Reducing the length of my DSMB

[raftingtigger]

I'm planning on taking Tech40 in the very near future. One area I need work on is in u/w deployment of the DSMB. I would like to inflate it with the zero buoyancy change trick of transferring as much air as you can from ONE breath into the DSMB and then letting go. The problem I have is my DSMBs take a little more than 3 really full breaths to fully inflate them. That means I can use this method if 70' or below when sending it up. I have shortened it with rubber bands (see photo) so it now only takes 2 breaths. That of course means that I can get it full from >35', a much more reasonable depth. With the modification the buoy now is 4' rather than the 5-1/2' before. Can you think of any reason this isn't a good idea?

BTW it does have a OPV on it

 

[Stoo]

The only reason that comes to mind is that it's 4', rather than 5.5'. The point of a DSMB is to be seen. By reducing the length, you're reduced it's usefulness. If you were adrift in rough seas, you might prefer to have that extra foot and a half.

This strikes me as another one of those examples of learning a skill to pass a course, rather than learning the skill properly, to become a better diver.

Just my $0.02 of course...

 

[raftingtigger]

The only reason that comes to mind is that it's 4', rather than 5.5'. The point of a DSMB is to be seen. By reducing the length, you're reduced it's usefulness. If you were adrift in rough seas, you might prefer to have that extra foot and a half.

Good point. However if that were to happen I could easily remove the rubber bands and put some more air in the tube.

This strikes me as another one of those examples of learning a skill to pass a course, rather than learning the skill properly, so become a better diver.
Just my $0.02 of course...

Another good point. I have heard a lot of people (at least in my circle) use a 1 meter bag, at least during most classes. Since I don't have a 1 meter bag, and prefer the longer bag for the reasons above, I'm trying to effectively create a 1 meter bag for a short time. I will continue to practice with the larger bag, but don't need the extra distractions when learning new stuff.

My question isn't which length should I use (1 meter vs 6'), but is there anything dangerous about the temporary modification? I did fill it from the tank with the LP hose and the OPV releases without affecting the rubber bands.

 

[Peter_C]

Get a Halcyon 3.3ft or whatever they are and be happy. There are many reasons you will see most of the tech divers at Lobos use them. Starting with a light breath to set the buoy, the next breath will fill the Halcyon enough to send it to the surface and stand upright. They also stand upright with very little downward force, unlike a bigger 6 footer. The Halcyon also packs easily into your pocket pre-rigged to a spool. I have seen many others try larger SMB's, and eventually spend the money to switch to a Halcyon DSMB.

 

[DevonDiver]

The necessary size/length of a DSMB should be dictated by the requirement to be visually obvious in the water conditions that you dive. If water conditions (current/swell/waves etc) dictated a larger DSMB, then that is what you should use; without modification.

The obvious solution is to consider basic dive physics/gas laws... and be sure to deploy your DSMB from sufficient depth that Boyle's Law does all the hard work for you.

1. Understand the volume of the DSMB.
2. Understand the volume of gas you can add in a single breath.
3. Understand the expansion necessary to increase that breath to match the volume of the DSMB at the surface.
4. Understand the depth of deployment necessary to permit that gas expansion on ascent.

If deployed from the correct depth, a single breath is ALL it's ever gonna take....

 

[Dr. Lecter]

Anything as small as a 1M bag is going to useless as a visual signal for rescue, and will not provide any meaningful support should your wing decide to the bed. Other than that, sure, opt for ensuring you can deploy the thing with fundies tech-pass form.

I prefer to get everything arranged where I want it for launch and then use an LP whip off a BO or deco tank to fire the SMB. From much past 150', even the 4.5' and 6' H markers come up on viagra with just a second or two squirt of gas. More importantly than how neat they are to deploy in terms of hovering motionless while doing so, they're easy to see and can support me even if my wing fails.

Since your question was more about what could be unsafe about it, though, I think the main issue is finding you need a bigger bag after you've already launched your modded one. Carry two, and you're good to go.

 

[raftingtigger]

The necessary size/length of a DSMB should be dictated by the requirement to be visually obvious in the water conditions that you dive. If water conditions (current/swell/waves etc) dictated a larger DSMB, then that is what you should use; without modification.

The obvious solution is to consider basic dive physics/gas laws... and be sure to deploy your DSMB from sufficient depth that Boyle's Law does all the hard work for you.

1. Understand the volume of the DSMB.
2. Understand the volume of gas you can add in a single breath.
3. Understand the expansion necessary to increase that breath to match the volume of the DSMB at the surface.
4. Understand the depth of deployment necessary to permit that gas expansion on ascent.

If deployed from the correct depth, a single breath is ALL it's ever gonna take....

I believe I covered all that in the original post. Since what is readily available has too much volume for what I want for some specific dives, I have come up with a possible way to reduce the volume of the DSMB that is readily available.

I guess for the class I can put an extra LP QD hose on my redundant tank and just shoot the big bag that way, but I would rather not.

Given the gas volume laws my preference is to use a 1 meter bag. My options are to buy one (Halycon) for $105, borrow one from a friend, or temporarily make mine a 1 meter bag. Choices 2 and 3 are more economical. Of course if I try choice #3 my instructor may object

 

[DevonDiver]

Given the gas volume laws my preference is to use a 1 meter bag.

Personally, I don't think it's a matter of 'preference'. It's a matter of necessity. What size bag do you need, on a given dive, to ensure adequate visual recognition by surface support?

I use a 1m bag quite often - when I am diving calm, tropical waters, with no significant current or waves...on a clear, sunny day...

When the weather deteriorates, current increases, waves rise, fog occurs etc etc ... I have to use a bigger bag. I'd 'prefer' a small, convenient to deploy DSMB... but what's the point? If nobody is gonna see it?

For *only* practicing skills and drills on a course - you might be doing repetitive deployments from a shallow depth. That'd make a 1m DSMB preferable. That, however, is a special case - because it's for training. You wouldn't use that bag for real dives, if the conditions/requirements dictated otherwise. You'd use a larger bag, and deploy (in the same way - one breath) from a deep depth.

My options are to buy one (Halycon) for $105, borrow one from a friend, or temporarily make mine a 1 meter bag. Choices 2 and 3 are more economical. Of course if I try choice #3 my instructor may object

Get the right tool for the job.

The 'job' being your given requirements, on a given day, in a given location.

There's much cheaper than Halcyon......

 

[OkByMe]

I don't think it is how big a breath you have as it is the type SMB and deployment technique. My SMB and lift bags have a skirt with a check valve and relief valve. The spool has 30' of 1/8" white braided line and on the end of the line is a brass swivel. A double end snap traps the line to the spool and the line end snap attaches the spool to the "D" ring. The SMB has a snap on the strap loop to attach to the "D" ring and a piece of surgical tubing to trap it when rolled up.

Go to 20-25' and get neutral, you don't want to be kicking around when you do this. Unsnap the spool and hold it in one hand with a finger through the center. Unsnap the SMB, don't unroll it, just attach the spool line end snap to the SMB snap. Put the double end snap on the line and drop the snap while unrolling some or all of the line. The double end snap keeps the line out of the way. Now you have the spool in one hand, the line dropped out of the way below you and the SMB in the other hand. Pull the tubing off the rolled up SMB and swap the SMB to the hand with the spool, holding the loop on the SMB with your thumb or finger - whatever is comfortable - keep in mind you are going to have to hold onto it somewhat forcefully, but can easily release it or you are going up with it. Now take the free hand to get your second stage , ether the one you are using or the spare and shove it up into the skirt. Hold both hands out from you and slightly above your chest, give one or two kicks to move away from the line and unroll the SMB, stop moving - check the line again and hit the purge button. It will fill quickly, nearly completely, before you start to ride up, release it.

Two things to watch for here... Make sure you have your hand with the regulator free of the strap and that the regulator does not catch the strap ether. What I do is put the back of my hand against the skirt and pull against the strap held by the other hand until the strap is nearly horizontal. The insures my hand with the regulator will be pulled clear when the SMB goes up. It is also a good idea not to have a dive computer or other device on the wrist of the hand your inflate with. Also, when you release it have the line between you and the SMB clear of you. When the SMB shoots to the surface, the double end snap will keep the line straight down unless you are in a current, If you are in a current, make sure you turn so the line and SMB wants to drift away from you. As the double end snap comes up, keeping slack out of the line, pay out or take in line until you have it in your hand. Snap it to the spool. Dump a little air to get slightly negative, riding the SMB, reel in line and go to your stop.

The second stage will put out more air than is needed. If you are kicking and get the line on a fin, it will promptly turn you upside down and make you feel stupid and give everyone who witnessed it something to talk about forever. If the SMB is overinflated at the surface, the relief valve will adjust that. Bear in mind that if you ride the SMB too heavy it will dump air. This spool doubles as a tender line at the safety stop when required by the operator, so I don't have to gang up with other divers on the down line in a current. That is why the line is heavy and short.

 

[waterpirate]

When it comes to tec40, rubber bands and duct tape are not good gear mods. We all bought gear that we paid dearly for, found it to be: instructor rejected, unsuited for the task at hand, or just really crap. Get what works and get skillfull with it.
Eric