How much weight with doubles

[dhaines429]

I am just putting together my back mount doubles setup and wondering how much weight I should start with. I know there will be buoyancy checks to do and a few shallow dives to get used to the new configuration but just looking for some ideas in where I should start. I currently dive backplate\wing with a single 130 and with the backplate am at 26 lbs. I doubled up 120s and the negative buoyancy for a 120 is almost the same. So how much weight should i be dropping with the doubles setup?

 

[tbone1004]

if it is the same backplate so nothing changed there, and assuming no STA which would change it by 2lbs

Bands-2 to 4lbs depending on how thick they are and what not
manifold-another 2 ish pounds
extra first stage-another 2 ish pounds
extra tank-another 2 ish pounds

I typically start with taking 10lbs off of the rig and go from there.

 

[northernone]

Roughly 4kg (9 lbs.) extra air. But since the added tank is already NEGATIVE, you'll be dropping lead weight to compensate for the tank's buoyancy characteristics.

Here's a good chart to do the exact rough math:
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

 

[Compressor]

Look at the link Norternone posted. Look at the buoyancy of the tanks and go from there comparing it to what you have now taking into account the other grear there such as bands, manifold . and extra 2nd stage as suggested above.

 

[tbone1004]

Roughly 4kg (9 lbs.) extra air. But since the added tank is already NEGATIVE, you'll be dropping lead weight to compensate for the tank's buoyancy characteristics.

Here's a good chart to do the exact rough math:
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

gas capacity doesn't alter lead carried, it only alters lift requirements of the wing

@compressor i assume that was supposed to read "second first stage" instead of second stage

 

[northernone]

gas capacity doesn't alter lead carried, it only alters lift requirements of the wing.

I must be misunderstanding something.

----- current understanding----
Haven't you been on a dive where half way through you discovered you didn't have enough lead along to compensate for the buoyancy swing of emptying an unfamiliar cylinder (compressed air weight) and had to pick up a rock or two to finish the dive comfortably?

If you initially fail to account for the cylinder's buoyancy change you could have a wing the size of a house and it won't keep you DOWN if you didn't have enough lead.

-------

What am I missing?

Cameron

 

[tbone1004]

I must be misunderstanding something.

Haven't you been on a dive where half way through you discovered you didn't have enough lead along to compensate for the buoyancy swing of emptying an unfamiliar cylinder (compressed air weight) and had to pick up a rock or two to finish the dive comfortably?

no because you do all of your weight checks with empty tanks, or compensate for the mass of gas in them.

I.e. with a single tank HP100 I need 30lbs of lead when empty. I go to doubles with 120's and I do a weight check and find out I only need 2lbs of lead, but they are full. I know that 120*2*.08=19.2lbs, so I have to add that back to my lead requirements so I will carry 21lbs of lead on that dive.

You have a chart that shows the empty buoyancy of the tank, but that doesn't matter since you did a weight check with those cylinders.
You know how much a cubic foot of gas weighs, you compensate that way. If you do a proper weight check, the only way you can get it wrong is if you don't need any lead when the tanks are full and you put too much lead on when they get empty. Not ideal, but you should never get to a point where you are underweighted unless you didn't do a weight check and didn't do the math.

Lead requirements are the sum of floaty stuff vs sinky stuff.
Lift requirements are the sum of floaty stuff that gets less floaty at depth and the mass of the gas you carry.

Lead requirements do not have anything to do with the mass of gas you carry *even with helium since helium doesn't make tanks float, it just makes them act less full than they are*, and lift requirements have nothing to do with how much lead you are carrying unless you have it all attached to the rig and then it needs to float itself at the surface *if all that lead is on a weight belt, then it doesn't matter, if it's integrated or attached to the rig, then it does*

 

[Compressor]

Interesting discussion above: I always saw how much weight to carry as the ability to hover around deco stops without the danger of floating to the surface. When the tanks are filled, they have x lbs of buoyancy (usually negative). When they are near empty their buoyancy changes to y. Y could be + or -. You do not want to be making a safety stop or deco stop and have + buoyancy.

 

[boulderjohn]

I thought I would just tell you what I wear when diving in freshwater and a dry suit with medium underwear for 60° water. I am 6-0 and weigh about 220.

With my Worthington LP 108 steel doubles, I use an aluminum backplate and no additional weight. (I do the same in salt water--still too heavy.)

If I am diving with AL 80 doubles, I use a steel backplate with a 5-pound soft v-weight. (I would carry a few more pounds in salt water--maybe 6.)

In really cold water, I would use thicker undergarments and need more weight.

 

[2airishuman]

I am just putting together my back mount doubles setup and wondering how much weight I should start with. I know there will be buoyancy checks to do and a few shallow dives to get used to the new configuration but just looking for some ideas in where I should start. I currently dive backplate\wing with a single 130 and with the backplate am at 26 lbs. I doubled up 120s and the negative buoyancy for a 120 is almost the same. So how much weight should i be dropping with the doubles setup?

tl;dr about 8 pounds.

I have measured the (empty) buoyancy of my LP72 twinset and find that it is -6.1pounds compared to a buoyancy of +0.5 pounds (each) for identical cylinders with standard (non-modular) valves. I have measure the (empty) buoyancy of my Faber HP100 twinset as -10.4 pounds. These have a manufacturer-reported empty buoyancy per cylinder of -0.6 (I haven't measured the individual cylinders).