Lift Bag Size

[markmantei]

Thanks for all the replies!

To clarify, I did TEC40 with a 3.3' Halcyon DSMB, which has only ~6lb of lift. My guess is that for the vast majority of my diving (drysuit) I will carry either that and/or my 6ft SMB. That said, my instructor would prefer further courses to be done with a lift bag with more significantly more lift than the SMB provides.

The bit about the backup buoyancy is related ONLY to diving in warmer climates where I may not be travelling with or using my drysuit. Without ditch-able weight, and sidemounting steel tanks I consider backup buoyancy to be a necessity in the unlikely case of a wing failure. If I wind up doing enough dives to justify a different wing with redundant bladders, I'll go that route, but for ~10 dives/year I have way more important gear to purchase.

---------- Post Merged at 04:42 PM ---------- Previous Post was at 04:30 PM ----------

What events specifically do you expect to occur that would require almost 40 kilos of lift?

I have never needed backup buoyancy of any type... my experience may reflect good luck... or pre-dive kit inspection... but you should not need more that a couple of kilos if that... if you do, you are overweighted.

Agreed regarding the 40kg of lift. I couldn't think of any reasons either, hence why I was leaning towards the 50lb bag.

I'm not overweighted, in cold water with a drysuit (carry 7lb non-ditchable on the BCD, plus 2 HP 130's). Warm water with a 3mm and I'll probably be overweighted with just 2 steel tanks.

 

[decompression]

Thanks for the info, fills in some gaps.

First, the SMB..
As you know the SMB is to provide a reference for accent during deco (if no up line), an easily seen surface marker and an alternate source of bouyancy In case the Flames make the play-offs! The 3.3 Halcyon is good for practice and use in ideal conditions, a larger SMB is useful for deco, large surface recognition, a lift bag for cool stuff and as a alt bouyancy device ( also a sword for my son!). The 25-50 lb SMB's fit that use (I have a great OMS just for that). However, if you dive a balanced rig your requirement for a second or third source of buoyancy that has 50 lbs of lift is way overkill. There maybe a very logical reason your instructor wishes you to carry a 50 lb lift bag, ask him the question and it may put an end to this. However, there are just so many reasons not to "carry" a 50 lb lift bag on a dive that doesn't specifically require one.

Warmer water diving...
if you are traveling to sunny climes and using a wetsuit (3mil?) the odds that you will use steel tanks and specifically 130's is unlikely. In cave country, Florida and Mexico, AL80s are the standard just for that reason (open water as well as caves). This is where the balanced rig comes in, in a catastrophic wing failure your weighting should allow you to continue your dives (including deco stops), no requirement for a large lift bag/SMB.


So, your instructor should be able to fully explain a balanced rig to you as well as the reasoning for a lift bag. That should narrow down your decision and choices for making your purchase and hopefully save you some money. Safe diving!

 

[GrimSleeper]

Diving wet with Al80 twins and a couple of Al80 slings, I do use a liftbag for redundant buoyancy. As Doppler and decompression have said, if your rig is anywhere close to balanced, you need very little air in a liftbag (or DSMB) to achieve neutral buoyancy and make a comfortable staged ascent. When I practise 'failed BCD' ascents, I never need much more air in the bag than you could contain by loosely cupping your hands, if that makes sense. Unless every tank is completely full, I don't really need the liftbag at all, frankly - lung volume does the job.

If your rig is so heavy that you need 100lbs of lift , the odds of successfully deploying a liftbag and making an ascent in the event of a BCD failure become vanishingly small, unless you've got a hard bottom to sit on while you do it.

 

[flots am]

here is one example, not sure why you would need a 100lb lift bag to stand up on the surface, and not sure why you would need to make your SMB 33lb buoyant at 99'

If you're questioning the reasoning for having your bag visible on the surface, it's in case you get blown off the dive site and need to let the boat know where you are so they can pick you up when you surface. The reason for a big bag is so it would be visible in heavy seas and the reason for filling it up is because only the part that stands above the waves is visible to the boat.

here is one example, not sure why you would need a 100lb lift bag to stand up on the surface, and not sure why you would need to make your SMB 33lb buoyant at 99'

If you're questining the numbers, it's bad math. 99' requires filling the bag 25% full to make it full on the surface. In any case, the results are the same. The very big SMBs are extremely difficult to properly fill underwater at reasonable depths.

flots.

 

[syntaxerrorsix]

We shot 50# lift bags in training for use as up lines when we drifted deco. I now use a 6' DR SMB for the same thing because it's much easier for the boat to track me and takes up roughly the same amount of room.

A 100 pound lift bag is huge comparatively and most certainly overkill in my opinion.

 

[nimoh]

If you're questioning the reasoning for having your bag visible on the surface, it's in case you get blown off the dive site and need to let the boat know where you are so they can pick you up when you surface. The reason for a big bag is so it would be visible in heavy seas and the reason for filling it up is because only the part that stands above the waves is visible to the boat.

You should use a DSMB for this, you don't need a bigger bag, you need a taller bag.

 

[markmantei]

Thanks for the replies, which have basically validated going with the 50lb bag. Sometimes its like buying a car, you can't have too much horsepower, therfore 50lb lift is good then 100 must better. I couldn't think of a reason for needing more lift and it sounds like the consensus is that most don't carry a large bag unless they specifically feel they'll need that much lift.

If your rig is so heavy that you need 100lbs of lift , the odds of successfully deploying a liftbag and making an ascent in the event of a BCD failure become vanishingly small, unless you've got a hard bottom to sit on while you do it.

If someone requires 100lbs of lift for just their rig, finding a wing large enough will be their biggest challenge

 

[flots am]

You should use a DSMB for this, you don't need a bigger bag, you need a taller bag.

I call my SMB a lift bag. Carter calls them "Personal Floats".

They range from 6' - 10' tall and 4" - 5" in diameter.

flots.

 

[nimoh]

I call my SMB a lift bag. Carter calls them "Personal Floats".

They range from 6' - 10' tall and 4" - 5" in diameter.

flots.

Even though I don't use that terminology, I can't come up with a reason as to why that would be wrong, so I guess the confusion I was attempting to clear up was my own

 

[decompression]

Gotcha, nimoh...

Very important.....lift bags and SMBs are very different..........