Ultrasonic flow measurement, also known as ultrasonic transmission principle, works on the basis of the propagation speed of sound waves, which changes in moving liquids with the flow speed of the transmission medium (see Fig. 2-15).
In this method, ultrasonic signals are alternately transmitted back and forth via the measuring probes mounted on the outside of the pipe. In resting media, the signal propagates with sound velocity c from A to B. If the liquid flows with the velocity v in the same direction as the signal, the resulting velocity is v+c. This means that the signal reaches probe B earlier than in resting media. This principle also works in the opposite direction under the same conditions. The resulting velocity is v-c. This means that the signal travels slower than in the resting medium.
From this it can be deduced that the time differences of these transit times are a measure of the speed v of the medium.
In order to determine the average velocity, the signal is sent at an angle φ from location A to B into the moving medium (see Fig. 2-15). Using the (eq. 2-24), the resulting speed of sound can be calculated from this.
The transit time tAB of the sound propagating from location A to B is obtained:
When sending in the reverse direction from probe B to A follows:
By equating and subtracting (eq. 2-24) and (eq. 2-26), with the insertion of (eq. 2-25) and (eq. 2-27), the media speed v:
By measuring the sound travel times tAB, tBA the velocity v of the medium and thus the volume flow can be determined.
For c≫v one can limit oneself to the measurement of the transit time difference ∆t=tAB-tBA. In this case it applies approximately:
A "pure" medium is a prerequisite for the application of the sound transmission principle. There must be no suspended particles in the fluid. It should also be noted that the transit time difference ∆t is dependent on material and temperature.
Vortex flow measurement
This measuring method works according to the principle of Karman's vortex street. Here, a baffle body (disturbance body, vortex body) is located transverse to the flowing fluid. Behind the body, vortices form at the same distance and alternate in opposite directions of rotation. A stable "road" can be guaranteed by the ratio of the width of the road a to the distance L of 0.2806 (aL=0.2806).
The separation of the vertebrae generates a bending vibration f (frequency of the vertebrae per second) in the vertebral body. This bending vibration is a measure for the flow velocity v. A key figure for determining the bending vibration is the Strouhal number (eq. 2-30).
From the (eq. 2-32 one can see that the time intervals t are inversely proportional to the flow velocity v and proportional to the width b.
The volume flow is calculated as follows:
This process is mainly used for gases, vapours and low-viscosity or non-conductive liquids. The accuracy of vortex flow meters is approx. 1 % of the measured value.
The vortex flowmeters require little maintenance as no moving parts are used. However, they are not suitable for small flow rates and require a long inlet and outlet section. In addition, the measuring range is limited by the Re-number and the viscosity.
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