digester vs immersion heater

digester

noun
  • A strong closed vessel in which bones or other substances may be subjected, usually in water or other liquid, to a temperature above that of boiling, in order to soften them. 

  • One who, or that which, digests. 

immersion heater

noun
  • A device for supplying domestic hot water in which a thermostatically-controlled heating element is immersed within a tank of water 

How often have the words digester and immersion heater occurred in a corpus of books? (source: Google Ngram Viewer )