Fast descents vs correct weighting

[Banana Joe]

Hello again,

So today I dropped a kilo, this meant I had minimum loop vol, comfy drysuit and just a squirt of air in the wing - bliss. Made the ascent very easy too, I could probably shave another 500g off so I don't need to use the wing at all but I will wait another five hours or so before I try that.

However, with the extra kilo I could descend at my usual 20m/min easily. Today it was around half that, it was also a free descent so I didn't have shot line to 'aid' my speed.

Really I am just asking weather I should compromise on comfort and descent speed. I was sooo comfy on this dive that I am perfectly happy to put up with a slower descent but I would like to hear opinions on this for sub 40m dives.

Cheers, Joe

Slowly filling the noob forum one thread at a time :sarcy:

 

[tecdiverdude]

sounds like you're on the right track... If I want to speed my descent.. I swim down...
It amazing what 1kg does to a CCR rig... the difference between a mongel of a dive and a good one

 

[matt]

For warm water I go with what you're suggesting (I guess warm in a dry bag is 18C or over?), and in wet-suit the same; just enough to be neutral.

For cold water I like to be a bit over so I can have some extra air in the suit to keep me warm (I guess cold is less than 12C - which makes we wonder what to call 12-18, lol!).

Everything's a compromise so perhaps keep the extra 500g and get used to the slower descents? Or add back 500g and see if that's a better deal?

Matt.

 

[depth]

Hello again,

So today I dropped a kilo, this meant I had minimum loop vol, comfy drysuit and just a squirt of air in the wing - bliss. Made the ascent very easy too, I could probably shave another 500g off so I don't need to use the wing at all but I will wait another five hours or so before I try that.

However, with the extra kilo I could descend at my usual 20m/min easily. Today it was around half that, it was also a free descent so I didn't have shot line to 'aid' my speed.

Really I am just asking weather I should compromise on comfort and descent speed. I was sooo comfy on this dive that I am perfectly happy to put up with a slower descent but I would like to hear opinions on this for sub 40m dives.

Cheers, Joe

Slowly filling the noob forum one thread at a time :sarcy:

Please add to the equation that you will have to be weighted with empty or no BO cylinders. If the shit hits the fan and you have to use your BO gas or hand over your cylinders to dive-buddies, you need to be weighted correctly to stay submerged!
This does not only apply at sub40m!

Safe diving

/Christian

 

[Iain3]

However, with the extra kilo I could descend at my usual 20m/min easily. Today it was around half that, it was also a free descent so I didn't have shot line to 'aid' my speed.

I try to be weighted so that at the surface I empty suit wing and loop, breathed out and need a back kick to pull me under. That is diving without stages, not fussed about back gas depletion, I don't use that much.

If I want to descend fast I go head down and fin, or pull myway down the shotline.

 

[Explorer]

I try to be weighted so that at the surface I empty suit wing and loop, breathed out and need a back kick to pull me under. That is diving without stages, not fussed about back gas depletion, I don't use that much.

If I want to descend fast I go head down and fin, or pull myway down the shotline.

If I want to drop faster I prefer to point my scooter down and go......

 

[Mark Davies]

Please add to the equation that you will have to be weighted with empty or no BO cylinders. If the shit hits the fan and you have to use your BO gas or hand over your cylinders to dive-buddies, you need to be weighted correctly to stay submerged!
This does not only apply at sub40m!

Safe diving

/Christian

Good point. Two or three floaty stages when yor weighting is on the limit would be a nightmare. Personally if there is any gas left in them I would prefer to hang onto them for air breaks.

 

[Peter L]

Good point. Two or three floaty stages when yor weighting is on the limit would be a nightmare. Personally if there is any gas left in them I would prefer to hang onto them for air breaks.

Claire made a comment on another thread that she considers an empty Ali 80 neutral aka not to add weight for it due to the regs etc on them. She has had more experience swimming them than I ever want to have.

I also have a photo of a friend who acting as deep support had 10 of them on him after doing a collection from some divers coming back from 150M on OC. You could not weight for that many tanks on a random basis so I suspect she may just be right.

 

[keri.lewis]

Claire made a comment on another thread that she considers an empty Ali 80 neutral aka not to add weight for it due to the regs etc on them. She has had more experience swimming them than I ever want to have.

From the UK Scuba Buoyancy Calculator

When a 14.3kg AL80 cylinder is COMPLETELY empty it in seawater is has 2.59kg positive buoyancy (based on material density for Alu, Sea water etc.). This is the absolute worst case possible. This sounds like a lot, but when you add on a cylinder pillar valve, a regulator, jubilee clips, bolt snaps etc. it does bring it back to almost neutral.

Using air in the cylinder, the contents weigh 2.76kg (obviously less for Helium). Add in 30 bar or so (30/210 = 400g) and it is neutral to all intents and purposes.

I would venture that if you've got less than 30 bar in the cylinder then you have quite a serious problem.. and buoyancy is only part of it!:hehehm:

A "GOOD" Alu80 is neutral for most calculations. There are some that are negative, some that are positive. It depends on the manufacturer and composition. You can work it out for any cylinder using the calculator in the link above.

P.S. For those that require complete assumption lists... the 14.3kg cylinder is one of my Luxfers.

 

[Iain3]

Claire made a comment on another thread that she considers an empty Ali 80 neutral aka not to add weight for it due to the regs etc on them. She has had more experience swimming them than I ever want to have.

But many divers when they add weight to their gear (torch, bigger back cylinder etc) will drop weight accordingly. Full stages will add weight, but don't drop lead to compromise or you could have problems.

 

[Peter L]

But many divers when they add weight to their gear (torch, bigger back cylinder etc) will drop weight accordingly. Full stages will add weight, but don't drop lead to compromise or you could have problems.

The conversation is around adding weight in case you breath it to empty which in this instance I suspect is not the right thing to be doing. I agree with the remainder of your points, you have to weight for minimal config aka unit alone because there will be times where your stages come off.

 

[YobazEfendi]

Why not try to make everything neutral?

I am now toying with the idea of attaching some foam to the bailout tanks to make them neutral. If i need to use them, i can just release the foam as needed making the tank neutral again.

 

[Peter L]

Why not try to make everything neutral?

I am now toying with the idea of attaching some foam to the bailout tanks to make them neutral. If i need to use them, i can just release the foam as needed making the tank neutral again.

Your foam would loose volume with depth ala neoprene unless you make it ridgid.

People are / were putting hollow capped PVC Pipe on the bottom of rEvo CCR's. They were great for trim until circa ~90M where upon they would implode with a rather distressing crunch according to Paul R who had it happen.

 

[Mike Ferguson]

Your foam would loose volume with depth ala neoprene unless you make it ridgid.

People are / were putting hollow capped PVC Pipe on the bottom of rEvo CCR's. They were great for trim until circa ~90M where upon they would implode with a rather distressing crunch according to Paul R who had it happen.

you need to use syntactic foam. basically encapsulated hollow glass or plastic beads. you dont really want to have one atm canisters imploding near your kidneys. one of my friends had a torch implode at 90m while holding it. He likened the experience to being punched in the head and the lights went out.

we once had some 10" steel pipe implode at 1300m on a piece of kit because someone forgot to include the flooding hole. nobody there to hear it but I assume it went with a bit of a bang.

 

[YobazEfendi]

Did he faint or did he flooded torch just refuse to work

 

[Bythesea]

Claire made a comment on another thread that she considers an empty Ali 80 neutral aka not to add weight for it due to the regs etc on them. She has had more experience swimming them than I ever want to have.

this is a good little too....> UK Scuba - Cylinder Buoyancy Calculation

It gives around +2.5KG for an empty ali80 with a dry weight of 14.3Kg

So 2.5Kg, add up the weight of your first stage, clips etc, I bet it is not for off 2.5 making it nice and neutral when empty.


I thought I would trim off a couple of Kg while diving the other evening, I have a 3ltr ali bailout, it was a shallow dive, so was happy to weight with that as it is the smallest stage I will carry and it weighs nothing.

I took 2Kg off {individual 1Kg weights with clips} what I would normally use, clipped them to a reel my buddy had for use on a permanent surface marker with the intention of adding them, if needed, one at a time. I explained to my buddy my intention to add one Kg, then the other if I needed it, I felt I could drop at least one of them.

We got on the surface, signalled to start the descent, I then watched as my buddy plummeted out of site with my spare weight while I stayed on the surface, turns out I needed one of those kilos I did rip the pi55 much when he came back up looking for me.

I repeated the experiment today with a more attuned buddy and I was right, I can drop one kilo, any more and I simply stay on the surface, the only question now is.... was it worth it for one kilo?

 

[Bythesea]

So you don't think there is an argument for a 'comfort kilo'? i.e. a kilo over ideal weight? You know, to speed up descents and to reduce potential floatyness if a little stressed on a stop?

 

[matt]

So you don't think there is an argument for a 'comfort kilo'? i.e. a kilo over ideal weight? You know, to speed up descents and to reduce potential floatyness if a little stressed on a stop?

For cold water yes for warm water no.

Matt.

 

[PeterN]

So you don't think there is an argument for a 'comfort kilo'? i.e. a kilo over ideal weight? You know, to speed up descents and to reduce potential floatyness if a little stressed on a stop?

Well, hang on a min!

I have been playing with this for a while. It is all very well to dial yourself in with a full bail out tank, what happens when it is near empty?

I expect most of us dry suit divers have vacume packed ourselfs when near the end of deco and finding out that a little more lead might have beeen nice.

As Monday is a holiday here, I will be diving. My plan for the dive is very modest. I will however blow off my Al 40 at the end of the dive to see how I do. I rairly dive with just the one bail out tank, so I will be taking advantage of the chance to tune up my weight.

I have noticed that an Al 80 with 20/35 is just slightly negative when full. As I am too cheap to blow it off, I can only wonder just how floaty that would be when nearly done. Never mind Al 80's with 10/50 or some such mix.

Peter

 

[YobazEfendi]

You should be able to calculate that.

Helium is 0.1785 kg/m3
and air is 1.225 kg/m3

so more or less 1kg/m3 difference for practical purposes.


If you ask google "80 cubic feet in m^3" it will tell you:
80 (cubic feet) = 2.26534773 m^3

So for your tank of 20/35 would be
2.26534773 *0.35 = 0.8 kg difference compared to air when full.

and for the 10/50 that would make a difference of 1.13kg compared to air when full.

Atleast in theory...


Teoman