A method of measuring the level of a phase from a plurality of phases of a conductive or semi-conductive mixture within a vessel, the method comprising: providing (510, 610) a vessel containing a conductive or semi-conductive mixture including a plurality of phases, the vessel comprising a sidewall; providing a process accretion layer (122, 234) between the vessel sidewall and the conductive or semi-conductive mixture; characterized by: transmitting (520, 620) at least one electromagnetic and/or eddy current signal into the vessel sidewall at a plurality of different levels so that each of the transmitted electromagnetic and/or eddy current signals impinges upon the conductive or semi-conductive mixture at substantially the same level at which each electromagnetic and/or eddy current signal is transmitted stimulating at least one corresponding reflected electromagnetic and/or eddy current signal; receiving (530, 630) the at least one corresponding reflected electromagnetic and/or eddy current signal, the reflected electromagnetic and/or eddy current signal originating at the boundary between the vessel sidewall and the process accretion layer and at the boundary between the process accretion layer and the conductive or semi-conductive mixture; and processing (540, 650) the at least one reflected electromagnetic and/or eddy current signal to determine the level of at least one phase from the plurality of phases in the conductive or semi-conductive mixture and the thickness of the process accretion layer within the vessel. providing (510, 610) a vessel containing a conductive or semi-conductive mixture including a plurality of phases, the vessel comprising a sidewall; providing a process accretion layer (122, 234) between the vessel sidewall and the conductive or semi-conductive mixture; characterized by: transmitting (520, 620) at least one electromagnetic and/or eddy current signal into the vessel sidewall at a plurality of different levels so that each of the transmitted electromagnetic and/or eddy current signals impinges upon the conductive or semi-conductive mixture at substantially the same level at which each electromagnetic and/or eddy current signal is transmitted stimulating at least one corresponding reflected electromagnetic and/or eddy current signal; receiving (530, 630) the at least one corresponding reflected electromagnetic and/or eddy current signal, the reflected electromagnetic and/or eddy current signal originating at the boundary between the vessel sidewall and the process accretion layer and at the boundary between the process accretion layer and the conductive or semi-conductive mixture; and processing (540, 650) the at least one reflected electromagnetic and/or eddy current signal to determine the level of at least one phase from the plurality of phases in the conductive or semi-conductive mixture and the thickness of the process accretion layer within the vessel. transmitting (520, 620) at least one electromagnetic and/or eddy current signal into the vessel sidewall at a plurality of different levels so that each of the transmitted electromagnetic and/or eddy current signals impinges upon the conductive or semi-conductive mixture at substantially the same level at which each electromagnetic and/or eddy current signal is transmitted stimulating at least one corresponding reflected electromagnetic and/or eddy current signal; receiving (530, 630) the at least one corresponding reflected electromagnetic and/or eddy current signal, the reflected electromagnetic and/or eddy current signal originating at the boundary between the vessel sidewall and the process accretion layer and at the boundary between the process accretion layer and the conductive or semi-conductive mixture; and processing (540, 650) the at least one reflected electromagnetic and/or eddy current signal to determine the level of at least one phase from the plurality of phases in the conductive or semi-conductive mixture and the thickness of the process accretion layer within the vessel.