Question Calculating Detachable Weight?

[Dan G]

I am sure ditchable weights have been debated a ton here, but I cannot find any old threads on this. I dive a BP&W. When I am not using an HP100, I have been adding two 2lbs weights to trim pockets attached to my cam bands. Works great with an AL80. I prefer not to use an STA, because I far prefer how the single tank sits on the backplate compared to how it sits on an STA. Recently, I bought a new 3mm suit and after a couple of dives, I upped my weight for an AL80 from 4lbs to 6lbs. Seemed like the new suit was much more positively buoyant than my very old and worn 3mm. I wore a weight belt with 4 lbs and placed two 1lbs in the trim pockets.

My preference is to just put any needed weight in my trim pockets rather than using a belt. I suppose I could get these and see how it goes. I am wondering how much ditchable weight is good to have have compared to overall weight. Some questions:

• Is the amount of ditchable weight really just a matter of what each individual is comfortable with?
• Is there a way people like to calculate this beyond just "How much weight can you kick up with?" For example, "Make 50% of your weight ditchable"
• If I dive an HP100 and HP117 without any weight, then is my question over two additional pounds when diving an AL80 moot? Essentially, is putting two 3lbs in my trim pockets compared to two 2lbs about the same as diving an HP100 or HP117?
  • An AL80 weights about 31.5 lbs
  • An HP100 weighs about 34lbs
  • An HP117 weighs about 37.5lbs

 

[inquis]

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.)

 

[inquis]

For reference, my XL 3mm suit would lose at most 6 lb of buoyancy from surface to 100 ft. (Smaller sized suits would lose less.) I can compensate for that with a large breath.

That leaves the gas weight of 5 lb to kick up if needed, which is quite easy. (I weight myself to be neutral with 1 lb reserve, and an AL80 holds 6ish lbs.)

 

[inquis]

If I dive an HP100 and HP117 without any weight

You wouldn't if that was the only change. Empty buoyancies are: +4.4 lb (AL80), -0.6 lb (HP100), and +0.2 lb (HP117). The buoyancy difference from your AL80 case (which needs 6 lb lead) is 5 lb (HP100) or 4.2 lb (HP117). So you'd still want 1 or 2 lbs of lead, respectively, to maintain your current target buoyancy.

Realistically, though? I'd at least try the HP100 without any lead, as your recent 2 lb addition may have overshot a little. Nothing like an actual dive to work these things out.

 

[rick00001967]

many people i am sure will chime in to tell you, you don't need any ditchable weight, and shouldn't use any. i have no opinion on that today.

without opening that can of worms, let just assume you want to use some ditchable weight for your own reasons.

lets also assume we are talking about basic recreational diving in a 3mm wet suit since that was in your example.

as was mentioned above, you should be weighting yourself at 10 or 15 ft with low tank pressure (near empty) and an empty wing.

so the only difference between that scenario and your "worst case" is that you would have a full tank, and would be at max depth. lets assume your max depth might be 130 feet.

so the difference in weight would be the weight of the gas (which depends on the tank capacity) and the loss of buoyancy from compression of the 3mm suit.

an allum 80 for example will hold aprox 6lbs of gas. how much buoyancy would your suit loose at 130 feet? i have no idea. but lets assume 4lbs.

so if you want to experiment, you could go to your max depth with a buddy, in a spot where there is a bottom (not on a drop off lol), empty your wing, and hand the buddy 10 lbs. that would simulate no gas in the wing, max suit compression, and an empty tank. you could hand off small amounts at a time until you find how much is needed until you are neutral.

but here is the problem. as you ascend, your suit would become more buoyant. this will cause an out of control emergency ascent.

so you need to decide which situation you want to be in. do you want to dump enough to assure you will be able to begin your ascent and def make it to the surface even if you pass out during the ascent?
or do you want to be in control all the way to the surface? (this assumes you will either have enough gas to make it, or can exhale all the way up)

just fyi....my boss always has said "it is better to be bent on the surface, than dead on the bottom". this of course may depend on the type of diving someone is doing. clearly this does not hold true for someone doing advanced deco diving for example.

but pls be mindful.....according to the way i was trained, most problems occur AFTER the diver makes it to the surface. meaning that most divers are able to reach the surface but fail to dump enough lead to remain there.

so maybe in a perfect world we would actually have two means to ditch weight. the first allowing us to ditch just enough to get us off the bottom from max depth and perform a controlled swimming ascent. the second allowing us to ditch more at the surface.

most recreational divers have at least two weight pockets that can be dumped. so it is possible to be able to achieve this by ditching only one pocket on the bottom, performing a controlled ascent, then ditching the second at the surface if needed. but i don't ever recall seeing any discussions about this online. i have discussed this in classes many times.

 

[CT-Rich]

Most of my lead is ditchable. I have two pouches on my BCD AND 12 lb on a belt. IMHO, is at a minimum you should be able to dump enough to stay on the surface with a full tank and empty BCD. My steel 120 has 9 lbs of air when full. I could just drop the belt (12 lbs) to stay on the surface if I had to.

To swim a compressed wetsuit off the bottom? I could drop one pouch and offset about 6 lbs. I am not a fan of dropping weights to go up, because you lose control of your ascent. Staying at the surface is going to be your biggest priority and once there, there are almost no downsides to ditching weights to stay at the surface.

I had dropped weights twice, both were on the surface. Once was when I was about to pass out and once at night with a BCD where the shoulder dump was jammed open with a long surface swim, and nearly full tank (my buddy was low on air and I didn’t want to separate so I could swim in on the bottom).

 

[formernuke]

You only need enough ditchable to stay afloat on the surface in the event of a bc failure etc if you are diving ditchable weight.

 

[inquis]

if you want to experiment, you could go to your max depth with a buddy, in a spot where there is a bottom (not on a drop off lol), empty your wing, and hand the buddy 16 lbs. that would simulate no gas in the wing, max suit compression, an empty tank, and dumping 10 lbs.

Bad idea. If properly weighted they will be at least +10 lb buoyant at their planned neutral depth (even more above that). Any residual air in the wing will expand, further exacerbating the situation. They WILL be head down, kicking hard to stay down, likely unable to vent the BC since most new divers do not know how use the rear dump.

A better experiment is to NOT DROP ANY WEIGHT and just try to swim up with empty BC. It gets easier as the wetsuit expands. If you make it to the planned neutral depth, you will be negatively buoyant by 5ish lbs, easily compensated with light finning and breathing. Very safe.

If you cannot get off the bottom, hand off an amount of lead (if you have it) matching the weight of the air you haven't breathed (about 5 lb). Swim up and you will be approximately neutral at the planned neutral depth. Very safe.

If you still can't get off the bottom, that is a sign you are overweighted OR perhaps that shouldn't dive that tank/wetsuit combination.

Take a buddy in case of the unlikely event your BC fails for real and you cramp up during all this experimentation.

 

[CT-Rich]

many people i am sure will chime in to tell you, you don't need any ditchable weight, and shouldn't use any. i have no opinion on that today.

without opening that can of worms, let just assume you want to use some ditchable weight for your own reasons.

lets also assume we are talking about basic recreational diving in a 3mm wet suit since that was in your example.

as was mentioned above, you should be weighting yourself at 10 or 15 ft with low tank pressure (near empty) and an empty wing.

so the only difference between that scenario and your "worst case" is that you would have a full tank, and would be at max depth. lets assume your max depth might be 130 feet.

so the difference in weight would be the weight of the gas (which depends on the tank capacity) and the loss of buoyancy from compression of the 3mm suit.

an allum 80 for example will hold aprox 6lbs of gas. how much buoyancy would your suit loose at 130 feet? i have no idea. but lets assume 4lbs.

so if you want to experiment, you could go to your max depth with a buddy, in a spot where there is a bottom (not on a drop off lol), empty your wing, and hand the buddy 16 lbs. that would simulate no gas in the wing, max suit compression, an empty tank, and dumping 10 lbs.

then see if you are able to swim up. if you end up being neutral then this should be easy.

but here is the problem. as you ascend, your suit will become more buoyant. this will cause an out of control emergency ascent.

so you need to decide which situation you want to be in. do you want to dump enough to assure you will be able to begin your ascent and def make it to the surface even if you pass out during the ascent?
or do you want to be in control all the way to the surface? (this assumes you will either have enough gas to make it, or can exhale all the way up)

just fyi....my boss always has said "it is better to be bent on the surface, than dead on the bottom". this of course may depend on the type of diving someone is doing. clearly this does not hold true for someone doing advanced deco diving for example.

if you want to try a controlled ascent, then run the same experiment above but only ditch maybe 12 lbs. see how that goes. this would simulate no gas in wing, max suit compression, and empty tank, and dumping 6lbs instead of 10.

but pls be mindful.....according to the way i was trained, most problems occur AFTER the diver makes it to the surface. meaning that most divers are able to reach the surface but fail to dump enough lead to remain there.

so maybe in a perfect world we would actually have two means to ditch weight. the first allowing us to ditch just enough to get us off the bottom from max depth and perform a controlled swimming ascent. the second allowing us to ditch more at the surface.

most recreational divers have at least two weight pockets that can be dumped. so it is possible to be able to achieve this by ditching only one pocket on the bottom, performing a controlled ascent, then ditching the second at the surface if needed. but i don't ever recall seeing any discussions about this online. i have discussed this in classes many times.

NEVER hand your weight (16 lbs!) off at 130'. that is almost like attaching two one gallon milk jugs to your armpits. Your buddy is now massively overweighted and you may quickly find yourself on an elevator you can't get off and he will have no way to catch up with you. Suit compression isn't going to make a huge difference after 70 or 80'. Dump your BCD fully and see if you can swim off the bottom. If you can't, plan on making a portion of your lead dumpable at depth. If you have a dry suit, it can function as redundant buoyancy in the event of a BCD failure.

Some experiments are not worth the risk. Handing someone 16 lbs of lead at 130' is a terrible idea.

 

[grantmac]

Zero for me. But I'm dry in cold water occasionally with some overhead.

Warm water with a wetsuit I need ~4# which I'll put on a belt.