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This may be obvious to some and not very interesting to others, but the buoyancy changes of a twinset throughout a dive are enormous. According to my calculations derived from UK Scuba - Cylinder Buoyancy Calculation, the buoyancy change of a standard 12L 232 bar twinset from 232 to 50 bar is over 5 kg and from 232 to 0 bar is 6.6 kg!

I’m very particular about my kit and it how it feels in the water. It’s unavoidable, but 5 – 6+ kg of extra weight at the beginning of a dive seems like an awful lot to me. In warm water gear, I can tell if I’m 1 -2 kg overweighed (honestly!), so this may be one of the reasons that new twinset divers, me included have had a little trouble getting that feel good feeling in the water.

Any thoughts?

Andrew
 

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This may be obvious to some and not very interesting to others, but the buoyancy changes of a twinset throughout a dive are enormous. According to my calculations derived from UK Scuba - Cylinder Buoyancy Calculation, the buoyancy change of a standard 12L 232 bar twinset from 232 to 50 bar is over 5 kg and from 232 to 0 bar is 6.6 kg!

I’m very particular about my kit and it how it feels in the water. It’s unavoidable, but 5 – 6+ kg of extra weight at the beginning of a dive seems like an awful lot to me. In warm water gear, I can tell if I’m 1 -2 kg overweighed (honestly!), so this may be one of the reasons that new twinset divers, me included have had a little trouble getting that feel good feeling in the water.

Any thoughts?

Andrew


To be honest - if you are new to a twinset - then you probably won't see that range of difference in weight loss - due to gas breathed - purely because that is an awful lot of gas - the dive would need to merit that. On most recreational dives (standard nitrox mixes to say 30-35 metres) then you will probably do two dives with it - so it is not uncommon for divers to add a couple of kilos for the second dive.

Yes - you are right though - that is a lot of difference from full until they are dry. Not much you can do about it though - thats physics.. just get weighted with your dive gear and the worst case - say 20 bar and go from there.
 

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This may be obvious to some and not very interesting to others, but the buoyancy changes of a twinset throughout a dive are enormous. According to my calculations derived from UK Scuba - Cylinder Buoyancy Calculation, the buoyancy change of a standard 12L 232 bar twinset from 232 to 50 bar is over 5 kg and from 232 to 0 bar is 6.6 kg!

I’m very particular about my kit and it how it feels in the water. It’s unavoidable, but 5 – 6+ kg of extra weight at the beginning of a dive seems like an awful lot to me. In warm water gear, I can tell if I’m 1 -2 kg overweighed (honestly!), so this may be one of the reasons that new twinset divers, me included have had a little trouble getting that feel good feeling in the water.

Any thoughts?

Andrew
Yep.

When you dive a standard 12lt you tend to exit the water on 50 bar.
This works fine for most dives, but the main reason for diving twins is to
go deeper or longer. That means our deco penalty will be greater and that
reserve needs to do ours and our buddies. So let's say that we now have
a third as a reserve.

So even with worse case at 6kg (i'd say thats way over, 4kg more like),
You would have a 4kg difference.

Hang on though, dont we have 2kg difference at least on a single and
manage without probs? Of course we do, so the actual difference with
a twinset is the difference between a single (2kg and say 50 bar reserve)
vs a twin with that additional gas as part of rule of thirds.

So in practice the actual difference could be as little a 1.5kg between the
two. That's one lump on the belt.
 

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Yeah but Andy your best to calc your weight with 30 bar in the tins, and see what the minimum amount of lead needed is. Then add 1 maybe 2kg for the UK sea. You can tell instantly at 3m if you cannot hold a stop, because if you stop finning you will sink if over weighted.
 

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so this may be one of the reasons that new twinset divers, me included have had a little trouble getting that feel good feeling in the water.

Any thoughts?

Andrew
I think it's a fairly sweeping generalisation. I know lots of divers who transitioned from singles to twins with no problems at all, and even commented on how much more stable they felt, especially those transfering from standard BCD to a wing.

Myself I took 2 dives and then it clicked and felt fantastic but then I was transitioning from a 15L plus 3L pony clamped to the side (which I'd been compensating for for ages) up to the twins.

I think any new configuration will take some getting used to but I don't think that the bouyancy is the major issue, it's more the complete transition over to a new config that almost everyone has to go through.
 

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The smell of freshly turned delrin is more powerfu
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In warm water gear, I can tell if I’m 1 -2 kg overweighed (honestly!), so this may be one of the reasons that new twinset divers, me included have had a little trouble getting that feel good feeling in the water.

Any thoughts?

Andrew
most fit divers can dive with very little weight in thin wetsuit and an ally tank.
overweighted or not ? this can only be proved at the end of a dive when you can hold a stop or not. I have had long discussions about this, how much gas should be left in cylinder ect.

my problem is if you think 50 bar is a safe reserve to surface with you need to be able to use this gas. To use it you need to be able to hold a stop with your kit all of it empty and bouyant.

if you weight your self so your are neautral with 50 bar left you can never use this gas its just not there because..... so for all you normal dives everything is fine and dandy when that last reseve is needed you cant use it to stay underwater because you float :)

so your correctly weighted when you can hold a stop :)

David
 

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Discussion Starter · #8 ·
if you weight your self so your are neautral with 50 bar left you can never use this gas its just not there because..... so for all you normal dives everything is fine and dandy when that last reseve is needed you cant use it to stay underwater because you float :)

so your correctly weighted when you can hold a stop :)

David

very good point!
 

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Nigel Hewitt
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I’m very particular about my kit and it how it feels in the water. It’s unavoidable, but 5 – 6+ kg of extra weight at the beginning of a dive seems like an awful lot to me. In warm water gear, I can tell if I’m 1 -2 kg overweighed (honestly!), so this may be one of the reasons that new twinset divers, me included have had a little trouble getting that feel good feeling in the water.
This is physics.

The biggest snag is new twinset divers who start taking off the weight to get comfortable simultaneously with increasing their bottom time and hence their deco commitment.

Finally one day they can't hold the last stop because they are too light and they have a trip home with Coastguard Airways.

If you are bringing one third back but not carrying the weight to be able to breathe that gas (or have your buddy breathe it) it isn't a reserve. It's a passenger. You might as just only part fill the cylinders as it's never going to do anything for you.
 

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Forget all of that 50 bar nonsense.

If you have the gas in the tank you need to be able to breathe it in an emergency. So do a weight check with almost nothing in the cylinders. Weight yourself correctly for this - ie be able to hold a stop at 6m. And then go diving a lot. You will get used to the buoyancy characteristics of the set over time. there's no getting around the fact that 5 and a half thousand litres of gas weighs quite a bit so you will get considerably lighter as you breathe the set down. Ce la vie.
 

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aka Chimp 1 or Mavis...
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....50 and a half thousand litres of gas weighs quite a bit so you will get considerably lighter as you breathe the set down....
F*ck me Garf, where did you get the bands for the 5 J's :D
 

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Ce la vie
Objet d'art laboratoire garnier Rodney

The reality of this is actually quite convenient. At the beginning of the dive you're several kilos overweighted which helps you to drop quickly down the shot onto the wreck and avoid wasting gas. At the end of the dive when you need to control your bouyancy the most for deco on the ascent - you're almost spot on. Many people are diving overweighted - play around with emptyish tanks and try and get it spot on - it makes a hell of a difference. HTH
 

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The smell of freshly turned delrin is more powerfu
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Objet d'art laboratoire garnier Rodney

The reality of this is actually quite convenient. At the beginning of the dive you're several kilos overweighted which helps you to drop quickly down the shot onto the wreck and avoid wasting gas. At the end of the dive when you need to control your bouyancy the most for deco on the ascent - you're almost spot on. Many people are diving overweighted - play around with emptyish tanks and try and get it spot on - it makes a hell of a difference. HTH
now im confused.... Howard is sort of making sence..12.45 i could be drunk...garf you been putting drugs in the coffee again !

David
 
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