Level detector is an op-amp circuit that compares two inputs and provides a DC output indicating the polarity relationship between the inputs. This is also used to rectify the input signal and give a DC output proportional the peak or RMS level of the input signal. It is used for detecting a level of a liquid, which is provided with an elongated probe having opposed proximal and distal ends. Its distal end of the elongated probe is submerged beneath the level of the liquid with an unwetted length of the elongated probe located above the level of the liquid. It contains a thermal conductor or an electrical heater, which is used for maintaining a temperature difference between the proximal end of the elongated probe.