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In the red mud washing process, although the fork-type density meter performs well in clean liquids, it turns out to be very fragile under the extreme operating conditions of red mud washing, which are characterized by high wear and tear, tendency to form scabs, and high alkalinity. In contrast, the ultrasonic impedance densitometer has become the ideal choice for this application due to its unique principle and materials. I. Comparison of core performance: ① The fork density meter has extremely poor wear resistance, as it relies primarily on the vibration of two slender metal prongs for measurement. Red mud particles have high hardness and are known as industrial sandpaper; under high-speed scouring, the fork arms can easily become dull, worn down, or even broken, resulting in the destruction of the instrument. The ultrasonic acoustic impedance densitometer boasts excellent wear resistance. The measuring surface of its probe is made of high-purity alumina ceramic or sapphire (with a Mohs hardness of 9 or higher). This material acts like armor, protecting the probe from the intense erosion caused by red mud. As a result, the densitometer can last for up to 10 years—even as long as the pipeline itself. ②The anti-caking/fouling performance of tuning fork densitometers is extremely poor; they are highly sensitive to fouling. Once scaling occurs on the surface of the forks, it effectively increases their mass, resulting in significantly inflated readings. Severe scarring can even encase the tuning fork, preventing it from vibrating and causing it to “freeze up”. The ultrasonic densitometer exhibits strong anti-scaling/deposition capabilities. Based on an analysis of the propagation characteristics of sound waves, its intelligent algorithm can detect and filter out the effects caused by minor scaling on the probe surface, calculating only the true density of the flowing medium. As a result, its operational data remains stable over the long term. ③The tuning fork densitometer has poor resistance to bubble interference; bubbles attached to the prongs alter the vibration damping, resulting in erratic or low readings that do not reflect the actual solid content, which can lead to errors in controlling the washing efficiency. Ultrasonic density meters have an extremely strong ability to resist interference from bubbles; by utilizing acoustic impedance technology and specialized algorithms, they can effectively distinguish the acoustic properties of gases from those of solids/liquids, filter out bubble-related interference, and determine the true density of solids and liquids, thereby ensuring the accuracy of alkali recovery control. II. Cost Efficiency and Maintenance Costs: In addition to their overwhelming advantages in terms of performance, ultrasonic density meters also offer significantly better economic benefits compared to fork-type density meters: ① Maintenance costs are extremely low: With fork-type density meters, replacement may be necessary every few weeks or even days under sludge conditions, resulting in frequent maintenance and high costs for spare parts. Ultrasonic waves: It essentially achieves \"maintenance-free\" operation, requiring only occasional checks on probe wear, which significantly reduces labor and spare parts costs. ②Avoid production losses: Ultrasonic density meters typically support online plug-and-play installation, allowing for cleaning or replacement without shutting down the factory, thus preventing production losses resulting from downtime to address instrument failures. By providing real-time, accurate density data, the amount of washing water added and the number of washing stages can be optimized. This directly helps to reduce the alkali consumption per ton of alumina and improve the reuse rate of recycled water, and the cost savings resulting from this far exceed the purchase cost of the instruments themselves. III. Conclusion: In the critical process step of red mud washing, the fork-type density meter is completely unsuitable due to its fragile structure and susceptibility to interference from scabs and bubbles. With its advantages of accurate measurement (resistance to bubbles), durability (wear resistance), and low maintenance needs, the ultrasonic impedance densitometer has become the best solution for replacing traditional instruments, enabling cost reduction, improved efficiency, and precise control.