measuring flow rate is the most
often performed measurement in industrial applications, because fluids are moved
around in almost all technological processes and we need to know how
much fluid is moved (especially so if fluids are mixed to form a
mixture with prescribed composition - or simply whenever transported
fluid has to be paid for). The ubiquitous nature of this measurement has
led to development of a large number of methods, which form the subject | |
of extensive literature. Most methods
used in practice are indirect nature - which means that the flowrate sensors based upon them
must be first calibrated. Usually, the calibration is peformed by comparison with one
of the two direct methods:
Liquid flow: is measured by weighing the outflowing liquid collected
during a time period. It follows from the definition
that mass flow rate is evaluated from the measured
according to the formula 
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Gas flow:
the small weight in a much more heavy
vessel would be difficult to measure accurately.
It is, therefore, experient to measure the volume flow rate
by storing the captured outflowing
gas under the movable, water-sealed bell
according to
Fig.A-19. From the definition
follows that the mass flow rate is evaluated
as 
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