Question Another BPW weighting question…

[PerthPom]

Good day fellow plongeurs,

So I’ve purchased a new BPW set up, a HOG 23lb lift donut with S/S backplate and a DIR harness. I’ve got 2 weight pouches that a can utilise, however I’ve weighed myself on scales wearing my weight belt and come out at 91kg, then weighing myself with the BPW on, I come out at the same weight, 91kg again - I’m assuming that I don’t need to add any additional weight, is that a fair assumption?

I dive in a full 5mm with steel 12L.

Thank you all

 

[inquis]

Weighing yourself on a scale doesn't characterize buoyancy. That is the difference between your mass (weight on the scale) and the weight of the water you displace when submerged.

In fresh water, I'd say you probably don't need any lead because the components buoyancies sum to about zero. I'm assuming you're about neutral in a swimsuit (can sink if you exhale or float with a breath). That means the water you displace weighs about the same as you do. The 5mm wetsuit, though, has a buoyancy of about +3.7 kg at 5 m depth. Your steel BP is probably about -2.3 kg (negative meaning it sinks). The steel tank (empty) and regulator are about -1.5 kg. Add those up and you're close enough to 0.

In salt water, however, you would need to add about 3 kg because buoyancy depends on the water density, and salt water is about 2.5% more dense on average.

The above is just an estimate. The real test is done in the water. At a 5m safety stop with about 35 bar in your tank, you should be neutral with an empty wing. If you can't stay down, add some lead for next time. If you try to vent your wing and sink, use less time next time.

 

[landphil]

A regular 3mm steel backplate weighs around 5 lbs; and the wing/harness is probably around a pound. if your weight belt was right around 6 lbs you should have similar weighting with just the BPW. So, if you were weighted correctly with your belt and standard BC, it’s likely that you’ll be weighted correctly the BPW and no extra weight. There’s still some unknowns here around the positive buoyancy of your old BC. But you’ll end up pretty close

 

[PerthPom]

Weighing yourself on a scale doesn't characterize buoyancy. That is the difference between your mass (weight on the scale) and the weight of the water you displace when submerged.

In fresh water, I'd say you probably don't need any lead because the components buoyancies sum to about zero. I'm assuming you're about neutral in a swimsuit (can sink if you exhale or float with a breath). That means the water you displace weighs about the same as you do. The 5mm wetsuit, though, has a buoyancy of about +3.7 kg at 5 m depth. Your steel BP is probably about -2.3 kg (negative meaning it sinks). The steel tank (empty) and regulator are about -1.5 kg. Add those up and you're close enough to 0.

In salt water, however, you would need to add about 3 kg because buoyancy depends on the water density, and salt water is about 2.5% more dense on average.

The above is just an estimate. The real test is done in the water. At a 5m safety stop with about 35 bar in your tank, you should be neutral with an empty wing. If you can't stay down, add some lead for next time. If you try to vent your wing and sink, use less time next time.

Thank you, I’m on a club dive this Sunday, I’ll go for an extra 3kg and review on the deco stop.