Method of determining a measure of a concentration of components removable from fluid in the form of a liquid by a fluid treatment part of a fluid treatment device, including the use of a fluid treatment apparatus including: an inlet (1;34) for receiving untreated fluid; an outlet (2;35); and at least one fluid path between the inlet (1;34) and the outlet (2;35) including at least one fluid treatment part (3;36) in the form of a liquid treatment part for treating fluid to remove to at least a certain extent at least some types of components in fluid led through the fluid treatment part (3;36), the method including: obtaining at least two measurement values, each representative of a respective value of a parameter of the fluid depending partly on the concentration of components removable by the fluid treatment part (3;36), wherein the measurement values pertain to measurements carried out on fluid at different ratios of untreated fluid to fluid led through the at least one fluid path including a fluid treatment part (3;36); using the measurement values to obtain a difference value representative of a change in the value of the parameter due to treatment by the fluid treatment part (3;36); and converting the difference value into an output value representative of the concentration of components removable from fluid by a fluid treatment part (3;36), characterised in that the conversion is carried out in dependence on a further value of the parameter approximating absolute values of the parameter within the fluid treatment apparatus. an inlet (1;34) for receiving untreated fluid; an outlet (2;35); and at least one fluid path between the inlet (1;34) and the outlet (2;35) including at least one fluid treatment part (3;36) in the form of a liquid treatment part for treating fluid to remove to at least a certain extent at least some types of components in fluid led through the fluid treatment part (3;36), the method including: obtaining at least two measurement values, each representative of a respective value of a parameter of the fluid depending partly on the concentration of components removable by the fluid treatment part (3;36), wherein the measurement values pertain to measurements carried out on fluid at different ratios of untreated fluid to fluid led through the at least one fluid path including a fluid treatment part (3;36); using the measurement values to obtain a difference value representative of a change in the value of the parameter due to treatment by the fluid treatment part (3;36); and converting the difference value into an output value representative of the concentration of components removable from fluid by a fluid treatment part (3;36), characterised in that the conversion is carried out in dependence on a further value of the parameter approximating absolute values of the parameter within the fluid treatment apparatus. obtaining at least two measurement values, each representative of a respective value of a parameter of the fluid depending partly on the concentration of components removable by the fluid treatment part (3;36), wherein the measurement values pertain to measurements carried out on fluid at different ratios of untreated fluid to fluid led through the at least one fluid path including a fluid treatment part (3;36); using the measurement values to obtain a difference value representative of a change in the value of the parameter due to treatment by the fluid treatment part (3;36); and converting the difference value into an output value representative of the concentration of components removable from fluid by a fluid treatment part (3;36), characterised in that the conversion is carried out in dependence on a further value of the parameter approximating absolute values of the parameter within the fluid treatment apparatus.