Measuring core body temperature?

To measure core body temperature accurately, Andrew Huberman describes a method involving the use of an esophageal probe. This probe is inserted approximately 2 feet down the esophagus to the level of the heart, providing a direct measurement of core temperature. He notes that while tympanic (ear) temperature measurement is common, it often mixes tympanic and ear canal temperatures unless perfectly aimed at the tympanic membrane. Surface temperature measurements, such as those taken from the forehead, generally offer a more crude reading 1.