Optical dissolved oxygen sensors rely on an oxygen sensitive luminescent coating to perform the measurement.
Causes of optical cap degradation
Process exposure over time can degrade the luminescent coating. Sensor and DO cap life is heavily dependent upon specific conditions of the application, with elevated temperatures, pressures, and chemical exposure potentially accelerating the ageing process of both the sensor and ODO cap.
An integrated quality indicator is utilized to predict remaining sensor and cap life time. A verbal and corresponding numerical value of quality indicator are tabulated below:
|Quality Indicator||Cap Status|
|81 to 100%||Excellent|
|61 to 80%||Good|
|46 to 60%||Acceptable|
|26 to 45% (Advise replacement at 35%)||Poor|
|11 to 25%||Very Poor|
|0 to 10%||Replace|
Viewing optical DO cap quality
The quality indicator for the cap can be viewed using ArcAir software by using the Sensor Quick View option under the Info tab. Additional information such as operating hours, sterilization cycles, and other information can also be viewed.
A example of sensor health information provided by ArcAir
The sensor cap combination will perform reliably within published specifications within the “Excellent” and “Good” range, however may start to lose measurement robustness as cap quality continues to decrease. Lower cap quality sensors may experience more drift, have a lower stability signal, and require more frequent calibrations in order to maintain accurate measurement. At 35% cap quality, a continuous warning will be output by the Visiferm sensor, advising replacement. This is the recommended cap quality percent for replacement of the ODO cap outlined in the Hamilton Visiferm DO Manual.
Some customers may replace ODO caps as part of a preventative maintenance program before reaching 35% cap quality. Choosing when to replace the cap is dependent upon individual needs and process variables.