Thus, a stationary object and pressure sensor installed in the middle of a pipe section constitute
a form of flowmeter called a vortex flowmeter. Like a turbine flowmeter with an electronic “pickup”
sensor to detect the passage of rotating turbine blades, the output frequency of a vortex flowmeter
is linearly proportional to volumetric flow rate.
The pressure sensors used in vortex flowmeters are not standard differential pressure transmitters,
since the vortex frequency is too high to be successfully detected by such bulky instruments. Instead,
the sensors are typically piezoelectric crystals. These pressure sensors need not be calibrated, since
the amplitude of the pressure waves detected is irrelevant. Only the frequency of the waves matterfor measuring flow rate, and so nearly any pressure sensor with a fast enough response time will
suffice.
Like turbine meters, the relationship between sensor frequency (f) and volumetric flow rate
(Q) may be expressed as a proportionality, with the letter k used to represent the constant of
proportionality for any particular flowmeter: