em-tec's non-invasive flow measurement method is based on the ultrasonic transit-time method.
This method determines the transit-time difference of ultrasonic signals through a measuring section and is used to evaluate volumetric flow rates.
Depending on the sensor construction, this means that at least two sound transducers are needed in order to send and receive acoustic wave packets both in and against the flow direction.
For a higher accuracy, our Clamp-On Transducers incorporate two pairs of sound transducers. They consist of four piezo ceramics and are arranged in a X configuration around the tube containing the measured flow.
When sending ultrasonic sound signals through the measuring section, the transit time depends on the flow direction of the medium:
- If the ultrasonic sound signals are sent along the flow direction, the downstream signal needs less time.
- If the ultrasonic sound signals are sent against the flow direction, the upstream signal needs more time
- The evaluation of the received ultrasonic signals is based on the upstream and the downstream measuring section.
Consequently, the transit-time is measured for every pulse. The difference between upstream and downstream measurements is proportional to the volumetric flow rate.
The picture below shows the transmitting impulses and the resulting received signals. Through the difference in the received signals, the transit-time can be determined.
Since the difference in transit-time is very slight (picoseconds), the calculation is done with a multi-step correlation method, which is a rather elaborate mathematical process.
With an increasing flow, the transit-time difference of the received signals rises as well. By adjusting the sensor, it is possible to convert the transit-time into a flow.