If insert a bluff body vertically into the flowing fluid, vortexes will be generated alternatively at its sides. These vortexes follow together with the fluid to the down stream, and form series vortexes (Karman vortex street ,see figure1). The bluff body which the generates vortex is called as vortex shedder. Experiment proves that frequency of vortex is in directly proportional to flow velocity it can be shown as following formula:

　　　　　　　　　　Figure 1 Diagram of vortex formed
-Vortex frequency
d-Width of buff body which face against the flow
Sr-Strouhol number
V—Average flow velocity in the pipe
D— Inside diameter of pipe
The experiment proves that if the distance between two vortex series. h and The distance between two adjacent vortexes in same series L are satiated with The formula L/h=0.218, then the non-symmetric vortex series can be kept in steady status. If the renolds number Re of flow varies within 5000 to 150000, Sr will basically be kept unchanged. So when Sr and d are constant, f will be directly proportional to the average flow velocity of the fluid, i.e. f is directly proportional to the volumetric flowrate Q of the fluid, but has no relation to the parameters as pressure, temperature, density, etc..
When vortexes are generated at both sides of the bluff body, the bluff body is subjected by the a action direction, signal is induced. The alternating freguency of the ascending force is same as the frequency of the vortex.
After the sens or transmitted the signal to the converter, the signal is amplified and shaped, the impulse signal which is linearly proportional to the velocity is got. This signal is output directly or converted into a 4~20mA standard output. The relation ship between the volumetric flowrate Q and frequency f is: