The industry's most authoritative handbook on flow measurement provides a road map to the field of flow measurement. This best-seller discusses strategies for problem solving and puts the whole array of types of flowmeters at the reader's disposal. The text includes laminar flow elements, critical flowmeters, statistics for measurement, laboratory primary standards, and uncertainty in flow measurement. Emphasis is placed on the importance of accuracy in measurements and ways of ensuring accuracy and avoiding equipment damage through correct forecast of operating conditions, flowmeter selection, installation, calibration, and maintenance. Fundamental considerations such as mixed-phase flow, piping effects, and flow conditioning are examined at length. The problem of attaining a meaningful flow signal through linearization, compensation, and totalization is discussed. Join the thousands of engineers, technicians, managers, and salespeople that have found this reference text an invaluable resource.
TABLE OF CONTENTS Chapter 5 - Linearization, Compensation, and Totalization of Flow Signals
With few exceptions, the purpose of a flowmeter is to generate a signal that is
directly proportional to flow rate. Certain types of flow signals are not necessarily
proportional to flow, i.e., they are nonlinear. Furthermore, if fluid properties such
as density, viscosity, and temperature are not constant, the signals from many
flowmeters become incorrect and no longer represent true flow.
"Linearization" of a flow signal is the characterization of a nonlinear signal to
form a linear signal.
"Compensation" of a flow signal is the mathematical combining of the
flowmeter output signal with other signals representing the fluid properties in
such a way as to form a true flow signal.
Often one or both of these functions are required to attain a true linear flow
signal.
Linearization
Many types of flowmeters produce a linear signal, meaning that the magnitude
of the signal is directly proportional to the flow rate of fluid passing through the
meter. This group includes turbine meters, magnetic meters, positive displacement
meters, rotameters, and others. Figure 5-1 shows the output signal vs. flow rate
characteristic of these meters.
Certain types of flowmeters produce nonlinear signals. The most common of
these are the differential pressure meters in which the output signal varies as the
square of the flow rate. Figure 5-2 shows this relationship. Notice that a change in
flow at low rates produces a very small signal change. The same change in flow at
high rates produces a much greater signal change. For example, a 0 to 10% flow
gives 0 to 1% signal change, whereas 90 to 100% flow gives 81 to 100% signal
change. Open channel meters generate signals that are exponential curves. For
rectangular weirs and many flumes the exponent is approximately 1.5, and for
V-notch weirs the exponent is 2.5. Figure 5-3 illustrates some nonlinear flow signal
characteristics.
Flowmeters that normally produce linear or squared signals are sometimes used
to measure fluids with high viscosity. The output signal, although repeatable, may
not adhere to the theoretical linear or squared characteristic. These signals must
also be considered nonlinear. Linearization is the conversion of a nonlinear signal
to a linear one. It can be performed by a separate transducer, by components in the
receiving instrument, or by components located in the flow transmitter itself (see
Figure 5-4).
