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TABLE 19.3 Main Categories of Closed Conduit Flowmeter
Type 1—differential pressure flowmeters
Sharp edged orifice plate, chord orifice plate, eccentric orifice plate, Venturi,
nozzle, Pitot tube, elbow, wedge, V-cone, Dall tube, Elliot-Nathan flow tube,
Epiflo
Type 2—variable area flowmeters
Rotameter, orifice and tapered plug, cylinder and piston, target, variable
aperture
Type 3—positive displacement flowmeters
Sliding vane, tri-rotor, bi-rotor, piston, oval gear, nutating-disc, roots, CVM,
diaphragm, wet gas
Type 4—turbine flowmeters
Axial turbine, dual-rotor axial turbine, cylindrical rotor, impeller, Pelton
wheel, Hoverflo, propeller
Type 5—oscillatory flowmeters
Vortex shedding, swirlmeter, fluidic
Type 6—electromagnetic flowmeters
AC magnetic, pulsed DC magnetic, insertion
Type 7—ultrasonic flowmeters
Doppler, single path transit-time, multi-path transit-time, cross-correlation,
drift
Type 8—mass flowmeters
Coriolis, thermal
Type 9—miscellaneous flowmeters
Laser anemometer, hot-wire anemometers, tracer dilution, nuclear magnetic
resonance

The following sections will consider the most popular types of flowmeter from each of the eight main
categories in Table 19.3. For information on other flowmeters and those in the miscellaneous group see
one of the many textbooks on flow measurement such as [3–6].
Differential Pressure Flowmeter

The basic principle of nearly all differential pressure flowmeters is that if a restriction is placed in a pipeline,
then the pressure drop across this restriction is related to the volumetric flowrate of fluid flowing through
the pipe.
The orifice plate is the simplest and cheapest type of differential pressure flowmeter. It is simply a
plate with a hole of specified size and position cut in it, which can then be clamped between flanges in
a pipeline (Fig. 19.53). The volumetric flowrate of fluid Q in the pipeline is given by Eq. (19.66):

p
C
Q = ------------------- e --- d 2 2 p 1 –( p 2 ) (19.66)
-----------------------
1 b 4 4 r
–
where p 1 and p 2 are the pressures on each side of the orifice plate, ρ is the density of the fluid upstream
of the orifice plate, d is the diameter of the hole in the orifice plate, and β is the diameter ratio d/D
where D is the upstream internal pipe diameter. The two empirically determined correction factors are
C the discharge coefficient, and ε the expansibility factor. C is affected by changes in the diameter ratio,
Reynolds number, pipe roughness, the sharpness of the leading edge of the orifice, and the points at
which the differential pressure across the plate are measured. However, for a fixed geometry it has been
shown that C is only dependent on the Reynolds number and so this coefficient can be determined for
a particular application. ε is used to account for the compressibility of the fluid being monitored. Both
C and ε can be determined from equations and tables in a number of internationally recognized

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