A substance that may appear intentionally in another substance as an impurity. A method of calibrating flow meters involves the use of a controlled amount of extraneous material that is intentionally added to the process stream either upstream or downstream of the flow meter. A sample is then taken downstream and analysed from which the rate of flow of the process stream is determined, and relevant calibration adjustments made to the flow meter. Examples include the addition of dyes to water, brine to seawater, the addition of radionuclides and oxygen in chlorine.