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Discussion Starter · #1 ·
Had a nice solo bimble in about 8m on holiday in Mallorca,
about half way through the dive I heard a strange gurgling sound as I added a little air to my bc, but as I was not using much air in my bc anyway, (Alu hire tank left me a little underweighted- ) It carried on regardless.
Only on exiting did I notice my bc was full of water. this being due to my inflator tube splitting (not the hose) as it bends over.
Whilst this caused no problems on a light bimble like this- other than exiting with half the med in my bc. It could have been disasterous at depth, if the bc wouldn't fill with air to get me off the bottom. Particularly if solo and no buddy to help diagnose the problem or lift with a cbl.
Moral?
I guess: Check bc for wear and tear, there was probably a small hole that I could have seen before the dive.
Check everything when solo/independent diving as the consequences of failure of kit are severe.
Do a shallow check out dive at the start of every season.
Anything else that could be learnt?

Stuart
 

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Hi Stuart,

I was thinking about what else you could do and it often occurs to me that a dying BC is a real risk as we don't carry any real bouyancy redundancy when in warm water (and therefore no Drysuit). I am lucky in that I am a reasonably well insulated chap, and am therefore naturally bouyant, and so ditching weights would probably be plenty to get me off the bottom even at depth, there is also the dsmb, assuming you have one.

I also dive solo every once in a while so these things do cross my mind.

Worth bearing in mind, thanks for the post.

Andy
 

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Discussion Starter · #4 ·
I guess it does beg the question what if it had happened solo at 45m...although personally I haven't been solo much past 25m.
With a dry suit; bouyancy could be improved by adding more air to it, though it still won't get you off the bottom without ditching weights. A fine idea until you start accelerating on the way up like a Polaris missile. Though at least you have the option to dump some air unlike a wetsuit.
I'm not familiar with twin bag wing bc's I assume they have independent bags and surely not independent feeds? If not the hole was in my bc inflator hose at the top where it bends and would still dump all the air, particularly if sitting upright on the bottom trying to work out the problem.
I guess if the bc is completely stuffed and for whatever reason it cannot be inflated I guess you have inflate your DSMB/ hopefully DSMB's on a short leash and if necessary remove your weights one by one until you can push off the bottom and swim downwards once you loose control on the ascent, breath out and hope for the best. Quite a lot of task loading.
I for one will now test it throughly every dive.
Stuartr
 

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I guess it does beg the question what if it had happened solo at 45m...although personally I haven't been solo much past 25m.
With a dry suit; bouyancy could be improved by adding more air to it, though it still won't get you off the bottom without ditching weights. A fine idea until you start accelerating on the way up like a Polaris missile. Though at least you have the option to dump some air unlike a wetsuit.
I suppose that depends on how much weight you have on you and when you notice the total failure of the BC.

If you dive correctly weighted for a twinset of 12's without stages, the most you are going to be overweighted is about 5kg (the weight of the gas in the tins). If you are diving, this would be reduced to around 2.5-3kg. If you are diving multiple stages, then you are going to be heavier, but you can always ditch the stages to get you back to close to neutral.

I know that it won't take much air in my drysuit to get me moving (slowly) and then after it can be controlled with breathing and suit dumping.

The lesson here though is make sure you check all your critical equipment on every dive. If you don't, you might have one of those (final) "I wish I had done that check, how daft was I" moments at the bottom of the sea. Just hope it never happens.

Thanks for posting Stuart.

Regards
 

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I guess it does beg the question what if it had happened solo at 45m...although personally I haven't been solo much past 25m.
With a dry suit; bouyancy could be improved by adding more air to it, though it still won't get you off the bottom without ditching weights. A fine idea until you start accelerating on the way up like a Polaris missile. Though at least you have the option to dump some air unlike a wetsuit.
I'm not familiar with twin bag wing bc's I assume they have independent bags and surely not independent feeds? If not the hole was in my bc inflator hose at the top where it bends and would still dump all the air, particularly if sitting upright on the bottom trying to work out the problem.
I guess if the bc is completely stuffed and for whatever reason it cannot be inflated I guess you have inflate your DSMB/ hopefully DSMB's on a short leash and if necessary remove your weights one by one until you can push off the bottom and swim downwards once you loose control on the ascent, breath out and hope for the best. Quite a lot of task loading.
I for one will now test it throughly every dive.
Stuartr

Surely, if you are correctly weighted, then you can just swim up irrespective of depth ?
I dived with a guy in Fiji whose corrugated hose came away completely from his, (rented), BC. He just shrugged and carried on with his dive.
I had the hose come off my wing once, but fortunately it was on a shallow bimble and i used my D/S for bouyancy.
 

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Discussion Starter · #7 ·
Surely, if you are correctly weighted, then you can just swim up irrespective of depth ?
I dived with a guy in Fiji whose corrugated hose came away completely from his, (rented), BC. He just shrugged and carried on with his dive.
I had the hose come off my wing once, but fortunately it was on a shallow bimble and i used my D/S for bouyancy.
You need to be correctly weighted to hold a stop in the eventuality that you have to breath all your gas down, so if you are barely neutral on entry you are going to be stuffed when empty after an emergency with you trying to get as much deco as possible to avoid a bend. Therefore you ought to be carrying a little more to offset this IMHO one day you might need it. I used to dive with 12lb in my wetty and a 12 l but now I carry more for that reason.
The other problem is you may also elect to be a bit overwieght so you can put some more air in your suit to keep warm and also even if you are neutral on entry you could still find yourself negative on the bottom because of the compression of the gas in your suit (more so if its uncompressed neprene) and also your lungs.
So I'm really not sure how easy it would be to get off the bottom in those circumstances, perhaps it would be worth trying out though as an exercise.

Does this make sense?
Stuartr
 

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You need to be correctly weighted to hold a stop in the eventuality that you have to breath all your gas down, so if you are barely neutral on entry you are going to be stuffed when empty after an emergency with you trying to get as much deco as possible to avoid a bend. Therefore you ought to be carrying a little more to offset this IMHO one day you might need it. I used to dive with 12lb in my wetty and a 12 l but now I carry more for that reason.
The other problem is you may also elect to be a bit overwieght so you can put some more air in your suit to keep warm and also even if you are neutral on entry you could still find yourself negative on the bottom because of the compression of the gas in your suit (more so if its uncompressed neprene) and also your lungs.
So I'm really not sure how easy it would be to get off the bottom in those circumstances, perhaps it would be worth trying out though as an exercise.

Does this make sense?
Stuartr
Correctly weighted means just that 'at the end of the dive'.

This is what I described in the above post. If you are correctly weighted at the end of the dive, the difference between the weight at the start of the dive and the end is the gas you have used - that is all. If you use a single 12L then the gas weighs around 2.5kg, if you are using twin 12s then you have around 5kg of gas. If you have deco cylinders these can be ditched in the event of an absolute emergency. A lot of weight checks I see conducted are done at the start of the dive, if this is the case, then approximately 2kg must be added to take into account the gas used on the dive.

You shouldn't be adding extra gas in your suit to keep warm whilst on deco, because unless you have loads of gas, all it does is migrate to the back of the drysuit and not keep your front warm. Adding extra weight (and therefore more gas) means that there will be more gas to expand and this makes controlled ascents far more difficult. Your weight check should be conducted with the same amount of gas in your suit as you would use on deco.

Redundant bouyancy when diving with a wetsuit could be as simple as a SMB which will allow the diver to 'pull' themselves to the surface. By this I mean, launch the SMB and then use that to go up, not to launch the bag and hold on to it :)

However, the biggest learning point from this is make sure that your kit is serviceable before you jump in.

HTH
 

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surely if you are diving in a neoprene wetsuit the compression at depth would make you even more overweighted,

Or am i been daft :)
Nope you correct, you would be more 'overweighted' at depth, one of the reasons for taking redundant bouyancy on deep wetsuit dives.
 

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Discussion Starter · #11 ·
Nope you correct, you would be more 'overweighted' at depth, one of the reasons for taking redundant bouyancy on deep wetsuit dives.
I wonder how much the increased density of the air in you lungs effects bouyancy too. I reckon if deep enough even a fully functional bc would struggle to lift due to the increased density of the air in it. I don't know at what depth that could start to kick in though..

I never really thought of a DSMB as being a redundant bouyancy device. Which I guess is the point of this thread.
Another good reason for carrying one.
Though I shall be more thorough in checking my bc in future also. A good idea would be to put air in the bc and squeeze it with your arms and check for leaks perhaps...
Stuart
 

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I think you would have to be f**king deep to get that sort of issue occuring bearing in mind that guys in Wakulla are regularily down in the region of 100m with lots of stages and reels (scooters should be neutral).

Regards
 

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Discussion Starter · #13 ·
I think you would have to be f**king deep to get that sort of issue occuring bearing in mind that guys in Wakulla are regularily down in the region of 100m with lots of stages and reels (scooters should be neutral).

Regards
You're probably right, but there again they probably have enormous wings with loads of lift too.
Actually thinking of the Physics.......
The upthrust is equal to the density of the fluid displaced so therefore wouldn't change, but the density of the air would increase say 7 times at 60m which if you had say 20 litres would be equivalent to an extra 0.18 kg (1.29x7x0.02), less actually as the air is not behaving as an ideal gas...
So I guess I'm talking [email protected] :)
I agree if you were deep enough for it to make a major difference you would probably not be in a state for it to matter anymore.
 

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Some assumptions:
A BC gives 18kg of lift on average (ref)
1 cubic foot of air displaces 29kg of water
-> Avg BC holds about 0.62 cubic feet of air

Air density doubles approx every 10 meters

A cubic foot of air weighs around 37.65 grams on the surface

-> For a BC to stop working, the air inside it would have to weigh 18kg or more when full
On the surface, that would be around 478 cubic feet of air

-> My maths says that at around 95 meters your 18kg BC would not generate any lift even when full.

Does that sound right?
 

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However you would have other issues because at 95m you are likely to be carrying far more cylinders than a single BC could cope with on the surface and so you would use something like a 55 or 60lb wing.
 

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Air density doubles approx every 10 meters


Does that sound right?
No it doesn't, the increase in air density is linear not exponential. It only doubles in density over the first 10 meters, thereafter the increases in density become smaller.
 

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Discussion Starter · #18 ·
No it doesn't, the increase in air density is linear not exponential. It only doubles in density over the first 10 meters, thereafter the increases in density become smaller.
Physics geeks of the world unite.:)

Agreed.... at 20m it would be at 3 atmospheres and at 1/3rd its volume, at 30m 1/4 this is linear (density proportional to Pressure )up to when it starts to get so compressed it stops behaving like an "ideal gas."
 

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Discussion Starter · #19 ·
Some assumptions:
A BC gives 18kg of lift on average (ref)
1 cubic foot of air displaces 29kg of water
-> Avg BC holds about 0.62 cubic feet of air

Air density doubles approx every 10 meters

A cubic foot of air weighs around 37.65 grams on the surface

-> For a BC to stop working, the air inside it would have to weigh 18kg or more when full
On the surface, that would be around 478 cubic feet of air

-> My maths says that at around 95 meters your 18kg BC would not generate any lift even when full.

Does that sound right?
Eugh! Kg and cubic feet, messy units.
To compress 478 cubic feet into 0.62 cubic feet you would have to be at 771 atmospheres.. about 7.7km deep....Now you are definitely in the sh1t if diving a single 12litre and you should consider a shallower wreck for your second dive.
 
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