cooking utensil vs double boiler

cooking utensil

noun
  • Any utensil used for preparing, handling, or serving food. 

double boiler

noun
  • A large pan containing hot water, into which other smaller pans are set in order to cook food at low heat (below the boiling point) or to keep food warm. 

  • A pair of interlocking pots, the lower one filled with water which when boiling heats the contents of the upper pot. 

How often have the words cooking utensil and double boiler occurred in a corpus of books? (source: Google Ngram Viewer )