A method used to calibrate flow meters. It involves the addition of a concentrated extraneous material whose presence can be quantitatively determined by an analytical technique. The extraneous material may or may not be one that is already present as an impurity and is diluted into the flow of fluid whose flow rate is being measured. Examples are salt in water, ammonia in steam, and chlorine in oxygen. The procedure is to add steadily a known amount of extraneous material to the flowing process fluid over a known period of time. If the addition is made upstream of the meter, the latter can act as a mixing device during the period before samples of the fluid are taken downstream of the meter and then analysed. For the addition of extraneous material added upstream of the meter this can be expressed mathematically as:

where Q_i is the flow rate of the process fluid, Q_a is the flow rate of the extraneous material, C_i is the concentration of the extraneous material in the process fluid, C_a is the concentration of the added extraneous material, and C_o is the diluted downstream concentration. This equation can be rearranged to calculate the flow rate of the process fluid as:

If the addition of extraneous material is after the meter, then the meter measures Q_i and not Q_i+Q_a.