life ring vs preserver

life ring and preserver both are nouns.

Word	Noun	Adjective	Verb	Adverb
life ring	Yes	No	No	No
preserver	Yes	No	No	No

As nouns, preserver is a hypernym of life ring; that is, preserver is a word with a broader meaning than life ring:

life ring: a life preserver in the form of a ring of buoyant material
preserver: rescue equipment consisting of a buoyant belt or jacket to keep a person from drowning

Other hypernyms of life ring include flotation device, life preserver.

life ring (noun)	preserver (noun)
a life preserver in the form of a ring of buoyant material	rescue equipment consisting of a buoyant belt or jacket to keep a person from drowning
	a skilled worker who is employed to restore or refinish buildings or antique furniture
	someone who keeps safe from harm or danger
	a cook who preserves fruits or meat