PLBs Can Save Your Life

[chillyinCanada]

Just being inquisitive. I picked up a pair of canisters that I will convert to emergency packs. I was thinking that the risk of flooding might be less if the cavity was water filled having the side benefit of drinking water.

This would be awesome if it works.

@Dan ?

 

[GJC]

I wanted to share something that came to mind and see if there is anything I am missing. In regards to how to carry a PLB to depth in diving, there has been much discussion about what would make a good pressure vessel to protect the PLB. Rather than try to have a vessel that keep the PLB, and/or nautilis dry and withstand the change in pressure due to airspace. Since there are PLB's that are waterproof to 5 meters, and the Nautilus is waterproof far deeper than that. Why try to create a pressure vehicle? Why not find a seal-able canister that will work but does not have to be robust. Stuff the PLB/Nautilus, and any other emergency gear in it, then fill it with fresh water. When you seal it up, assuming you get all or nearly all of the air out then there is no need to have a robust pressure vehicle since water does not compress like air. The water in the container could also work as emergency drinking water.
Or am I missing something, like small air spaces in and ACR unit that would create issues. Or maybe that even though water will not compress like air it is still subject to the pressure changes of the weight of water above it? Just an idea, and wanted to see what others have to say.

Water, unlike air, is not compressible.

A canister that flexes a tiny bit at depth will only slightly increase the pressure inside of an air filled container.

A canister completely filled with water, that flexes the tiniest bit, will expose the contents to the pressure that is outside. Even if only the seals flex or compress a little, the contents are compressed.

Unless the contents have an air pocket, and then the air pocket would compress the amount equal to the volume that the canister flexes. The resulting pressure increase would be partially absorbed by the compressed air. The pressure on the contents then gets complicated to compute. The big factors being the amount of flexion and size of the air pocket.

I don't think it's a good idea to fill your canister with water. There is a risk of exposing the contents to the outside pressure and flooding the contents.

 

[Dan]

I don't get it. What would be the advantage of flooding the canister with water? You would still have bulky canister size. I would consult with the OEM technical support before doing something like that.

I doubt it's going to work when I look to my PLB1 with winding spiral flat / blade-like antenna. That kind of antenna would be hard to seal a slot cavity, especially at the edge. If you submerge it in the water for a long time, say for an hour, water would eventually seep through the antenna compartment and get into the electronic. Nautilus fix that problem by putting a sealed cap over the spiral antenna.

My PLB1 canister (2.5" OD x6" L) doesn't take much space in my BCD pocket. It is about the same size as my DSMB.

 

[aviator8]

The pressure on the contents then gets complicated to compute. The big factors being the amount of flexion and size of the air pocket.

This is the real problem I see! How to do you calculate it

I don't get it. What would be the advantage of flooding the canister with water?

To me the advantage would be this:
1 An air filled pressure vessel is great, but if it fails everything in it fails catastrophically and the PLB is rendered ineffective.
2 If it is water filled the failure rate could be mitigated and less likely to cause catastrophic failure in a seal breach.
3 There is a side benefit of having drinkable water in an emergency unless there was a seal breach.

 

[aviator8]

Also I should say benefit 4. not having to by a high priced bulky pressure vessel. A diy solution may work fine

 

[DandyDon]

1 An air filled pressure vessel is great, but if it fails everything in it fails catastrophically and the PLB is rendered ineffective.

Leaks can happen, especially with old or ungreased o-ring, and that can ruin a camera or a light, but not likely with a PLB as it's water resistant.

 

[Dan]

2 If it is water filled the failure rate could be mitigated and less likely to cause catastrophic failure in a seal breach.

Do you think there is no air space inside the PLB? Also I know the housing of my PLB1 is just a thin plastic sheet, especially around the button. With water-filled canister, as you go down to 100’ depth, the water column crushing pressure would be transmitted directly on to the canister surface. Since the water inside the canister is incompressible, that pressure would then be transmitted directly onto the PLB housing and ended up flexing the PLB housing and turn on the red button. On top of that, now you have water pressure inside the canister pressing against the PLB housing to beyond its rated value.

I tested this by putting my PLB1 in a soft case and taking it to 100’ depth. The red button turned on as the soft case crushed on the PLB1. See the red LED lights up in the picture, below.

If you have air inside a rigid canister and the canister leaks underwater, as long as there is still air in the canister cavity, the PLB would still be fine. The pressure inside the canister would still be below the rated value since the air is compressible.

Unlike the PLB1, Nautilus Marine Rescue GPS (MRG) alert buttons are placed inside the protective cap that would hold up to 425’ depth water pressure. That’s why you need a canister to take the PLB underwater beyond tge rated value. The PLB makers need to design the PLB with more rigid housing with buttons inside a protective sealed cap, just like the MRG.

 

[Dan]

A canister completely filled with water, that flexes the tiniest bit, will expose the contents to the pressure that is outside. Even if only the seals flex or compress a little, the contents are compressed.

This makes sense. My canister is sealed by a big o-ring on top. Just imagine you fill the canister up to the brim with water at 1 atmosphere, close the lid so it traps no air . If you take it down to 100’ (30m) or 4 atmospheres, the differential pressure across the o-ring seal is going to be 3 atmospheres (44 psi). That 44 psi pressure is going to squeeze the o-ring flatter and pressuring the water-filled cavity towards 4 atmosphere. The flat polyethylene lid and bottom acrylic plate would also cave in.

 

[Joshua Young]

If you’re just diving with a group, as well as a DM in a controlled setting, wouldn’t a PAB all you’d need?

 

[Dan]

To avoid the alert buttons from getting pressed by hydraulic pressure at depth, Nautilus MRG has those buttons inside an o-ring sealed air cap.