Thermal sensor and method for operating the thermal sensor

A method for operating a thermal sensor, wherein the thermal sensor comprises at least a first sensor element and an electronics, wherein the first sensor element is in thermal contact with a container, through which a measured medium flows at a flow velocity, the method comprising:

• periodically heating the first sensor element by applying an alternating voltage to the first sensor element while simultaneously detecting a temperature of the first sensor element over time;
• and
• determining a measured variable of the measured medium based on a comparison of a third harmonic oscillation of the alternating voltage applied to the first sensor element with a third harmonic oscillation of the temperature of the first sensor element using the electronics, wherein the comparison includes calculating a phase shift between the third harmonic oscillation of the alternating voltage and an amplitude of the third harmonic oscillation of the temperature,
• wherein a frequency of the alternating voltage is selected such that an emitted heat of the first sensor element extends into the measured medium to a penetration depth when the flow velocity of the measured medium is essentially zero.