A thermal increase system comprising: a radio frequency, RF, signal source (320,720,1120) configured to supply an RF signal; an electrode (170,340,740,11170) coupled to the RF signal source; at least one variable impedance network (334,370,734,772,1160) that includes at least one variable passive component having at least one current variable component value, wherein the at least one variable impedance network is coupled between the RF signal source and the electrode; and characterised by a controller (312,712,1130) configured to determine an initial estimated mass of a load that is proximate to the electrode based at least on the at least one current variable component value of the at least one variable impedance network, to determine one or more desired signal parameters for the RF signal, including a desired RF power level, based on at least the estimated mass of the load, to control the RF signal source to supply an initial-mass-estimate-based RF signal with the one or more desired signal parameters, to determine a rate of change of a parameter of the RF signal source while the initial-mass-estimate-based RF signal is supplied, to determine a refined estimated mass of the load based on at least the rate of change of the parameter, to determine one or more refined signal parameters for the RF signal based on at least the refined estimated mass of the load, and to control the RF signal source to supply a refined-mass-estimate-based RF signal with the one or more refined signal parameters, wherein the parameter is selected from a group consisting of: an S11 parameter, a voltage standing wave ratio, and a reflected power of the RF signal. a radio frequency, RF, signal source (320,720,1120) configured to supply an RF signal; an electrode (170,340,740,11170) coupled to the RF signal source; at least one variable impedance network (334,370,734,772,1160) that includes at least one variable passive component having at least one current variable component value, wherein the at least one variable impedance network is coupled between the RF signal source and the electrode; and characterised by a controller (312,712,1130) configured to determine an initial estimated mass of a load that is proximate to the electrode based at least on the at least one current variable component value of the at least one variable impedance network, to determine one or more desired signal parameters for the RF signal, including a desired RF power level, based on at least the estimated mass of the load, to control the RF signal source to supply an initial-mass-estimate-based RF signal with the one or more desired signal parameters, to determine a rate of change of a parameter of the RF signal source while the initial-mass-estimate-based RF signal is supplied, to determine a refined estimated mass of the load based on at least the rate of change of the parameter, to determine one or more refined signal parameters for the RF signal based on at least the refined estimated mass of the load, and to control the RF signal source to supply a refined-mass-estimate-based RF signal with the one or more refined signal parameters, wherein the parameter is selected from a group consisting of: an S11 parameter, a voltage standing wave ratio, and a reflected power of the RF signal.