We have a radar level transmitter (Emerson Rosemount 5400 series, 4" process seal type antenna), mounted on top of a water tank of height 1.9 m . Its level reading is stuck at 1500 mm irrespective of the actual level. When we dropped the transmitter from line and tested it in our lab with a metal obstruction, it sensed the distance accurately. But when we mounted it on the tank, its value is stuck again at 1500 mm and distance value at 400 mm. We checked the tank for obstructions in the path of the radar beam but it was all clear. The nozzle on which it's mounted is around 2.5" and the nozzle flange diameter comfortably seats the process seal antenna. Even when the tank is empty, the reading shows 1500 mm level and 400 mm distance. We even tested the transmitter in the field, near the tank with some obstructions. It was showing the correct distance values which we confirmed using a measuring tape. The level and distance reading is stuck only when we mount it on the tank nozzle. The tank has an agitator in the center and the transmitter is mounted near the outer wall, 350 mm away from the wall. The echo analyzer shows the strongest echo at 400 mm distance and the actual tank bottom echo's amplitude (when tank was empty) is comparatively very small. It has been working alright on the same tank for 3 years and suddenly got stuck last week. There's another tank of the same dimension with the same model radar transmitter. It's also in water service and works fine still now. Could any of you guys suggest what might be causing the false reading when the LT is mounted on the tank? Any suggestion would be helpful!