Measurement principle
Time difference method: there is a time difference when sound waves propagate the same distance in the fluid upstream and upstream, and the difference in propagation time is related to the flow speed of the measured flow body. The flow rate of the fluid is obtained by measuring this time difference, and then the flow rate is calculated.
Doppler method: Based on the Doppler principle in acoustics, the frequency offset between the reflected sound wave and the emitted sound wave is detected to determine the speed of moving particles in the fluid, so as to indirectly obtain the flow speed and flow rate of the fluid, requiring that there are suspended objects such as bubbles and solid particles in the water to reflect the ultrasonic beam.
Applicable fluid
Time difference method: suitable for the measurement of pure, less impurities of the fluid, such as crude oil, heavy oil, pure water, deionized water, chemical solvents, etc., if the fluid contains a large number of bubbles or solid impurities, will hinder the normal propagation of ultrasonic waves, affecting the measurement accuracy.
Doppler method: often used to measure cloudy fluids containing bubbles, solid particles and other impurities, such as sewage, mud, oil-water mixture, etc., widely used in natural waters, industrial wastewater treatment and other fields.
Measurement accuracy
Time difference method: the accuracy is relatively high, up to ±0.5%, and its measurement accuracy is affected by the accuracy of the signal processing electronic components and the stability of the speed of sound.
Doppler method: The accuracy is generally lower than the time difference method, usually about ±2%, because its measurement results are based on the scattered signal of suspended particles or bubbles in the fluid, and the distribution and motion of these particles may be uneven, which will have a certain impact on the measurement accuracy.
Piping requirements
The time difference method: the flow state of the fluid in the pipeline is more strict, the fluid needs to be more stable and uniform, and the straight pipe section has certain requirements, usually the length of the upstream straight pipe section is not less than 10 times the diameter of the downstream straight pipe section is not less than 5 times the diameter of the pipe.
Doppler method: The requirements for the pipe segment are relatively not strict, the installation position is more flexible, and the flow pattern in the pipeline is less affected.
Sensor structure and installation
Time difference method: one or more pairs of sensors are usually used, respectively installed in the upstream and downstream of the pipeline, and the transmitting and receiving direction of the sensor needs to maintain a certain Angle with the pipeline axis to ensure that the transmission time of the ultrasonic signal is accurately measured. During installation, the sensor must be aligned and fixed to ensure the stability of signal transmission.
Doppler method: The general use of a single-head sensor, including sending and receiving piezoelectric crystals, can be installed on the outer wall of the pipeline, or can be inserted directly in contact with the fluid, easy to install, and will not cause damage to the pipeline.
Anti-interference capability
Time difference method: easy to be affected by pipeline vibration, fluid temperature and density changes and other factors, pipeline vibration may lead to sensor position deviation, affecting the measurement accuracy, and the change of temperature and density will cause the change of sound velocity, temperature compensation and other measures are needed to improve the measurement accuracy.
Doppler method: strong anti-interference ability, insensitive to pipeline vibration, and because its measurement principle is based on Doppler frequency shift, and the temperature and density of the fluid has little relationship, so it can still maintain good stability under complex working conditions.
Measuring range
Time difference method: The measurement range is wide, can measure low flow rate to high flow rate fluid, suitable for a variety of pipe diameters, but for large pipe diameters, high flow rate measurement, may require higher transmission power and more complex signal processing technology.
Doppler method: generally suitable for low and medium flow rate fluid measurement, for high flow rate fluid, because the relationship between Doppler frequency shift and flow rate is not linear, may lead to increased measurement error.
Post time: Dec-09-2024