A method of measuring a liquid volume of a sample solution in a vessel, characterised in that the method comprises the steps of: a. adding to the vessel a known volume of diluent including a known concentration of a diluent dye which absorbs light at a wavelength unique to the diluent; b. adding a volume of the sample solution to the vessel, wherein the sample solution includes a known concentration of one or more dyes, wherein the one or more dyes are selected to produce multiple distinct detectable light absorbance spectral features at a corresponding number of distinct wavelengths, each of which is distinct from the wavelength unique to the diluent, and wherein the sample solution does not include the diluent dye; c. mixing the diluent and the sample solution in the vessel to produce a mixture of the sample solution and the diluent; d. measuring the absorbance of the mixture of the sample solution and the diluent at one or more wavelengths at which the distinct detectable light absorbance spectral features can be detected so that they provide an absorbance within a defined measurable absorbance range; and e. calculating the volume of the sample solution added to the vessel based on the measured absorbance, absorbance per unit pathlength of the diluent and the sample solution, and the known volume of the diluent, in a manner that when a relatively large volume of the sample solution is added, a light absorbance spectral feature with a low absorbance per unit pathlength is used and when a small volume of the sample solution is added, a light absorbance spectral feature with a high absorbance per unit pathlength is used as the basis for calculating the volume of the sample solution added to the vessel. a. adding to the vessel a known volume of diluent including a known concentration of a diluent dye which absorbs light at a wavelength unique to the diluent; b. adding a volume of the sample solution to the vessel, wherein the sample solution includes a known concentration of one or more dyes, wherein the one or more dyes are selected to produce multiple distinct detectable light absorbance spectral features at a corresponding number of distinct wavelengths, each of which is distinct from the wavelength unique to the diluent, and wherein the sample solution does not include the diluent dye; c. mixing the diluent and the sample solution in the vessel to produce a mixture of the sample solution and the diluent; d. measuring the absorbance of the mixture of the sample solution and the diluent at one or more wavelengths at which the distinct detectable light absorbance spectral features can be detected so that they provide an absorbance within a defined measurable absorbance range; and e. calculating the volume of the sample solution added to the vessel based on the measured absorbance, absorbance per unit pathlength of the diluent and the sample solution, and the known volume of the diluent, in a manner that when a relatively large volume of the sample solution is added, a light absorbance spectral feature with a low absorbance per unit pathlength is used and when a small volume of the sample solution is added, a light absorbance spectral feature with a high absorbance per unit pathlength is used as the basis for calculating the volume of the sample solution added to the vessel. <EOS>