The method comprises the following steps: a) Providing a sonotrode (1) formed from an essentially inert material with respect to the liquid metal, such as a ceramic, and preferably a silicon nitride or a silicon oxynitride, such as SIALON, or a metal essentially inert to said liquid metal, b) Immersing at least partially the sonotrode (1) in a bath of said metal, c) Applying to the sonotrode (1) power ultrasounds, particularly ultrasounds having a power greater than 10 watts to obtain the wetting of said sonotrode by said metal, d) Applying continuously to the sonotrode (1) measurement ultrasounds, also known as testing ultrasounds, particularly ultrasounds wherein the frequency is between 1 and 25 MHz, e) Applying intermittently to the sonotrode (1) power ultrasounds, particularly ultrasounds having a power greater than 10 watts, to maintain said wetting.