Why the clamp-on ultrasonic flowmeter sensor may not be used well in occasions where IP68 is required?

When the external clamp sensor is installed, the coupling agent is used to couple the sensor and the pipe, but when working in an IP68 environment, the sensor and couplant are both immersed in water, and the couplant works in water for a long time, which affects the measurement effect of the external clamp sensor , Increase maintenance workload.