Steel tanks and wetsuits...

[Scubakevdm]

I never would have thought that. I dive 400g and I would have figured 6 to 8# but not 20#.

I dive a trilam with a weezle and require 24 lbs of negativity to dive. A single HP100 on a ss backplate cut the belt down to 12 lbs. Manifolded Double hp 100's and a can light cut it down to two pounds.

 

[limeyx]

I never would have thought that. I dive 400g and I would have figured 6 to 8# but not 20#.

400G (DUI) for me is about 6-8 pounds more than the GUI 200G or a 300G fleece
but my buddy routinely uses 6-8 pounds less than me for most identical configurations...

so the only way is to hop in and see I guess.

For sure, in fresh water with just my drysuit (TLS350) 400G and 6 pound weightbelt, I wasn't going anywhere. the 400 is floaty (but warm)

in fact, with a HP119 tank (-2) empty, I need about 20 pounds of weight (including steel BP) to sink a 200G thinsulate.

 

[PerroneFord]

Ok, so my numbers aren't far off the mark then, thinking 24# or so for a 400g and a trilam. I'll be in a weezle which is pretty lofty.

 

[limeyx]

I dive a trilam with a weezle and require 24 lbs of negativity to dive. A single HP100 on a ss backplate cut the belt down to 12 lbs. Manifolded Double hp 100's and a can light cut it down to two pounds.

2 pounds + SS BP for a total of 8 ?
I definitely need significantly more than that to sink same configuration in salt water.

 

[Scubakevdm]

2 pounds + SS BP for a total of 8 ?
I definitely need significantly more than that to sink same configuration in salt water.

2.5 for the can
2.6 (1.3 each) for the hp 100's
2 for the manifold
6 for the plate
2 on the belt

for a total of 15.1 empty with about a 15 lb swing.

 

[bradshsi]

Lets say for the sake of argument that you are wearing a 2-piece 5mm wetsuit. It requires about 12 pounds to sink. And lets say you intend to dive with a pair of LP85s, which offer 10 pounds of negative buoyancy. And you have a can light at 2.5 pounds. And the regs/manifold offer another 2 pounds. I am now 2.5# negative with full tanks. My gas swing is going to be about 12 pounds if I drain the tanks. I am going to be WAY light at the end of a dive in this gear. So I have to wear a weightbelt of about 10 pounds to be sure I can stay down if necessary.

If I have to ditch at the beginning of the dive, and I drop my belt, An easy breath will bring me up. Rig is fairly well balanced.

This isn't rocket science.

You need to take into account wetsuit compression. This is the main reason (that I have seen) given to justify the argument that steel doubles and a wetsuit are bad.

Taking a look at your numbers and consider the scenario where you dive to 100 ft. Your wetsuit is now probably less than 1/3rd it's original thickness. Your wetsuit buoyancy is now maybe 3 lbs max.

You are now -2.5# -(12-3)# = -11.5 lbs at the start of your dive. If your wing did fail and you ditched weight you would still be 1.5 lbs negative. Likely you could swim up against this, but it illustrates the point.

If you have steel tanks that are more negative and a thicker wetsuit, the problem can be worse.

 

[PerroneFord]

You need to take into account wetsuit compression.

I have.

You are now -2.5# -(12-3)# = -11.5 lbs at the start of your dive. If your wing did fail and you ditched weight you would still be 1.5 lbs negative. Likely you could swim up against this, but it illustrates the point.

Correct. Most people could fairly easily swim up 11.5 pounds, and anyone could inhale and lift 1.5. However, as the diver comes shallow, the 11.5 pounds negative will reduce as my suit comes back, and by the time I hit the surface, I should actually be positive. Remember it took 12 pounds to sink the suit, and I have also burned some gas in the tanks. To my thinking, this is a balanced rig.

If you have steel tanks that are more negative and a thicker wetsuit, the problem can be worse.

Correct. Which would require rethinknig whether the rig was balanced or not.