This Standard provides guidance for the design and application of models by those concerned with the extension or supplementation of prototype tests of equipment using ASME Performance Test Codes. Where there are test codes in existence covering specific equipment, the guiding principles, instruments and methods of measurement from such codes shall be used with only such modifications as become necessary by virtue of the fact that a model is being tested instead of a prototype. A model is a device, machine, structure or system which can be used to predict the behavior of an actual and similar device, machine, structure or system which is called the prototype. A physical model may be smaller than, the same size as, or larger than the prototype. PTC 19.23 consists of (a) General discussion of model testing; (b) Example problems; and (c) Theoretical background.