Continuous measurement of the concentration of a predetermined ingredient in a diluted liquid

The invention provides a method of monitoring the concentration of a predetermined ingredient in a first liquid (8), wherein the concentration of the predetermined ingredient in a mixed liquid obtained by mixing together the first liquid and a diluent (9) is measured, respective tube pumps (3,2) acting on two of the first liquid, the diluent and the mixed liquid to effect transfer of the mixed liquid to measuring means (6) which measures the concentration of the predetermined ingredient in the mixed liquid, the method comprising: providing the mixed liquid at two or more dilution ratios (i.e. ratio of the mixed liquid to the first liquid) by controlling at least one of the tube pumps to change the rate of supply of at least one of the first liquid and the diluent to the mixed liquid and measuring the concentration of the predetermined ingredient in the mixed liquid before such change of dilution ratio and resulting from the change; determining from the change or changes in the rae of supply of the first liquid and/or a diluent and from the measured concentrations at least one value of the dilution ratio before or resulting from the change or changes; and determining from such determined dilution ratio the concentration of the predetermined ingredient in the first liquid. The invention further provides apparatus for carrying out the method.