Buoyancy, Balanced Rigs, Failures and Ditching – a comprehensive tool

[anchochile]

Man! You must not have very much body fat, or are bringing a steel black plate with you on your trip. Without the spreadsheet in front me that looks pretty close. I think you're on the right track.

Ha... all of the above. I use a 5lb steep backplate, plus steel cam buckles and waist buckle, so my BP/W is probably 7 lb negative. Was planning to bring it on this trip, unless it would make me dangerously overweighted.

Do you think it’s safe to go with zero lead for my first warm water dives or is there a chance I won’t be able to sink and should start with 2-4 lb just in case?

 

[rsingler]

Lol, I'm not surprised! Well, from what you've described I don't think you need any lead at all. Remember that at the beginning of a dive you will also have 5 pounds of air that you're carrying, so you'll have no trouble getting down. If you start to feel floaty at the very end that should be enough warning that you need a touch more lead. From the numbers you've quoted I highly doubt it. I think it's going to be exactly as you described : that you'll be slightly heavy throughout. But that amount of overweight doesn't sound dangerous at all. You can easily Fin up 7 pounds excess if your BCD fails.

What you are describing sounds exactly like what I just did in Grand Cayman. I dive a 5 pound Freedom Contour backplate and cam buckles with steel at home . I traded my 30 pound wing for an 18 pound wing, but otherwise brought the whole kit to GC. And switching from 7 mm to 3 left me with only 2 pounds lead for the whole trip.

You're doing this right.

 

[johndiver999]

A steel backplate and an aluminum tank and an old 3 mm suit, should make you about perfect in saltwater - would be my guess. So I would take a weightbelt and carry 4 lbs on it for the first dive just to make sure - with the understanding that somebody on the internet told you that "you probably won't want it for the second dive".

Carrying 4 lbs "too much" lead on a weightbelt that can be dropped in an emergency is not going to decrease your safety if you are skilled at buoyancy control.

 

[AfterDark]

Carrying 4 lbs "too much" lead on a weightbelt that can be dropped in an emergency is not going to decrease your safety if you are skilled at buoyancy control.

Right there is the take away from this entire thread.

 

[anchochile]

Lol, I'm not surprised! Well, from what you've described I don't think you need any lead at all. Remember that at the beginning of a dive you will also have 5 pounds of air that you're carrying, so you'll have no trouble getting down. If you start to feel floaty at the very end that should be enough warning that you need a touch more lead. From the numbers you've quoted I highly doubt it. I think it's going to be exactly as you described : that you'll be slightly heavy throughout. But that amount of overweight doesn't sound dangerous at all. You can easily Fin up 7 pounds excess if your BCD fails.

Thanks! Good to hear. I've also heard that some operators in Key Largo, where I'm going next month, offer larger steel tanks (HP100s) for the deep wreck dives. I'm thinking I should probably turn those down to avoid being dangerously overweighted at depth in this rig, right?

 

[rsingler]

My HP100 is -0.6lb empty. Better than a LP80 steel! So don't worry too much unless it's a Heiser.

You can use the spreadsheet to see what your loss of buoyancy at depth due to wetsuit compression might be. Just bring an SMB if you want redundant lift.

 

[rsingler]

Here's a screenshot of your data with a Worthington 100 (-2# empty) and a 3mm suit (no hood), with no extra lead. -7# for your backplate and cam bands.

10# suit buoyancy; only 2.5# buoyant at 100'. Your rig is -18.5# by itself.
You are -8# at surface at dive start with a full tank. That becomes -16# at 100' due to suit compression. Can you swim that much up? Need an SMB just in case.

But you're only -9# at dive end, at 100'.
You are -5# at 15' at dive end, and -2# at the surface, with a near-empty tank.

Yeah, you can dive a HP100 easily with that rig in a 3mm. But bring an SMB, just in case your bcd fails at the very beginning.

 

[guyharrisonphoto]

I used your tool and here is my practical result. My issue was that I am a Florida diver that uses a stretchy 3 mil suit. I was going to dive in California and use a much less stretchy 7 mil semi-dry with hood, gloves, boots, etc. I currently carry 12 pounds but that is intentional overweighting to hold bottom in our currents for my photography, so I had no real starting point for an estimate. I needed to dial in my weight before California because my first dives were boat dives and I had to have it right. I also knew i would not need to overweight because the conditions in California did not involve currents.

After using the calculator and estimating the weight of my equipment, the estimated weighting was 20 pounds. I then went to an easy shore dive spot with the tank I used for calculation (steel hp 100) geared fully up (looked ridiculous in 82 degree water), carried 25 pounds in my thigh pockets just in case the calculator was light, dove to 16 feet, drained the tank to 500 psi, and then began shedding weight. I was an anchor at 25 pounds. I was still heavy at 20 pounds. Ultimately, I was nice and neutral with 15 pounds. I think my regulator (a double hose) and some of my other gear combined weighs several pounds more than the default weights in the calculator.

But, in summary, the calculator was great and pretty right on. It was a perfect starting point when otherwise I would have had no idea. If I had actually weighed my gear (reg, light, reel, etc) then I bet it would have been even closer to the 15 pounds.

I also figure that I could easily drop from 12 pounds to 6 pounds for my Florida diving unless I want to intentionally overweight.

 

[rsingler]

But, in summary, the calculator was great and pretty right on. It was a perfect starting point when otherwise I would have had no idea. If I had actually weighed my gear (reg, light, reel, etc) then I bet it would have been even closer to the 15 pounds.

Glad to hear it! We need all the data points we can get. Yes, there's room in Initial Data Entry for "Other Fixed Weight", in cell D20, for other items that increase your negative buoyancy. Make sure you enter them as negative numbers. Also, consider a correction to Personal Buoyancy if you have more or less than an average amount of body fat.
Thank you for the feedback! If we can come within a couple of pounds of correct for a blind estimate, that's as much as I can hope for.

 

[guyharrisonphoto]

My "real" feedback is: WELL DONE! I can't believe all of the hard work and thought you put into this extremely useful tool.