retailer site for solid info. I was a poli sci major and only took "physics for poets" so maybe that has something to do with the confusion too...

Tom

 

[TomR]

As a follow-up, I did my rig balancing check out dive in Redondo last night. Here's what I was wearing:

PST119 filled to 3200psi
CF200 w/300ct. undergarment
Halcyon Backplate (6+lbs) and single tank adapter (6lbs)
8lbs extra weight
Small primary light, backup lights, and full drysuit pockets

Went down to 20 ft, emptied the bladder and suit and started swimming upwards. It was quite easy so I stopped after swimming up 10 ft. At the end of the dive, I got the tank down to 1000psi and was slightly less than neutral. Anyway, I think I'm balanced and none of my weight needs to be ditchable.

Tom

 

[jonnythan]

The Halcyon STA is only 1.5 to 2 lbs negative, not 6.

 

[Jasonmh]

The Halcyon STA is only 1.5 to 2 lbs negative, not 6.

I believe the weighted STA is 6, is that what you were using Tom?

 

[dbulmer]

Halcyon do a weighted STA which is about 6 pounds.