Appendix B 389
This section discusses the most common liquid flow measurement de-
vices. In general, flow sensors can be grouped into two different types
of meters:
1 Intrusive Flow Meters (Differential Pressure and Obstruction)
2 Non-Intrusive Flow Meters (Ultrasonic and Magnetic)
Choosing a flow meter for a particular application requires know-
ing the type of fluid being measured, how dirty or clean it is, the highest
and lowest expected flow velocities, and the budget.
Differential Pressure Flow Meters
The calculation of fluid flow rate by measuring the pressure loss
across a pipe restriction is perhaps the most commonly used flow
measurement technique in building and industrial applications. The
pressure drops generated by a wide variety of geometrical restrictions
have been well characterized over the years, and, these primary or
“head” flow elements come in a wide variety of configurations, each
with specific application strengths and weaknesses. Examples of flow
meters utilizing the concept of differential pressure flow measurement
include Orifice Plate meter, Venturimeter, and Pitot Tube meter. Accu-
racy of differential pressure flow meters is typically in the vicinity of
1-5% of the maximum flow for which each meter is calibrated.
Obstruction Flow Meters
Several types of obstruction flow meters have been developed that
are capable of providing a linear output signal over a wide range of
flow rates, often without the severe pressure loss penalty incurred with
an orifice plate or venturi meters. In general, these meters place a small
target, weight or spinning wheel in the flow stream that allows fluid
velocity to be determined by the rotational speed of the meter (turbine)
or by the force on the meter body (vortex).
Turbine meters
They measure fluid flow by counting the rotations of a rotor that is
placed in a flow stream. Turbine meters can be an axial-type or insertion-
type. Axial turbine meters usually have an axial rotor and a housing
that is sized for an appropriate installation. An insertion turbine meter