Guided Wave Radar Level Measurement

I was just going through Emerson Specifications for Guided Wave Radar Level Transmitters.
They do not recommend a carbon steel chamber above 4" nozzle size due to signal damping effect. I could not understand the correlation, could someone help me in this.

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As per the below emerson resource, carbon steel are not suitable for non-contact radar level transmitters as the pipe material effects the radar signal, so level measurement will be affected.

Still pipe requirement

Plastic, acrylic glass, or other non-metal materials do not shield the radar from outside disturbances and offer minimal, if any, application benefit. Due to high signal damping effect, carbon steel pipes are not suitable for non-contacting radars in pipes larger than 4 in.

Other requirements include:

  • Pipes should be an all-metal material.

  • Pipe should have a constant inside diameter.

  • The inner surface must be smooth and clear of any rough edges. (Smooth pipe joints are acceptable, but may reduce accuracy.)

  • The end of the pipe must extend beyond the zero level.

  • Avoid deposits, rust, gaps, and slots.

  • Maximum hole diameter is 1 in. (25 mm).

  • Minimum distance between holes is 6 in. (150 mm).

  • Holes should be drilled on one side only and deburred.

  • Drill one hole above maximum product surface.

  • Ball valve or other full port valves must be completely open.

  • Level will not be measured accurately below the pipe opening.

  • In heavy condensation applications, insulate the pipe part/nozzle which is outside the tank atmosphere.

Failure to follow these requirements may affect the reliability of the level measurement.