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Bobbins are a component of the automatic inflation mechanism commonly found in auto-inflating PFDs. (Another device sometimes used is an actual pill-like tablet.)
These are the components which dissolve when immersed in water, allowing a spring-loaded puncture mechanism (which they have been holding back) to pierce the CO₂ inflation cylinder and inflate the PFD.
Inflatable PFD manufacturers recommend that both bobbins and tablets be replaced on a regular schedule to avoid inadvertent inflation. Replacement is required, of course, anytime the PFD inflates during use.
Always refer to the label inside your inflatable PFD to determine or confirm the type of mechanism used in your particular inflatable PFD.
Comes in a pack of 3