Turbine meters
It is a rotating-wheel type magnetic flow meter which is used to measure water flows in rivers and streams. As the fluid moves through the meter, it causes a rotation of the small turbine wheel. A permanent magnet is encased in the rotor body such that the change in permeability is noticed when the rotor blade passes through the pole of the coil. The change in the permeability of the magnetic circuit produces a voltage pulse at the output measuring terminal. The rotor motion is proportional to the volume flow rate and is captured by an inductive coil. A flow coefficient 
for the turbine meter may be defined based on the flow rate 
and kinematic viscosity of the fluid.
 |
(7.4.2) |
This particular device indicates the flow accurately within ±0.5% over wide range of flow rates.
Vortex flow meters
When a flow stream encounters an obstruction in its path, the fluid separates and swirls around the obstruction. This leads to formation of vortex and it is felt for some distance downstream. It is a very familiar situation for turbulent flows and a short cylinder placed in the flow sheds the vortices along the axis. If the vortices are periodic in nature, then the shedding frequency is proportional to the average flow velocity. In other words, the flow rate can be determined by generating vortices in the flow by placing an obstruction along the flow and measuring the shedding frequency. For an incompressible fluid, a dimensionless parameter known as Strouhal number 
is defined as function of vortex shedding frequency 
, characteristic dimension of the obstruction 
and the velocity of flow 
impinging the obstruction.
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(7.4.3) |
Fig. 7.4.2: Operation of a vortex flow meter.
A vortex flow meter works on the above principle is shown in Fig. 7.4.2. It consists of a bluff body placed in the flow that serves as vortex generator and a detector placed at certain distance downstream on the inner surface of casing records the shedding frequency. A piezoelectric sensor mounted inside the vortex shedder detects the vortices and subsequently amplified to indicate either instantaneous flow rate or total flow over selected time interval. With prior knowledge of calibration constant 
and characteristic length dimension of the bluff body, the average flow velocity can be obtained. The vortex flow meters operate reliably between the Reynolds numbers of 104 to 107 with an accuracy of 1%. It is generally not suitable for use of high viscous liquids.