Calibration Flow Meter - Weighing Method (5)

International Standard 4185

Measurement of liquid flow in closed conduits -

Weighing method

First edition - 1980-12-15

Ref. No. ISO 4185-1980 (E)

Accuracy of the method

Overall uncertainty on the weighing  measurement

The weighing method gives an absolute measurement of flow which in principle requires only mass and time measurements. Provided that the precautions listed in 2.2.2 are taken, this method may be considered as one of the most accurate of all flow-rate measuring methods, and for this reason it is often used as a calibration method. When the installation is carefully constructed, maintained and used, an uncertainty of ± 0,1 % (with 95 % confidence limits for the random part of that uncertainty) tan be achieved.

 Requirements for accurate measurements

The weighing method gives an accurate measurement of flow rate provided that :

• there is no leak in the flow circuit and unmetere d leaka ge flow acro lss the diverter;
• there is no accumulation ior depletion) of liquid in a part of the circuit by thermal contraction (or expansion) and there is no accumulation (or depletion) by Change of vapour or gas volume contained unknowingly in the fiow circuit;
• necessary corrections for the influence of atmospheric buoyancy are made; this correction may be made when calibrating the weighing apparatus;
• the weighing machine, the timer and means for starting  and stopping it achieve the necessary accuracy;
• the time required by the diverter for traversing is small with respect to the filling time, the timer being started and stopped while the diverter is crossing the hydraulic centre line;
• in the case of the dynamic weighing me thod the effects of the dynamic phenomena are sufficiently small