Lift Bag/Signal Marker Buoy background…
Signal marker buoys (SMBs) and lift bags are used for a variety of purposes, all of which are essential: SMBs and lift bags can be used as signaling devices launched from depth while ascending or from the surface, they can both serve as a deco platform, either could be used for emergency buoyancy in the case of wing failure, and lift bags can be used to salvage items from the sea floor. “Bags” is a slang term used to include both SMB and lift bags.

Open or Closed
SMBs and lift bags can be either open or closed cell. Open cell bags are open at the bottom, where they are meant to be filled by purging the second stage of a regulator (or another source of air).  Many open cell bags have no way to seal the bottom, so they can sometimes tip over at the surface and lose their air if sent up from depth.  If inflated at the surface, they must be held closed with a hand to keep air from escaping.  Some open cell bags are self sealing, like Carter Bags, and become airtight once filled.

Closed cell bags are totally sealed except for a one-way filling valve that mates to a low pressure inflator hose, such as a drysuit hose or LP BC hose. The primary advantage of closed cell designs is that they can be quickly filled with a low pressure hose, and do not require the user to manipulate regulators to fill the bag. They can also be made totally full at the surface quite easily (even large bags) using an inflator hose.  The one-way valve can also be used for oral inflation.

Oral fill valve
Some open cell bags are equipped with one-way oral inflation valves, as in Carter Bags. With this design, a diver can launch an open cell bag from depth with a regulator, and then top it off once at the surface using the oral inflation valve. Once at the surface, it’s important to be able to easily fill the bag to maximum capacity for highest visibility.  For smaller bags, the oral fill valve can also be used at depth. All of our Cater offerings are equipped with oral fill valves.

Over Pressure Valve (OPV)
An OPV allows bags to be launched from depth.  As a bag travels to the surface, the air will expand, possible to a volume that exceeds the bag’s capacity and the OPV will release this gas. Some inexpensive closed cell bags do not have an OPV, and are designed for surface use only. The Carter Bags we carry all have OPVs, allowing them to be used both at the surface and from depth. 

Visibility
SMB/Liftbag color is an interesting issue. Fluorescent yellow is by far the most visible color for signaling devices. However, orange is a more popular color (even though less visible), possibly because it looks cooler to the technical crowd.  Some dive sites and boats have designated yellow as an emergency color.  Although yellow is the outright visibility winner, there is one situation where orange is more visible than yellow. If the diver were on the surface of the water between the boat and sun, a darker color such as orange is more visible. Because of this, some SMB have yellow on one side and orange on the other. We carry some of this design.

When it comes to visibility, unfortunately, size matters. I like to carry a marker in my left pocket all the time, usually a smallish unit that can serve as a deco stop platform and surface signal in normal conditions.  This unit would be from about 4 feet long. This size is a reasonable mix of compactness when folded, ease of inflation and visibility (in our diving environment). 

When diving in areas where being seen by a boat or plane is a more of a life or death matter, I carry a 6 foot or larger unit.  This might be in my pocket, or stowed in my backplate pack.