Measuring the flowrate with force

In flowrate measurements, metrological traceability is assured by using flowmeters as transfer standards. These transfer standards are, for example, used to compare the test facilities of the petroleum industry, of the industry for liquid food and of the drinking water supply companies metrologically with the respective national primary standard. In Germany, the hydrodynamic test field of PTB has been developed as a primary standard for this purpose. Up to now, commercially available flowmeters - where the material parameters of the liquid flow to be determined frequently influence the measurement result - have been used as transfer standards. The temperature and, in particular, the liquid viscosity may falsify the measurement values by several tenths of a per cent, which sometimes leads to inadmissibly large measurement uncertainties in the traceability chain.

A patented flowmeter, which has been developed at PTB, shall put things right. By means of this flowmeter, the flowrate of liquids is - for the first time - determined by measuring the force of the mass flow which acts directly on a target with the aid of a load cell. The force of the mass flow is proportional to the mean velocity of the flow. To become independent of the material properties of the liquids, a defined steady-state partial flow of the same liquid is fed to and evacuated from the steady-state mass flow. During this process, the force of the total mass flow is measured. When these partial flows are known, it is possible to determine the mass flow independently of the material properties of the liquid to be measured.

On the basis of the sensitive and exact force measuring technique, the newly developed flowmeter represents a precision measuring instrument which considerably increases the accuracy for the traceability of the mass flow. In first measurements, relative standard deviations of the order of 10-5 could be achieved under repeatability conditions. Another advantage of the device is its extremely compact design and its large measurement range. In future, the application of force measurement will offer the potential for an additional extension of the measurement range.