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[Instrumentation Table of Contents]
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Flow meters operate on the principle of
placing a restriction in the line to cause a differential
pressure head. The differential pressure, which is caused by
the head, is measured and converted to a flow measurement.
Industrial applications of flow meters incorporate a
pneumatic or electrical transmitting system for remote
readout of flow rate. Generally, the indicating instrument
extracts the square root of the differential pressure and
displays the flow rate on a linear indicator. There are two
elements in a flow meter; the primary element is the
restriction in the line, and the secondary element is the
differential pressure measuring device. Figure F1 shows the
basic operating characteristics of a flow meter.
Figure F1
The flow-path restriction, such as an
orifice, causes a differential pressure across the orifice.
This pressure differential is measured by a mercury manometer
or a differential pressure detector. From this measurement,
flow rate is determined from known physical laws.
The flow meter actually measures volume flow
rate rather than mass flow rate. Mass flow rate is easily
calculated or computed from volumetric flow rate by knowing
or sensing temperature and/or pressure. Temperature and
pressure affect the density of the fluid and, therefore, the
mass of fluid flowing past a certain point. If the volumetric
flow rate signal is compensated for changes in temperature
and/or pressure, a true mass flow rate signal can be
obtained. In Thermodynamics it is described that temperature
and density are inversely proportional, while pressure and
density are directly proportional. To show the relationship
between temperature or pressure, the mass flow rate equation
is often written as either Equation E-1 or E-2.
E-1
E-2
The flow coefficient is constant for the
system based mainly on the construction characteristics of
the pipe and type of fluid flowing through the pipe. The flow
coefficient in each equation contains the appropriate units
to balance the equation and provide the proper units for the
resulting mass flow rate. The area of the pipe and
differential pressure are used to calculate volumetric flow
rate. As stated above, this volumetric flow rate is converted
to mass flow rate by compensating for system temperature or
pressure.
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