Life Rings
The purpose of life preservers, now commonly referred to as "personal flotation devices," is to help a person (or animal) stay afloat if they end up in deep water. Life preservers are made from various materials that are substantially lighter than water and thus provide buoyancy. Life preservers such as the life ring, or life buoy, require the user to cling to it, and thus are effective only for conscious individuals.
Life Jackets
A life vest can help keep you afloat in troubled waters.
Another form of life preserver is the life vest or life jacket. As its name suggests, the device is worn as a vest.
A simple U-shaped life vest may fit around the back of the neck with the two legs of the "U" extending down either side of the chest. The legs may be connected together by straps and buckles, hook-and-loop fastener tape (such as Velcro), or zippers. Typically, the vest includes a belt that encircles the body and that attaches to the bottom ends of the legs. The belt, and any other straps or fasteners, should be tightened securely around the body to help keep the vest in place.
If the entire "U" of the life jacket contains flotation material, including the area behind the neck, the device may turn the user face up in the water and prevent the head from submerging. That is important if the user may be unconscious, as may occur as the result of a boating or water-skiing accident.
The life jacket shown in the image does not include substantial flotation material behind the neck. Although that style of life jacket does provide a righting force to turn the body face up, it is not as effective as the one described above for keeping the face clear of the water.
Flotation Materials
Life jackets use a variety of flotation materials.
Ordinary life jackets use a closed-cell plastic foam.
Compact life jackets used by airlines and some passenger vessels are inflated with air when in use. The user must inflate the life jacket either by blowing it up through a check valve or by using an inflation device. The inflation mechanism is typically activated by pulling on a lanyard. This action results in the release of compressed gas, such as carbon dioxide, from a canister attached to the vest.
Inflatable life preservers do not provide buoyancy until they are inflated. That may be an advantage if the user has to submerge under water to access an escape door, such as when an airplane is sinking. The flotation material in an ordinary life jacket might hinder the user from submerging to escape from sinking vehicles.
