Purpose of an SMB?

[TSandM]

Tortuga, I'm confused. 100 fsw is 4ATA, right? 30 fsw is 2 ATA, right? So the one liter you put in at 100 fsw becomes 2 liters at 30, because you have half the pressure, so twice the volume? I don't think I have this screwed up

So, if you want the SMB taut and full at the surface, you can fill it one quarter full at 100 fsw. If it holds, say, two liters of gas, you can put in 500 ccs at 100 fsw. That gives you the equivalent of 500 ccs of lift to manage. Obviously, the bag will expand on the way up, but at that point, you are allowing the line to unspool without hindrance, so it doesn't matter to your stability where you are.

On the other hand, if you want the bag completely full at the surface, you have to fill it half full at 30 fsw. You've now displaced twice as much water, which makes the bag, prior to you releasing it, twice as buoyant and more difficult to manage (especially since your own buoyancy is more delicate that shallow). Again, what happens after you let go of it is irrelevant.

It is easier to stay completely stable during a bag shoot, when you are new to it, if you do it deeper; on the other hand, it's much more dangerous to lose control of your buoyancy or get caught in the line if you shoot the bag deep when you aren't accustomed to doing it. For me, the bottom line is to put up with the instability in the shallows -- and having to deal with it will very soon teach you how to shoot a bag efficiently and quickly, so you don't get unstable.

 

[Crush]

dougchartier,

For the reasons mentioned by D_B and others, I find it easier to deploy a rescue sausage below 40 feet than at, say, at 20 feet. The deeper you are, the lesser the volume or air (at depth) you have to put into the SMB to have it fully inflated at the surface (the air expands as it rises). This means that you will have to deal with a smaller buoyancy change at depth (owing to the rescue sausage trying to pull you up as you hold onto it to fill it). Having said that, I am not advocating that you first try to deploy your rescue sausage at a challenging depth.

Tortuga68, TSandM,

Perhaps I was unclear in my wording. An example to illustrate: let's take a rescue sausage with 20 lbs of lift.

• Deploying said rescue sausage at approx. 30 feet will allow you to fill the rescue sausage to half its capacity at that depth. As it rises the air will expand. Thus, at the moment of release at 30 feet the half filled rescue sausage is buoyant by half of its capacity, or 10 lbs.
  
• Deploying said rescue sausage at approx. 60 feet will allow you to fill the rescue sausage to one-third of its capacity at that depth. At the moment of release at 60 feet the one-third filled rescue sausage is buoyant by one-third of its capacity, or about 6.7 lbs.
  
• Deploying said rescue sausage at approx. 90 feet will allow you to fill the rescue sausage to one-quarter of its capacity at that depth. At the moment of release at 90 feet the one-quarter filled rescue sausage is buoyant by one-quarter of its capacity, or about 5 lbs.

In all cases the rescue sausage will reach its full lift capacity of 20 lbs at the surface. This is not to say that you could not fill the rescue sausage to capacity at depth, and in such case, it's lift would be a constant 20 lbs as it ascends and vents.

 

[Crush]

mpetryk, I'm not sure what distinction you are making between an SMB and a rescue sausage. I haven't seen the latter term before. To me, it's a DSMB if it can be deployed at depth, and a safety sausage if it is designed to be inflated on the surface. A DSMB can be used for a variety of things, including towing as a dive marker (not sure if it's a legal dive "flag" everywhere or not) or deploying at the end of the dive as a signal to the boat.

TSandM,

You are perfectly correct. In an earlier post in this thread I correctly defined SMB and rescue sausage. I have since gone on to use the term SMB incorrectly - my bad. I will be re-editing my posts now to clear this up.

To me a rescue sausage is a DSMB - is my usage of the term incorrect?

Thanks!

 

[Tortuga68]

Tortuga, I'm confused. 100 fsw is 4ATA, right? 30 fsw is 2 ATA, right? So the one liter you put in at 100 fsw becomes 2 liters at 30, because you have half the pressure, so twice the volume?

Sorry Lynne & mpetryk, I have a bad habit of posting then editing as I decide what it was I actually meant to say

Especially at 03:50am

I think there was some confusion over whether we were talking about the buoyancy of the SMB or the diver

Anyway, I think we all agree on the principles as stated

 

[Crush]

But unless you **** up, that lift won't be applied to you

Either way, you won't have a "smaller buoyancy change" to deal with at depth

I am confused - perhaps I am using my rescue sausage/DSMB the wrong way. If I fill the 20# rescue sausage to 1/4 of its capacity at 90 feet, at the instant before I let go of it, it is pulling me up with 5# lift. Once I let go, that force is no longer acting upon me. However, at the moment before release there was a buoyancy change. If the fill and release are done quickly, the change in buoyancy may have only a negligible effect on my depth.

Oops - I stated typing before I saw your response, Tortuga68. It is all good.

 

[TSandM]

I'm still confused. A deployable surface marker buoy (DSMB, or as commonly used, SMB) is a device designed to be inflated either underwater or on the surface. It is commonly attached to a spool, and deployed at the end of the dive as a marker for the boat or others. Some people tow one as a dive flag (I do not know if this meets the dive flag requirements in all locations). An SMB can be designed to be orally inflated, inflated with a low pressure hose, or inflated with a purged regulator. They come in a variety of sizes -- lengths, widths, and amounts of lift. Some have OPVs and others do not.

There ARE open-ended tubes which are only designed to be inflated and waved at the surface. They have no mechanism for attaching them to a spool or reel. They are almost always just open-ended. I've seen this type of thing referred to as a "safety sausage".

It's the difference between your SMB and your "rescue sausage" that has me stumped. What would one see as the design differences between the two, or are you simply using two different terms to distinguish the intended PURPOSE of the same device?

 

[Tortuga68]

NP mpetryk

On deep dives I will brake the reel and use the SMB to help me ascend to the first stop, at the appropriate rate

As someone said earlier, it's nice for the boat crew to see the balloons pop and know we are ok

 

[Tortuga68]

It's the difference between your SMB and your "rescue sausage" that has me stumped. What would one see as the design differences between the two, or are you simply using two different terms to distinguish the intended PURPOSE of the same device?

There are so many different terms used here on SB, but I think in most cases they mean the same thing. I mostly call them an SMB here (even though I nearly always use mine as a DSMB), but balloon, blob, sausage etc are pretty common

But yes it can be confusing

 

[Crush]

I'm still confused.

TSandM, it is likely me who is confused.

This is what I referred to as a rescue sausage:
click here

However, it now seems that everyone but me calls this a surface marker buoy. Since it resembles a sausage to me, I will continue to refer to it as such in my head. However, to minimize confusion, I will now refer to it publicly as an SMB.