Ultrasonic flow meters are commonly applied to measure the velocity of liquids that allow ultrasonic waves to pass, such as water, molten sulfur, cryogenic liquids, and chemicals. Transit time designs are also available to measure gas and vapor flow.
Be careful because fluids that do not pass ultrasonic energy, such as many types of slurry, limit the penetration of ultrasonic waves into the fluid. In Doppler ultrasonic flowmeter, opaque fluids can limit ultrasonic wave penetration too near the pipe wall, which can degrade accuracy and/or cause the flowmeter to fail to measure.
Transit time ultrasonic flowmeters can fail to operate when an opaque fluid weakens the ultrasonic wave to such an extent that the wave does not reach the receiver. Transit Time flowmeters must have a pair of transducers, each containing a piezo-electric crystal. One transducer transmits sound while the other acts as a receiver.
Transit Time flowmeters measure the time it takes for an ultrasonic signal transmitted from one sensor, to cross a pipe and be received by a second sensor. Upstream and downstream time measurements are compared. With no flow, the transit time would be equal in both directions. With flow, the sound will travel faster in the direction of flow and slower against the flow.
Because the ultrasonic signal must cross the pipe to a receiving transducer, the fluid must not contain a significant concentration of bubbles or solids. Otherwise, the high-frequency sound will be attenuated and too weak to traverse the pipe.
Which is a concern when we apply the ultrasonic flow meter be sure that the fluid can adequately conduct ultrasonic waves because the flowmeter will not measure when the ultrasonic waves cannot penetrate the flow stream between the transducers. Similarly, ultrasonic waves must be able to penetrate the fluid for Doppler flowmeters to operate accurately.
When the fluid is relatively opaque and does not penetrate the fluid, Doppler flowmeters tend to measure the velocity of the fluid at or near the pipe wall, which can cause significant measurement error and/or cause the flowmeter to fail. Kind Of Ultrasonic Transit Time Flow Meter