DP Level control on Distillation Column Sump


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We have a Rosemount DP level transmitter installed on a bridle on the side of a distillation column. It tends to read consistently with the installed sightglass for long periods of time, but will experience episodes where it deviates quite badly. The Bridle is open to both the liquid and vapor side of the sump. I was wondering if anyone knew of any issues that would make the levels deviate. The nozzles on the complete system are steam traced to prevent freezing in the system.
Wet leg? Dry leg? Remote seal(s)?

"Deviates quite badly" means?

Always low by 15%? Always high by 22%? High in daytime? Low at nighttime? Good after turnaround, error gradually goes negative over course of days? DCS reading jumps around every time the compressor turns on? Every time it rains the DP signal loses 3"? Do the welders hang their hoses over the 3 valve manifold valve handle tees?

Trend it and look for patterns.