Working out weight required

[alexxred]

Hi,

Could someone remind me of the calculations that you need to do to work out roughly how much weight you need. I'm going to be switching to twin steel tanks, faber 10.5L, which I think are like 12kg ea and wearing 7mm suit in salt water and weight about 75kg.

If anyone could give me the rundown on the adds and subtracts that you use for the different mm or rubber and tank sizes/types, steel vs. al that would be great

Cheers

Alex

 

[utnapistim]

Hi,

Could someone remind me of the calculations that you need to do to work out roughly how much weight you need. I'm going to be switching to twin steel tanks, faber 10.5L, which I think are like 12kg ea and wearing 7mm suit in salt water and weight about 75kg.

If anyone could give me the rundown on the adds and subtracts that you use for the different mm or rubber and tank sizes/types, steel vs. al that would be great

Cheers

Alex

i don't know how helpfull this will be to you but if you have the occasion play with the gear in the pool.

it's much better than a bunch of numbers and it's more play than work

for example i didn't get a clear idea of my suit's buoyancy until i started putting weights on top of it in the cleaning tank after a dive.

 

[markfm]

Getting a baseline, in whatever suit you're wearing, plus tank and BC, is kind of important.
Once you do that, to switch to a different tank, use a buoyancy-empty table and adjust for the delta:
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

For example, if I have a Catalina S80 aluminum, it's +4 buoyant empty. If I switch to a PST E-7 100 steel, it's -1 pounds -- I need 5 pounds less weight, in a given exposure suit, when switching to that tank.

(Note that in the steel tank section of the table, only the PSTs have the valve weight included, for the other steel tanks think of them as being about -2 pounds compared to the numbers shown)

For a switch from fresh water to salt, for a given exposure suit, tank, BC, it's a 2.5% of total land weight delta (1 pound in 40). I, all my gear (including weights), and a full AL80 are about 240 pounds dry weight. When I go to salt water I need 240/40 = 6 pounds more weight.

Getting that baseline is all important -- pool time works. I get my baseline fresh water weight for
diveskin
5mm core warmer only
full 5mm
drysuit

As to the baseline, people recommend doing a check, holding a safety stop at 15 feet with 500 psi in your tank. For myself, I'm quite content to just get neutral at the surface, with a full AL80, then add 5 pounds. The reason is that the weight of 2500 psi of air in an AL80 is 5 pounds, so if I'm neutral with a full tank, by definition I'll be +5 at the end of the dive when down at 500 psi, need to compensate that much.

(You can see the air weight by looking in the two buoyancy columns on the chart, the AL80 swings from buoyancy full to buoyancy empty by just about 6 pounds, which means that it's about 1 pound per 500 psi, in that specific tank size.

(To tell the truth, I actually compensate 6 pounds, so that if there's an oh-heck situation that totally empties my tank I'll still be neutral)

You may also notice your weight needs dropping over time. I started at 18 pounds in a full 5 mm, fresh water, dropped to 12 pounds over 8 or 10 dives, just recently dropped 2 pounds, expect that 10 will be my weight going forward. Comfort level and suit break-in...

The fun part is there's focus on dropping weight , skinning it down as far as possible. Get to a Rescue class, or go beyond, and you may well then hear about purposefully carrying a bit of weight beyond your minimum need, to help control other divers from darting to the surface. It's then a purposeful over-weighting.

 

[Steele]

Getting a baseline, in whatever suit you're wearing, plus tank and BC, is kind of important.
Once you do that, to switch to a different tank, use a buoyancy-empty table and adjust for the delta:
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

For example, if I have a Catalina S80 aluminum, it's +4 buoyant empty. If I switch to a PST E-7 100 steel, it's -1 pounds -- I need 5 pounds less weight, in a given exposure suit, when switching to that tank.

(Note that in the steel tank section of the table, only the PSTs have the valve weight included, for the other steel tanks think of them as being about -2 pounds compared to the numbers shown)

For a switch from fresh water to salt, for a given exposure suit, tank, BC, it's a 2.5% of total land weight delta (1 pound in 40). I, all my gear (including weights), and a full AL80 are about 240 pounds dry weight. When I go to salt water I need 240/40 = 6 pounds more weight.

Getting that baseline is all important -- pool time works. I get my baseline fresh water weight for
diveskin
5mm core warmer only
full 5mm
drysuit

As to the baseline, people recommend doing a check, holding a safety stop at 15 feet with 500 psi in your tank. For myself, I'm quite content to just get neutral at the surface, with a full AL80, then add 5 pounds. The reason is that the weight of 2500 psi of air in an AL80 is 5 pounds, so if I'm neutral with a full tank, by definition I'll be +5 at the end of the dive when down at 500 psi, need to compensate that much.

(You can see the air weight by looking in the two buoyancy columns on the chart, the AL80 swings from buoyancy full to buoyancy empty by just about 6 pounds, which means that it's about 1 pound per 500 psi, in that specific tank size.

(To tell the truth, I actually compensate 6 pounds, so that if there's an oh-heck situation that totally empties my tank I'll still be neutral)

You may also notice your weight needs dropping over time. I started at 18 pounds in a full 5 mm, fresh water, dropped to 12 pounds over 8 or 10 dives, just recently dropped 2 pounds, expect that 10 will be my weight going forward. Comfort level and suit break-in...

The fun part is there's focus on dropping weight , skinning it down as far as possible. Get to a Rescue class, or go beyond, and you may well then hear about purposefully carrying a bit of weight beyond your minimum need, to help control other divers from darting to the surface. It's then a purposeful over-weighting.

Thank you Mark for an insightful post. That actually helped me a bit. My wife was having problems getting correctly weighted and with this info I'll take it into consideration. I think it also has something to do with distribution and the type BCD she's wearing.