Weighting 15l + Ali80

Leg of Salmon · January 12th, 2008, 02:04 PM

Hi

I couldnt find a definitive answer so here goes:

If a diver, properly weighted for a 15l steel cylinder, adds a side slung Al80 to use when the wotsit hits the fan, does he need to make any changes to the weight carried?

As I understand it the Al80 is neutral, more or less, at 150 - 160 bar. So if the diver starts to breath this it will become buoyant (about 2.5kg when empty).

Does this mean the diver should add 2.5kg?

Cheers
Chris

**keri.lewis** · January 12th, 2008, 02:10 PM

Wouldn't it be better to breath the Al80 first - stage style ?

That way you have a nice ful 15l steel at the end of the dive to offset the buoyancy changes.

I'm assuming that you are not proposing decompression dives here as we are effectively talking about independent double tanks on a DIR forum..

Leg of Salmon · January 12th, 2008, 02:23 PM

Quote: (Originally Posted by keri.lewis)

Wouldn't it be better to breath the Al80 first - stage style ?

That way you have a nice ful 15l steel at the end of the dive to offset the buoyancy changes.

That sounds like a good idea.

Quote: (Originally Posted by keri.lewis)

I'm assuming that you are not proposing decompression dives here as we are effectively talking about independent double tanks on a DIR forum..

Was hoping nobody would notice

I wasn't sure where to post but wanted to know how people here would do it if they did it...

Thanks

**keri.lewis** · January 12th, 2008, 02:32 PM

Good... and I should think so too! Independent doubles... mutter mutter mutter

More seriously - you're much better off with a twinset if you're going down that path, and they don't cost that much..

**neilh** · January 12th, 2008, 03:14 PM

Quote: (Originally Posted by Leg of Salmon)

If a diver, properly weighted for a 15l steel cylinder, adds a side slung Ali80 to use when the wotsit hits the fan, does he need to make any changes to the weight carried?

Turning this question on it's head a bit - what are you trying to solve by doing this? If you have a catastrophic gas failure, then you have at least one team mate with you with enough gas to get you both out (that's your minimum gas planning). If you're diving in a 3 then there's two people with enough gas to get you out.

You would have to lose all your team (oops!) and have a complete gas failure (unlucky) before you would need to worry...

If you're concerned about any kind of cylinder/reg failure taking out your one and only source of breathing and inflation gas then perhaps manifolded matched doubles would be a better option than a 15L and an 11L (ali 80)

Leg of Salmon · January 12th, 2008, 03:25 PM

I'm not trying to solve anything. I wasn't planning on doing this myself I was just reading around about gas management, equipment, weighting, and so on and wondered what the weight/buoyancy implications of adding aluminium stage cylinders.

Go easy on me, I'm only new

**neilh** · January 12th, 2008, 03:34 PM

Quote: (Originally Posted by Leg of Salmon)

Go easy on me, I'm only new

Hehe - no problem, was just trying to point out that you can think about it another way

Quote: (Originally Posted by Leg of Salmon)

I'm not trying to solve anything. I wasn't planning on doing this myself I was just reading around about gas management, equipment, weighting, and so on and wondered what the weight/buoyancy implications of adding aluminium stage cylinders.

Fair enough, you would work out your weighting without them

Leg of Salmon · January 12th, 2008, 03:42 PM

Quote: (Originally Posted by neilh)

you would work out your weighting without them

In my example, I understand that you could breath the stage first, so you still have back gas and this sorts out the buoyancy.

But what happens in other situations? If you are breathing an aluminium stage bottle for decompression, or on the bottom, for example it will become more buoyant so how do you combat this. I am possibly being a bit dumb about this.

**neilh** · January 12th, 2008, 04:02 PM

Quote: (Originally Posted by Leg of Salmon)

But what happens in other situations? If you are breathing an aluminium stage bottle for decompression, or on the bottom, for example it will become more buoyant so how do you combat this. I am possibly being a bit dumb about this.

Well using bottom stages is beyond my training so I can't comment on that, but as for backgas you plan a reserve for your deco gas. So yes the deco bottle is going to get more buoyant, but if all goes to plan it's not going to be unmanageable. Also you've still planned your backgas based on minimum gas, but your weighting based on almost empty backgas cylinders - so you have a bit of weight left there as well.

In the worse case scenario where you've lost all your backgas and you've just used all your deco gas then you can always clip the deco bottle to the SMB line and let it head off to the surface while you go onto someone's long hose.

**keri.lewis** · January 12th, 2008, 04:04 PM

As an example... Wetsuit - warm water but still relevant to sorting out the set-up before trying it out safely in shallow water and then diving for real.

Start Scenario
Twin Al80 back gas set, 5mm wetsuit, standard DIR Rig - no weight needed to hold a 30bar 3M stop (I'm lucky I am a sinky person - I know some people need a few KG of lead either as a v-weight or on the belt.)

Next scenario
Add 1*Al80 stage and Al40 deco bottles (1*50%, 1*Oxygen)

OK - first we'll deal with the Al40s - even empty of gas, with a stage set (reg, clips/straps and gauge) on them they are pretty much neutral in sea water - no extra weight needed.
Now the stage - the Al80 with Mix (say 21/35) and a stage set when empty is about 3Kg negative when full and 1 - 1.5kg positive when absolutely empty. (I know someone with the exact gas densities can calculate it, but with say a DS4 and ATX50 as the stage reg this is close enough) with 60 bar in them they are pretty much neutral - again exact calcs may say a little either side.

So

if you are planning to leave 3rds in the stage - no buoyancy issue
if the stage is "all useable" you may have a slight buoyancy issue to manage, but it is not extreme. (If you are weighted for empty wing neutral at 30 bar you will be slightly negative at 70 bar thirds.)

In practice, if I have a couple of stages that I plan to breath to empty I usually add a couple of KG to my weighting in a non-ditchable form like a v-weight although a belt with the weight positioned near the left hip D-Ring also works well by positioning the weight near the point of lift from the empty stage.

Troubleshooting
If you can't maintain the stop there are a couple of ways of dealing with it in the last resort.

1) Clip the stage to the DSMB line and let it go - it will be under the balloon when you get there..
2) Just drop lose the stage - better lost gear than bent.. who knows you may be able to find it later

Many others will have opinions, and this is just one way of dealing with it. Hope that helps..

January 12th, 2008, 02:04 PM	#1 (permalink)
Leg of Salmon(Offline) New Member Join Date: Dec 2007 Location: Belfast Posts: 139	Weighting 15l + Ali80 Hi I couldnt find a definitive answer so here goes: If a diver, properly weighted for a 15l steel cylinder, adds a side slung Al80 to use when the wotsit hits the fan, does he need to make any changes to the weight carried? As I understand it the Al80 is neutral, more or less, at 150 - 160 bar. So if the diver starts to breath this it will become buoyant (about 2.5kg when empty). Does this mean the diver should add 2.5kg? Cheers Chris Last edited by Leg of Salmon; January 12th, 2008 at 03:21 PM.

January 12th, 2008, 02:10 PM	#2 (permalink)
keri.lewis (Online) Back in the water.. Join Date: Nov 2007 Location: Hampshire Posts: 386	Wouldn't it be better to breath the Al80 first - stage style ? That way you have a nice ful 15l steel at the end of the dive to offset the buoyancy changes. I'm assuming that you are not proposing decompression dives here as we are effectively talking about independent double tanks on a DIR forum.. __________________ Even a "bad" day diving beats a day in the office... Keri The more I learn.. the more ignorant I realise I am..

January 12th, 2008, 02:32 PM	#4 (permalink)
keri.lewis (Online) Back in the water.. Join Date: Nov 2007 Location: Hampshire Posts: 386	Good... and I should think so too! Independent doubles... mutter mutter mutter More seriously - you're much better off with a twinset if you're going down that path, and they don't cost that much.. __________________ Even a "bad" day diving beats a day in the office... Keri The more I learn.. the more ignorant I realise I am..

January 12th, 2008, 03:25 PM	#6 (permalink)
Leg of Salmon(Offline) New Member Join Date: Dec 2007 Location: Belfast Posts: 139	I'm not trying to solve anything. I wasn't planning on doing this myself I was just reading around about gas management, equipment, weighting, and so on and wondered what the weight/buoyancy implications of adding aluminium stage cylinders. Go easy on me, I'm only new Last edited by Leg of Salmon; January 12th, 2008 at 03:29 PM.

January 12th, 2008, 04:04 PM	#10 (permalink)
keri.lewis (Online) Back in the water.. Join Date: Nov 2007 Location: Hampshire Posts: 386	As an example... Wetsuit - warm water but still relevant to sorting out the set-up before trying it out safely in shallow water and then diving for real. Start Scenario Twin Al80 back gas set, 5mm wetsuit, standard DIR Rig - no weight needed to hold a 30bar 3M stop (I'm lucky I am a sinky person - I know some people need a few KG of lead either as a v-weight or on the belt.) Next scenario Add 1Al80 stage and Al40 deco bottles (150%, 1Oxygen) OK - first we'll deal with the Al40s - even empty of gas, with a stage set (reg, clips/straps and gauge) on them they are pretty much neutral in sea water - no extra weight needed. Now the stage - the Al80 with Mix (say 21/35) and a stage set when empty is about 3Kg negative when full and 1 - 1.5kg positive when absolutely empty. (I know someone with the exact gas densities can calculate it, but with say a DS4 and ATX50 as the stage reg this is close enough) with 60 bar in them they are pretty much neutral - again exact calcs may say a little either side. So if you are planning to leave 3rds in the stage - no buoyancy issue if the stage is "all useable" you may have a slight buoyancy issue to manage, but it is not extreme. (If you are weighted for empty wing neutral at 30 bar you will be slightly negative at 70 bar thirds.) In practice, if I have a couple of stages that I plan to breath to empty I usually add a couple of KG to my weighting in a non-ditchable form like a v-weight although a belt with the weight positioned near the left hip D-Ring also works well by positioning the weight near the point of lift from the empty stage. Troubleshooting* If you can't maintain the stop there are a couple of ways of dealing with it in the last resort. 1) Clip the stage to the DSMB line and let it go - it will be under the balloon when you get there.. 2) Just drop lose the stage - better lost gear than bent.. who knows you may be able to find it later Many others will have opinions, and this is just one way of dealing with it. Hope that helps.. __________________ Even a "bad" day diving beats a day in the office... Keri The more I learn.. the more ignorant I realise I am..

January 12th, 2008, 02:23 PM	#3 (permalink)
Leg of Salmon(Offline) New Member Join Date: Dec 2007 Location: Belfast Posts: 139	Quote: (Originally Posted by keri.lewis) Wouldn't it be better to breath the Al80 first - stage style ? That way you have a nice ful 15l steel at the end of the dive to offset the buoyancy changes. That sounds like a good idea. Quote: (Originally Posted by keri.lewis) I'm assuming that you are not proposing decompression dives here as we are effectively talking about independent double tanks on a DIR forum.. Was hoping nobody would notice I wasn't sure where to post but wanted to know how people here would do it if they did it... Thanks

January 12th, 2008, 03:14 PM	#5 (permalink)
neilh(Offline) Haemoglobin on the bus... Join Date: Nov 2005 Location: Barnsley, UK Posts: 1,900	Quote: (Originally Posted by Leg of Salmon) If a diver, properly weighted for a 15l steel cylinder, adds a side slung Ali80 to use when the wotsit hits the fan, does he need to make any changes to the weight carried? Turning this question on it's head a bit - what are you trying to solve by doing this? If you have a catastrophic gas failure, then you have at least one team mate with you with enough gas to get you both out (that's your minimum gas planning). If you're diving in a 3 then there's two people with enough gas to get you out. You would have to lose all your team (oops!) and have a complete gas failure (unlucky) before you would need to worry... If you're concerned about any kind of cylinder/reg failure taking out your one and only source of breathing and inflation gas then perhaps manifolded matched doubles would be a better option than a 15L and an 11L (ali 80)

January 12th, 2008, 03:34 PM	#7 (permalink)
neilh(Offline) Haemoglobin on the bus... Join Date: Nov 2005 Location: Barnsley, UK Posts: 1,900	Quote: (Originally Posted by Leg of Salmon) Go easy on me, I'm only new Hehe - no problem, was just trying to point out that you can think about it another way Quote: (Originally Posted by Leg of Salmon) I'm not trying to solve anything. I wasn't planning on doing this myself I was just reading around about gas management, equipment, weighting, and so on and wondered what the weight/buoyancy implications of adding aluminium stage cylinders. Fair enough, you would work out your weighting without them

January 12th, 2008, 03:42 PM	#8 (permalink)
Leg of Salmon(Offline) New Member Join Date: Dec 2007 Location: Belfast Posts: 139	Quote: (Originally Posted by neilh) you would work out your weighting without them In my example, I understand that you could breath the stage first, so you still have back gas and this sorts out the buoyancy. But what happens in other situations? If you are breathing an aluminium stage bottle for decompression, or on the bottom, for example it will become more buoyant so how do you combat this. I am possibly being a bit dumb about this.

January 12th, 2008, 04:02 PM	#9 (permalink)
neilh(Offline) Haemoglobin on the bus... Join Date: Nov 2005 Location: Barnsley, UK Posts: 1,900	Quote: (Originally Posted by Leg of Salmon) But what happens in other situations? If you are breathing an aluminium stage bottle for decompression, or on the bottom, for example it will become more buoyant so how do you combat this. I am possibly being a bit dumb about this. Well using bottom stages is beyond my training so I can't comment on that, but as for backgas you plan a reserve for your deco gas. So yes the deco bottle is going to get more buoyant, but if all goes to plan it's not going to be unmanageable. Also you've still planned your backgas based on minimum gas, but your weighting based on almost empty backgas cylinders - so you have a bit of weight left there as well. In the worse case scenario where you've lost all your backgas and you've just used all your deco gas then you can always clip the deco bottle to the SMB line and let it head off to the surface while you go onto someone's long hose.