Fig. 1.
Zero-heat-flux systems consist of two thermometers separated by an insulator, covered by a heater. One thermometer is positioned between the skin surface and the lower surface of the insulator, and the other between the upper surface of the insulator and the heater. The temperature of the heater is servo-controlled to keep both thermometers at the same temperature. Because there is no temperature gradient across the insulator, there can be no flow of heat, making the entire system a perfect insulator. Normally, there is a thermal gradient from the body core, where most heat is generated to the skin surface. In the presence of a zero-heat-flux thermometer, heat flowing toward the covered surface cannot escape through the perfect insulator and therefore accumulates below the device. In theory, after some minutes of equilibration, a column of tissue at the same temperature will extend from the core to the skin surface below the zero-heat-flux thermometer. When these conditions are met, temperature of the core and thermometer are identical. Because of lateral convection of heat by blood, the column typically extends only about a centimeter below the skin surface—but on the forehead, that is deep enough to approximate core temperature.