KROHNE OPTISENS PH 9100 EN - Manual (Page 37)

SERVICE
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OPTISENS PH 9100
www.krohne.com 09/2012 - 4002314801 - MA OPTISENS PH 9100 R02 en
5.1 Maintenance
5.1.1 Cleaning
• Slight dirt residues or dust: Rinse the sensor tip with tap water and clean it with a soft tissue.
• Oily and greasy coatings: Remove with a warm soap solution and rinse with water.
• Hardness deposits or metal hydroxide deposits: Remove with 10% citric acid or hypochloric
acid and rinse with water.
5.1.2 Aging and re-calibration
During operation, but already during storage, pH senors age due to poisoning effects of the inner
buffer system. Therefore it is important to re-calibrate the sensors in regular intervals as
described.
When the sensor becomes too old to provide reliable measurements, the signal converter
displays an error message after the calibration procedure. In this case, the sensor has to be
exchanged.
Aging effects pH sensors:
• Decrease of slope due to abrasion, drying, corrosion of glass bulb coating and leaching: The
slope should be > 50 mV/pH. An optimal value is 59 mV/pH at 25°C / 77°F. When the slope
drops below 50 mV/pH, an error message is displayed and the sensor has to be exchanged.
The slope of the sensor is displayed after each calibration procedure.
• Shift of zero point due to leaching / contamination of reference half cell or increased
resistance between glass bulb and reference half cell: The zero point should lie between
-58...+58 mV at pH 7. An optimal value is 0 mV at pH 7.
The zero point of the sensor is displayed after each calibration procedure.
The following figure shows the aging effects:
Figure 5-1: Aging effects of sensor
1 Horizontal shift of slope
2 Vertical shift of slope
3 Decrease of slope