Portable Electronic Thermometer (PET) is the term used by API to describe a hand held electronic thermometer instrument with an associated display that is usually not part of a monitoring system.
The use of the word PET presently implies a battery operated device that can be used for independent temperature measurement of petroleum products or systems associated with vessels, pipelines, or process systems. As an independent measuring device that may be used for commercial purposes, verification or calibration, it should meet the specifications of an applicable standard, whether that is API, ASTM, etc.
The specifications for a PET usually include accuracy tolerance and other calibration requirements. Since a PET is most likely being used in an environment where flammable fluids may be present (Hazardous Location) it should be designated as an Intrinsically Safe device by a recognized testing and certification organization such as UL, BASEEFA, Intertek, TIIS etc.