1. A method for detecting the ingress of air into a system for liquid comprising: measuring and analyzing a pressure signal in the liquid; spectrally breaking down the measured pressure signal into at least one function, said at least one function having at least one coefficient; monitoring the at least one coefficient of the at least one function; comparing the at least one coefficient of the at least one function to respective preset limiting values; and determining that there has been an ingress of air into the system for liquid if at least one of the coefficients of the at least one function exceeds or drops below the respective preset limiting values.