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In alumina production processes such as red mud washing and sedimentation, the choice between acoustic impedance type (usually referring to plug-in or straight-tube types) and external clamping type ultrasonic density meters is essentially a trade-off between \"measurement accuracy and durability\" and \"ease of installation\". Although both belong to ultrasonic technology, their applicability differs significantly under operating conditions involving red mud, which is highly abrasive, prone to scaling, and has a high concentration. I. Core selection comparison ① The measurement principle of acoustic impedance-type (insertion type/straight pipe section) devices is that sound waves pass directly through the medium, allowing for the analysis of its acoustic impedance characteristics. The measurement principle of the external clamping (contactless) type is that sound waves pass through the tube wall and the medium, and calculations are performed based on the time difference or attenuation. ②The acoustic impedance densitometer has extremely high wear resistance; its probe is made of alumina ceramic/sapphire, with a Mohs hardness of 9+. The external clamp-type densitometer has strong wear resistance; the probe does not come into contact with the medium, being separated by the tube wall. ③The acoustic impedance densitometer has excellent resistance to scarring and residue accumulation; its algorithm can filter out minor scarring, and the ceramic surface does not tend to accumulate residues. External clamp-type densitometers have poor resistance to scarring and residue accumulation; scarring on the inner wall of the pipeline can severely hinder sound wave transmission, leading to data drift. ④The acoustic impedance densitometer features high measurement accuracy, direct contact with the medium, strong signals, and strong resistance to bubble interference. The external clamp-type densitometer has moderate measurement accuracy; it is affected by the material and thickness of the tube walls as well as the coupling agent, and is prone to interference from bubbles. ⑤The acoustic impedance densitometer is easy to install, features flange connections, and supports online plugging and unplugging (maintenance without shutting down production). The external clamp-on densitometer requires no drilling; it can be attached directly, making it the quickest to install. ⑥The acoustic impedance densitometer is suitable for all pipe diameters (DN50-DN1000+). The external clamp type densitometer is suitable for small pipe diameters; accuracy decreases for larger diameters (>DN600). II. Selection recommendations for \"red mud operating conditions\": 1. Preferred option: Impedance type (insertion type or straight pipe section type). Application scenarios: Bottom flow in red mud sedimentation tanks, red mud washing pipelines, leaching slurry, and other key process control points. ①Specifically designed to address \"scarring\" and \"wear\" – red mud tends to form scars on the inner walls of pipes. External clamping ultrasonic waves must penetrate the pipe wall; if there is a layer of scale on the inner surface of the pipe, the sound wave signals will be significantly attenuated, resulting in failed measurements. Acoustic impedance probes (especially insertion-type ones) are typically made of alumina ceramic or silicon carbide, which have extremely high hardness; they are not only wear-resistant but also have smooth surfaces that prevent material from sticking to them. Even with slight scarring, its dedicated chirp algorithm can identify and filter out interference signals, ensuring data authenticity. ②Anti-bubble interference is common during the washing of red mud. By analyzing the ratio of reflected energy of sound waves at an interface, impedance technology can effectively distinguish between gases and solid particles, allowing for the determination of the actual densities of solids and liquids ； In contrast, the external clamping type is prone to signal scattering when encountering a medium containing bubbles, resulting in fluctuating readings. ③Although the maintenance procedure for non-shutdown operation is similar to that of plug-in devices, modern acoustic impedance densitometers are typically equipped with dedicated ball valves. These allow the probe to be removed for cleaning or replacement without having to shut down operations or drain the pipeline, thereby addressing the difficulty associated with maintaining traditional contact-type instruments. 2. Alternative: External-clamp ultrasonic densitometer. Applications: temporary measurements, old pipelines where drilling holes is not permitted, or non-critical monitoring of clean water/return water. If you can’t perform hot welding on your existing pipelines, or if you simply want to conduct a rough trend monitoring, the clamp-on method is the quickest installation option. However, under red mud operating conditions, the biggest enemies of external clamping are “pipe wall scaling” and “lining”. If the inner wall of the pipe is already scaled, external clamps are practically unusable. If a pipe has a lining (such as a rubber lining for corrosion protection), ordinary external clamp-on sensors cannot be used for measurement. III. Summary of recommendations: In the stages of red mud washing and sedimentation, to ensure stability in process control and economic efficiency over the long term, it is highly recommended to prefer density meters using wear-resistant ceramic probes of the acoustic impedance type (insertion type or straight-tube type). Although its installation is a bit more complicated, it can truly resolve the issues of “accurate measurement” and “reliability in use”.