Please evaluate my weight calculation, Buoyancy

[rnln]

Hi everyone,
I am back with another question on tanks/weights in general. Can someone help correct me if my numbers are not right. Note, I need to fill air at a scuba shop; therefore, overfill will not be the case.

Going from AL80 to LP72, I can save 4 lbs on my waist
From LP72 to LP85, I save another 2 lbs (breathing capacity is about the same as the AL80)?
From LP85 to HP100, I can save another 3lbs (how breathing capacity is 15cf more)?
Thanks again.

 

[James79]

Hi everyone,
I am back with another question on tanks/weights in general. Can someone help correct me if my numbers are not right. Note, I need to fill air at a scuba shop; therefore, overfill will not be the case.

Going from AL80 to LP72, I can save 4 lbs on my waist
From LP72 to LP85, I save another 2 lbs (breathing capacity is about the same as the AL80)?
From LP85 to HP100, I can save another 3lbs (how breathing capacity is 15cf more)?
Thanks again.

The buoyancy difference from al80 to lp72 is right, but I'm not familiar on the others. However, do not worry about tank capacity when looking at weighting... Look at tank buoyancy. Depending on how a tank is made (size, shape, and pressure) will determine it's buoyancy. The buoyancy calculator has a pretty good chart for tank buoyancy characteristics.

Respectfully
James

 

[stepfen]

The tool below is made exactly to answer this kind of questions:
Optimal Buoyancy Computer
You should be able to get basic estimations/suggestions (i.e. quick results) within minutes.

 

[rongoodman]

I think I use 18# in salt water with a similar set-up but I only weigh about 170#. Get flat or a little head down at your safety stop and reach around to feel how much air is in the bottom of your wing. If there's more than a fistful or so, drop a little weight and check again on the next dive. If you need to have the wing shrink-wrapped around the tank in order to hold the stop, you're probably too light.

 

[rnln]

Hi everyone, I am back on the same topic. So, I had a chance to test my "new gears" (new to me), BP/W and steel tank 85. I hope some expert can have time to read and input. Let me remind everyone without reading back from the beginning, I am ....

weight: between 200 to 205 lbs depend on the day. Height: 5.9". Have some belly.

Previous dives:
- I had full jacket BCD, AL 80, 7 mm farmer John wet suit (so, let say 8mm). I wore 32 lbs weight. Very hard to decent, and hard to stay put on bottom
- Another dive I had same setup but back inflate BCD and 30 lbs weight. Still hard to decent and stay put on bottom, but little better than the full BCD.

New gears:
Back plate is SS DSS, 7 lbs I heard.
Wing 35 lbs capacity.
Steel tank LP 85
I wore 5mm top/jacket, 3mm bottom/pants, open cell freediving wetsuit. So, let say 4mm wet suit
6 lbs weight attached to the tank, and 13 lbs at the belt. That is 19 lbs total.

The test:
4 full blown by mouth to inflate the wing before I got in the water. After several seconds swimming out from the rocky shore, probably 10' dept, when I looked up, with some kicking and breathing, water level started at my eyes level and sinking slowly. Then I inflated the wing to the max, with some kicking and breathing, water level was at my mouth level and still sinking slowly. Now, I am sure that I am over weight, but how much? Can anyone "guess" how much I was over weight by the above?

Calculation 1: base on steel tank 80 (I have LP 85)
divebuddy.com gave me 19 lbs for 5mm and 15 lbs for 3mm = 17 lbs for 4mm wet suit.

Calculation 2: base on previous set up...
32 lbs - 7 lbs for the back plate = 25 lbs
- 6 lbs from AL 90 to steel 85 tank = 19 lbs
- 8 lbs from 7mm farmer John (let say 8mm) to 4mm I wear now (5mm top and 3mm bottom) = 11 lbs

Does that mean I should wear only 11 lbs of weight, including the back plate is 18 lbs total?
Does the 8 lbs over weight did the above test result?

Thanks everyone.

 

[formernuke]

Personally your overthinking it. You did your rough calculations and tried it. You feel your over so take off 2 pounds try it again.

Everyone is different and will need different amounts even with same gear and same weight. Thats why the calculator is just an estimate to give a starting point. Actual correct weight is trail and error.

 

[Barnaby'sDad]

Hi everyone, I am back on the same topic. So, I had a chance to test my "new gears" (new to me), BP/W and steel tank 85. I hope some expert can have time to read and input. Let me remind everyone without reading back from the beginning, I am ....

weight: between 200 to 205 lbs depend on the day. Height: 5.9". Have some belly.

Previous dives:
- I had full jacket BCD, AL 80, 7 mm farmer John wet suit (so, let say 8mm). I wore 32 lbs weight. Very hard to decent, and hard to stay put on bottom
- Another dive I had same setup but back inflate BCD and 30 lbs weight. Still hard to decent and stay put on bottom, but little better than the full BCD.

New gears:
Back plate is SS DSS, 7 lbs I heard.
Wing 35 lbs capacity.
Steel tank LP 85
I wore 5mm top/jacket, 3mm bottom/pants, open cell freediving wetsuit. So, let say 4mm wet suit
6 lbs weight attached to the tank, and 13 lbs at the belt. That is 19 lbs total.

The test:
4 full blown by mouth to inflate the wing before I got in the water. After several seconds swimming out from the rocky shore, probably 10' dept, when I looked up, with some kicking and breathing, water level started at my eyes level and sinking slowly. Then I inflated the wing to the max, with some kicking and breathing, water level was at my mouth level and still sinking slowly. Now, I am sure that I am over weight, but how much? Can anyone "guess" how much I was over weight by the above?

Calculation 1: base on steel tank 80 (I have LP 85)
divebuddy.com gave me 19 lbs for 5mm and 15 lbs for 3mm = 17 lbs for 4mm wet suit.

Calculation 2: base on previous set up...
32 lbs - 7 lbs for the back plate = 25 lbs
- 6 lbs from AL 90 to steel 85 tank = 19 lbs
- 8 lbs from 7mm farmer John (let say 8mm) to 4mm I wear now (5mm top and 3mm bottom) = 11 lbs

Does that mean I should wear only 11 lbs of weight, including the back plate is 18 lbs total?
Does the 8 lbs over weight did the above test result?

Thanks everyone.

Include the back plate in your weight calculation. As the others were saying though...it’s trial and error. Whenever you switch buoyancy devices, wetsuit thickness, Water type (fresh, brackish, salt), you’ll have to tinker with your weight again. Take notes in your logbook so that you know where your starting point is for each scenario and you don’t have to start from scratch (an online calculator) each time you make a configuration change.

The easiest thing to do is start a bit heavy and then start removing weight as you go. If you think that you’re WAY overweight, remove 4lbs and try again. If you think that you’re in the ballpark, remove 2lbs and try again.

 

[rongoodman]

Hi everyone, I am back on the same topic. So, I had a chance to test my "new gears" (new to me), BP/W and steel tank 85. I hope some expert can have time to read and input. I

You're overthinking it, it isn't rocket surgery. You need to be able to hold a stop with a nearly empty tank at the end of a dive. It will be easy for you to check with your new BP. Go a little head down on the safety stop on your next dive and reach around to feel how much air is left in the wing. If it's more than a fist full or so, drop a couple of pounds before your next dive. Repeat as necessary.

 

[formernuke]

I inflated the wing to the max, with some kicking and breathing, water level was at my mouth level and still sinking slowly. Now, I am sure that I am over weight, but how much? Can anyone "guess" how much I was over weight by the above?

Forgot to add if your wing is fully inflated and your sinking you are way over. Even a little over with a fully inflated wing you should be comfortably floating without movement.

 

[Bob DBF]

If you can't get down with a full tank, you won't be able to hold a safety stop.

Not necessarily. I dive a 7mm and have to duck dive to submerge, after the dive all the trapped air is out of my rig and the suit does not rebound to its initial buoyancy, I have no problem holding a stop. If I put another tank on and get back in the water, I have no problem submerging.

It depends on all conditions, not just initial weighting. In most cases I do a weight check on every dive at the safety stop. Hovering at 15', about 500# in the tank, with an empty BC.