Non-Contact Radar level sensor doesn't show the actual level, stuck at a particular value

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!
 
> 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.

What about swapping the bad boy radar with the working radar on the other water tank and vice versa?

If the tanks and radar nozzles are the same, the working radar should work on the other tank and you can see what happens when the bad boy goes on the other tank.
 
> 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.

What about swapping the bad boy radar with the working radar on the other water tank and vice versa?

If the tanks and radar nozzles are the same, the working radar should work on the other tank and you can see what happens when the bad boy goes on the other tank.
We don't have a spare radar LT lying around just in case the other one too goes bad. So, our team doesn't want to take the risk of disturbing the other one. If it's a transmitter failure as you suggest, I don't see why it's working outside the tank or in the lab.
 
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!

What is the MOC of your tank?? And at what frequency is the transmitter working?? Also do an echo curve mapping and check if it solves the problem. Another thing I want to say is that for a 1.9m tank, your range for a non contact radar tx can never be 1.9m. Any Tx employing the time of flight measurement technique, there is always a blind zone both at the top and at the bottom which the Tx cannot sense. It varies for diiferent antenna type and OEM but you need to take that into account in your span calculation.
 
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