This International Standard specifies a method of liquid flow- rate measurement in closed conduits by measuring the mass sf liquid delivered into a weighing tank in a known time interval. lt deals in particular with the measuring apparatus, the pro- cedure, the method for calculating the flow-rate and the uncer- tainties associated with the measurement. The method described may be applied to any liquid provided that its vapour pressure is such that any escape of liquid from the weighing tank by vaporization is not sufficient to affect the required measurement accuracy. C!osed weighing tanks and their application to the flow measurement of liquids sf high vapour pressure are not considered in this International Stan- dard. This International Standard does not cover the cases of cor- rosive or toxic liquids. Theoretically, there is no limit to the application of this method which is used generally in fixed laboratory installations only. However, for economic reasons, usual hydraulic laboratories using this method tan produce flow-rates of 1.5 m% or less. Owing to its high potential accuracy, this method is often used as a primary method for calibration of other methods or devices for mass flow-rate measurement or volume flow-rate measure- ment provided that the density of the liquid is known accurately. lt must be ensured that the Pipeline is running full with no air or vapour pockets present in the measuring section.