From the sound of it you will need to work with very high frequencies - 50 MHz or higher, consider that the resolution limit is correlated with the ultrasonic wavelength. At the same time you will face high attenuation in the fluid medium. Therefore you might not be able to execute such measurement depending on the pipe diameter.

Hello Wan Muhammad Syazwan Wan Musa ,

If your ultrasonic measurement depends on sand particles impact on the walls of the pipe, then the frequencies generated by such impacs are in the order of 100 to 500kHz. You can refer to :

Brown, G.K., Davies, J.R., Hamblde, B.J. 2000. Solids and Sand Monitoring- An Overview, NACE International, 2000, Houston, Texas, NACE-00091

Also there are commercial passive ultrasonic sensors for the oil and gas industries:

https://www.clampon.com/products/

https://www.aquip.com.au/products/clampon-dsp-acoustic-sand-detector/

And if you are interested on the computational model for sand contents to develop a contactless sand production instrument:

https://openpetroleumengineeringjournal.com/VOLUME/10/PAGE/1/FULLTEXT/

Regards

Thank you for the answers.

Martin, such product is very expensive due to the application is mostly used for oil&gas industry. For now, it is best for me to build my own sensor to detect the particle and hopefully can discover new things through research.