Question Calculating Detachable Weight?

[rick00001967]

i dont think you guys understood the point my post. but i certainly understand why. i am not sure why i put those numbers. my bad.

i was not suggesting the diver rocket to the surface leaving a buddy holding 16 extra pounds. lol but

only trying to suggest what amount might make him neutral at max depth with an empty tank.

i will edit the post to reflect my errors.

 

[johndiver999]

Worst case your wing will fail at depth with a full tank. Weighting yourself to be neutral at, say, 10 ft at minimal tank pressure with EMPTY wing, you'll obviously be negatively buoyant with a full tank (and busted wing). By how much? Probably less than most new divers think: Gas weight + wetsuit compression. It's just not that much WHEN you're correctly weighted.

Plus, rarely is a wing failure catastrophic (yielding zero lift). Either a bit head up or down depending on where the failure is will usually work. However, we plan for total loss as a worst case.

This tool will help evaluate:
Optimal Buoyancy Computer

The biggest unknown is your wetsuit buoyancy, but that can be measured in a pool with some lead bricks.

While you're at the pool, see how much lead you can hold (on a weight belt) while treading water (finning). Subtract the lead needed to make you neutral. (No need to put your full rig on for this demo.)

A diver can easily be 15-20 lbs negative at depth due to suit compression and weight of gas in tank.

The calculator should help.

As for the OP's question: It is advisable to carry enough ditchable lead so that you can swim to the surface with a deflated wing. This is not a ratio of your total lead, but is dependent on how strong you are and how much suit compression you have (which is dependent on depth and suit size and thickness).

As others mentioned, you can test this out by deflating the BC and trying to swim to the surface with all ballast in place.

Also it would be nice if you could actually stop your ascent at maybe 30 feet after ditching lead and do a little safety stop (with the blown wing). If you have a thick suit, it will still be compressed quite a bit at 30 feet or so. Maybe play with the calculator to find what you are comfortable with?

I dive in a 3 mm suit often and with a steel tank and a pony bottle and an aluminum backplate, I need no additional ballast. Sometimes I will carry an additional 5-7 lbs on a belt, just so I can be really heavy on the bottom and can crawl against the current.

It took me a long time to get used to having no ditchable lead, but I still think it is safer to have a belt you can drop (if you are using a lighter tank) - like an 80. Also, I personally have an smb that I could probably use to get off the bottom in an emergency, assuming the scooter wasn't working and I didn't want to kick too hard. I've had a few BC failures at depth and made it to the surface by winding myself up a reel attached to the smb - no dropped lead.

If your total ballast is like 4-6 lbs, that amount is easily and comfortably carried on a weight belt and ditching that much lead is not going to send you rocketing to the surface, no matter how many internet experts tell you it will. In all likelihood, you will reach neutral buoyancy around 30 - 40 feet, and you can rest and pause your ascent there and then slowly ascend, with limited lung volume and you should have no problem controlling the ascent, especially if you relax, lay back and flair out.

 

[SlugLife]

Given this is a basic-scuba thread, you probably should have ditchable weight, because you're probably a little over-weighted. That's how most people start out, which is fine.

The ideal longer-term goal is to be very neutral. Specifically, try to be neutral at 15ft (5-meters) with about 500psi (35bar) left. You also want to be able to fin to the surface, from depth, with a completely empty BCD and not much finning effort. Once you're confident you're about as neutral as possible, and can easily fin to the surface (BCD empty), then zero ditchable weight is perfectly valid.

But staying within the context of basic scuba, I'd probably suggest a maximum of 4lbs of ditchable weight. Perhaps closer to 2lbs. The idea is you're probably slightly over-weighted, and when ditching weights, you're going from negative, to slightly positive. You don't want to go VERY positive, which would force you into an uncontrolled ascent to the surface.

The best calculator is to actually try it out. See how your buoyancy responds when removing 1, 2, 3, 4, etc lbs. Perhaps hand it to a friend or have a railing you can hold onto, so you can retrieve those weights.

 

[Dan G]

I appreciate the feedback. I put this in Basic, because I felt as if this is a fairly basic level question, not an advanced level question.

Some of this variation I most recently experienced may have a bit to do with who my buddy was when I dove the AL80s. This most recent trip I was diving with my wife who seemingly has gills. This was the first time in years she and I dove AL80s together so perhaps I was feeling the impact of an AL80 below 1000 psi for the first time in a while. With other buddies and I was the first to the turn pressure we had HP100s. When it was my buddy who turned the dive first we would be using AL80s and I had over 1000 pounds remaining. Therefore, I never got any feedback I might be underweighted. All this plus the fact that I wear a 3/5mm hooded vest occasionally, I get quite a bit of variation as to how buoyant I am. The spreadsheet will come in handy once I find out how buoyant my new 3mm is.

I like the idea of swimming different amounts of weight up from the deep end of a pool to see how that feels. I may also just need to get a pair of ditchable weight pockets to add to my waistbelt and see how the different distributions feel once back in salt water. Fortunately, this is all just couple of pounds in either direction.

 

[CT-Rich]

I appreciate the feedback. I put this in Basic, because I felt as if this is a fairly basic level question, not an advanced level question.

Some of this variation I most recently experienced may have a bit to do with who my buddy was when I dove the AL80s. This most recent trip I was diving with my wife who seemingly has gills. This was the first time in years she and I dove AL80s together so perhaps I was feeling the impact of an AL80 below 1000 psi for the first time in a while. With other buddies and I was the first to the turn pressure we had HP100s. When it was my buddy who turned the dive first we would be using AL80s and I had over 1000 pounds remaining. Therefore, I never got any feedback I might be underweighted. All this plus the fact that I wear a 3/5mm hooded vest occasionally, I get quite a bit of variation as to how buoyant I am. The spreadsheet will come in handy once I find out how buoyant my new 3mm is.

I like the idea of swimming different amounts of weight up from the deep end of a pool to see how that feels. I may also just need to get a pair of ditchable weight pockets to add to my waistbelt and see how the different distributions feel once back in salt water. Fortunately, this is all just couple of pounds in either direction.

There's yer' problem... The dive characteristics of AL80 are terrible once you've dialed in on steel. Every time I dive AL, I feel like the last 25% of the dive like i am wearing styrofoam floaty on my back.

 

[formernuke]

There's yer' problem... The dive characteristics of AL80 are terrible once you've dialed in on steel. Every time I dive AL, I feel like the last 25% of the dive like i am wearing styrofoam floaty on back

I only AL for bailout now for just this reason.

 

[Dan G]

As someone who needs to fly to 99% of the dive site we go to, we do not always get to choose which type of cylinder we use. We use an op in Cozumel who uses HP. When I dive with a buddy who lives in FL, I usually have access to HP. Otherwise, it is usually AL.

 

[CT-Rich]

As someone who needs to fly to 99% of the dive site we go to, we do not always get to choose which type of cylinder we use. We use an op in Cozumel who uses HP. When I dive with a buddy who lives in FL, I usually have access to HP. Otherwise, it is usually AL.

Once you have the weights for one tank dialed in, you can look up the buoyancy characteristics of pretty much any tank and add or subtract lead based the difference of of the tanks empty. For example you dive a steel tank at home that is 2 lbs negative and go on a trip where you are going to use AL80, which are 4.5 positive empty. You are going to need 6.5 of additional lead and you will be about the right weight. You can start you weight check there. The amount that is ditchable doesn’t really change a lot, unless you change from a wetsuit to a rash guard or renting a BCD.

For tropical diving, I make all the lead ditchable. I need to almost no adding or dumping of air in my BCD. If the pooch was totally screwed and my BCD was not holding air and the boat not coming back, I could drop lead and stay at the surface indefinitely with no effort. Suit compression is a non issue. A buoyant ascent is the absolute last ditch effort to get to the surface.