Abstract: An inflator for automatically inflating an inflatable article, such as a personal flotation vest, upon immersion in water. A plunger and a piercing pin are movably mounted within a housing and are movable with respect to a pressurized gas canister attached to the housing. A force generating mechanism, mounted within the housing, acts on the plunger to forcibly move the plunger and the piercing pin to pierce the canister to release the pressurized gas therefrom through the housing to an inflatable article attached to the housing. A pair of spaced water sensor probes mounted in the housing provide a signal to a control device indicating that the housing is immersed in water. The control device activates a release mechanism which rotates a cam to release a spring-biased ram in the force generating mechanism from a first, retracted position such that the ram engages the plunger and urges the plunger and the piercing pin under force into the sealed end of the gas canister.