Method for operating a device for treating an aqueous fluid, for example drinking water, wherein the device comprises an apparatus for changing the concentration of at least one specified ion species, which apparatus has a limited treatment capacity, and wherein the device comprises a blending apparatus (8, 14) for blending fluid treated by the apparatus with fluid which has not been treated by the apparatus, wherein a blending proportion related to volume of the non-treated fluid in a mixture can be changed, wherein values are measured of a parameter which is a function of the concentration of at least the specified ion species, wherein measurements of a first type are repeatedly undertaken on fluid which is supplied to an outlet of the device and constitutes at least a part of the fluid treated by the apparatus, and wherein measurements of a second type are undertaken on only fluid consisting of fluid which is not treated by the apparatus, and wherein at least one measurement value obtained from a measurement of the second type in reaction to a determination that the value obtained in the measurements of the first type was outside specified limits, is used in order to specify whether a state of the apparatus is present which is related to the treatment capacity, characterised in that the measurements of the second type are carried out with a sensor arranged downstream of a merging point for merging the fluid which is not treated by the apparatus and the fluid treated by the apparatus, wherein for obtaining one of the at least one measurement values obtained in a measurement of the second type in reaction to a determination that the value obtained in the measurements of the first type was outside specified limits, a measurement of the second type is carried out and, for carrying out the measurement of the second type, the blending proportion is changed. wherein the device comprises an apparatus for changing the concentration of at least one specified ion species, which apparatus has a limited treatment capacity, and wherein the device comprises a blending apparatus (8, 14) for blending fluid treated by the apparatus with fluid which has not been treated by the apparatus, wherein a blending proportion related to volume of the non-treated fluid in a mixture can be changed, wherein values are measured of a parameter which is a function of the concentration of at least the specified ion species, wherein measurements of a first type are repeatedly undertaken on fluid which is supplied to an outlet of the device and constitutes at least a part of the fluid treated by the apparatus, and wherein measurements of a second type are undertaken on only fluid consisting of fluid which is not treated by the apparatus, and wherein at least one measurement value obtained from a measurement of the second type in reaction to a determination that the value obtained in the measurements of the first type was outside specified limits, is used in order to specify whether a state of the apparatus is present which is related to the treatment capacity, characterised in that the measurements of the second type are carried out with a sensor arranged downstream of a merging point for merging the fluid which is not treated by the apparatus and the fluid treated by the apparatus, wherein for obtaining one of the at least one measurement values obtained in a measurement of the second type in reaction to a determination that the value obtained in the measurements of the first type was outside specified limits, a measurement of the second type is carried out and, for carrying out the measurement of the second type, the blending proportion is changed.