A device which measures the instantaneous difference in the composition of the column effluent.
Sources:
Orange Book, 2nd ed., p. 99 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 849 (https://doi.org/10.1351/pac199365040819)