Most measurements are based on indications provided by measuring instruments or measuring systems. An indication (e.g. an instrument signal or response) and a corresponding value of the quantity being measured are not necessarily values of quantities of the same kind . In many cases, the indication provided by the measuring instrument or measuring system will be a value related to a quantity different from the measurand.
In most cases in analytical science, analysts rely on the measurement of physical quantities such as mass of precipitate, volume of titrant, or the change in electrical signal produced by the absorption of radiation of a particular wavelength. These indications are then converted, using well known stoichiometric relationships or a calibration curve, to quantities related to the amount of substance. In chemical analysis it is quite common to observe an instrument response for a series of reference solutions and then for the test sample, so in this context the term ‘instrument response’ is generally used for indication. Similarly blank indication refers to the instrument response for a test material where the analyte of interest is believed to be absent (in VIM 3 the phrase ‘supposed not to be present’ is used).
REF: Terminology in Analytica Measurement Introduction to VIM 3
