Rosemount Dissolved Oxygen Analyzer Calibration


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Hi Everybody
In Our Thermal Power Plant Facing A Problem With Desolved Oxygen Analyzer.

Recently DO Analyzer in our SWAS Analysis Room Showing More Value in local meter >50 ppb. so i went to SWAS Lab to Calibrate the Sensor. Here i have Checked in 3 Methods

1. Air Calibration: Here i Removed my Sensor from Process and Made dry, later Air Calibration Done. Message Came "Calibration Success," but DO Sensor after kept in Process it went to negative value.

2. Zero Calibration: Here i Have taken 5% of Sodium Hydroxide Solution and Dipped Sensor in it and checked to have stable value and stuck at 35ppb then zero calibration done. Message came "Calibration Success" later after some time observed in display negative value.

3. Process Calibration: Here according to the process DO value to be entered after informing to Chemist. So Value entered, then it showed a Message "Calibration Error."

Out of these 3 Procedures in the Manual problem is not solved still it is showing -ve value. so i made factory reset for the analyzer and repeated the same process but no result.

I'm unable to find the solution. Can anyone Please tell me what does mean this -ve ppb display and how to rectify it?