50 lb lift bag

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I have and have used open bags/SMBs, semi-closed bags/SMBs and closed bags/SMBs over the course of 25 years of technical diving, and my general preference for most cases is a semi closed bag.

But there are pros and cons.

I have an 85 pound closed circuit Halcyon SMB and it's nice on a deep dive in south Florida where you might shoot the bag coming off a 200' wreck to do your drift deco. At that depth you don't have to put much gas in it to get it fully inflated at the surface, and a long and comparatively skinny SMB goes up a little smoother than a lift bag.

A positive is that any inflator hose can use used to inflate the bag as the it has a QD fitting, but without the race that allows the ball to lock onto the fitting. So in effect, you just press the inflator hose against the fitting and hold it there long enough for the SMB to inflate as much as needed. From 200 ft, you only need to put about 12-14 pounds of gas in the SMB at depth to get full inflation at the surface.

Another positive is that the SMB is fairly low drag when standing upright on the surface incurrent, so it can be smaller and not get carried way down current before it finally has enough lift to stay on the surface.

The downside is that you need to keep line tension on a large and fully inflated SMB to keep it standing upright on the surface, and if you are close to neutral at the end of the dive, that's hard to do. You'll want to weight accordingly.

At the other extreme, an open ended bag will be wider at the top than at the bottom, so they hunt a little from side to side going up, as the draggiest area of the bag is the top. In some cases if you don't maintain some tension on the line while you're shooting it, it can invert and dump some or all of the gas. Once on the surface, if there isn't tension on the line it can lay down on it's side and in some cases lose gas.

An open ended SMB generally launches better, but has is even more likely to lose lift if it lays down on the surface. On the other hand, a small skinny SMB is fairly low drag in current so you can get by with a smaller SMB, and that makes launching from shallower depths much easier.

The sweet spot is the semi closed design, in either bag or SMB. Some models allow oral inflation on the surface, and at depth you can easily will the bag with a regulator orally or with the wing inflator. The SMB or bag won't dump on the way up if it inverts and it won't dump on the surface if it lays down.

My preference is to dump gas from the wing, into the semi-closed SMB or bag, so that my change in total lift is zero. Exhaling let's you put another 5-6 pounds of gas in the bag or SMB as well while still being neutral. Once the bag is released, I inhale and get on the power inflator to refill the wing, while the bag or SMB is going up, and net change in depth is maybe a foot. The caution here is the need for a clean configuration and good line and bag management skills so that nothing gets tangled, but if that does happen you are still effectively neutral so you aren't going up with the bag.

Another thing you can do with a semi-closed bag with a dump valve on top is clip it to your scooter D-ring and use it as an alternate lift device in the event of a wing failure. You fill the bag from the bottom with a reg, or via a QD fitting, and then use the dump valve on the top to control ascent rate and get neutral when you reach a stop depth. If you have to swim horizontally, you can place the bag underneath your torso once you're neutral.
 
My preference is to dump gas from the wing, into the semi-closed SMB or bag, so that my change in total lift is zero. Exhaling let's you put another 5-6 pounds of gas in the bag or SMB as well while still being neutral. Once the bag is released, I inhale and get on the power inflator to refill the wing, while the bag or SMB is going up, and net change in depth is maybe a foot. The caution here is the need for a clean configuration and good line and bag management skills so that nothing gets tangled, but if that does happen you are still effectively neutral so you aren't going up with the bag.
My head is spinning. Seriously, I‘ve always believed “there‘s more than one way to skin a cat“, and so I‘m at the moment filling my bathtub .... and ... looking for a cat.

I prefer the closed circuit bags (especially down the road for RB's). IMO, Halcyon's are a little more expensive, but they are the best (design, quality, construction, materials used, easy of use) CC Lift Bag and CC SMB on the market.
I dig the Halcyon closed bags, too. Use the orangle 40 lbs. and yellow 6 lbs. SMB, and waiting for the 85 lbs. lift bag on order.

I was taught to position the SMB under the body in the event of a wing failure. Orally inflate and dump as needed. This technique frees the diver‘s hands to execute tasks to finish the dive safely ie. gas switches, tank rotations.
 
I have and have used open bags/SMBs, semi-closed bags/SMBs and closed bags/SMBs over the course of 25 years of technical diving, and my general preference for most cases is a semi closed bag.

But there are pros and cons.

I have an 85 pound closed circuit Halcyon SMB and it's nice on a deep dive in south Florida where you might shoot the bag coming off a 200' wreck to do your drift deco. At that depth you don't have to put much gas in it to get it fully inflated at the surface, and a long and comparatively skinny SMB goes up a little smoother than a lift bag.

A positive is that any inflator hose can use used to inflate the bag as the it has a QD fitting, but without the race that allows the ball to lock onto the fitting. So in effect, you just press the inflator hose against the fitting and hold it there long enough for the SMB to inflate as much as needed. From 200 ft, you only need to put about 12-14 pounds of gas in the SMB at depth to get full inflation at the surface.

Another positive is that the SMB is fairly low drag when standing upright on the surface incurrent, so it can be smaller and not get carried way down current before it finally has enough lift to stay on the surface.

The downside is that you need to keep line tension on a large and fully inflated SMB to keep it standing upright on the surface, and if you are close to neutral at the end of the dive, that's hard to do. You'll want to weight accordingly.

At the other extreme, an open ended bag will be wider at the top than at the bottom, so they hunt a little from side to side going up, as the draggiest area of the bag is the top. In some cases if you don't maintain some tension on the line while you're shooting it, it can invert and dump some or all of the gas. Once on the surface, if there isn't tension on the line it can lay down on it's side and in some cases lose gas.

An open ended SMB generally launches better, but has is even more likely to lose lift if it lays down on the surface. On the other hand, a small skinny SMB is fairly low drag in current so you can get by with a smaller SMB, and that makes launching from shallower depths much easier.

The sweet spot is the semi closed design, in either bag or SMB. Some models allow oral inflation on the surface, and at depth you can easily will the bag with a regulator orally or with the wing inflator. The SMB or bag won't dump on the way up if it inverts and it won't dump on the surface if it lays down.

My preference is to dump gas from the wing, into the semi-closed SMB or bag, so that my change in total lift is zero. Exhaling let's you put another 5-6 pounds of gas in the bag or SMB as well while still being neutral. Once the bag is released, I inhale and get on the power inflator to refill the wing, while the bag or SMB is going up, and net change in depth is maybe a foot. The caution here is the need for a clean configuration and good line and bag management skills so that nothing gets tangled, but if that does happen you are still effectively neutral so you aren't going up with the bag.

Another thing you can do with a semi-closed bag with a dump valve on top is clip it to your scooter D-ring and use it as an alternate lift device in the event of a wing failure. You fill the bag from the bottom with a reg, or via a QD fitting, and then use the dump valve on the top to control ascent rate and get neutral when you reach a stop depth. If you have to swim horizontally, you can place the bag underneath your torso once you're neutral.

What is your preferred semi-closed bag?
 

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