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Operational Reliability Paramount
Whether a purification plant manager is upset over higher energy consumption, because the oxygen measurement in the aeration tanks indicates values that are too low, or the fermentation in a pharmaceutical operation fails because the oxygen sensor, newly sterilized at 130°C, suddenly stops reporting useful values, every such problem is one too many.
The most common malfunction with classical Clark Cells is damage to the mechanically sensitive oxygen membrane. If the membrane is seriously damaged, chances are good that a visual check will catch the problem before use. But if only a small, unrecognizable defect arises on the membrane, this becomes apparent only as the electrolyte leaks out and the sensor stops working.
The very small electric currents (nAs) represent another typical problem area, since these are transmitted to a measurement amplifier via cable in a rough operational environment, even slightly moist or dirty contacts from sweaty fingers, can result in chaotic or irreproducible signals. In addition, small fluctuations in temperature or vibrations can alter cable resistances noticeably. Over time, cables that have become damp, and especially damp cable connections are often the cause of problems in oxygen measurement, and the measurement of other parameters such as pH.
Measurement results from Visiferm can be exported from the sensor as 4-20 mA or digitally. Both types of signal are clearly more tolerant of difficult process conditions than the sensitive (nA) signals of a classical dissolved oxygen sensor.
Signal Availability
A measuring point that delivers no signal can cause great damage. The signal for process control depends on all the components necessary for that signal. For the classical oxygen measurement these are: the sensor, sensor cable, measurement amplifier, its power source, the cable from the measurement amplifier to process control, and usually a separate amplifier for galvanic separation and/or voltage surge protection. The weakest link contributes the most to system failure. In classical systems this is clearly the membrane-covered sensor first and secondly the cable to the measurement amplifier. With Visiferm, the especially vulnerable small currents are dispensed with and a considerably insensitive analog mA or digital signal is supplied as the measurement amplifier is built right into the sensor. Instead of a disturbance-vulnerable membrane, Visiferm has a robust sensor cap as a sensitive and selective element. Visiferm even tolerates low to medium damage to the sensor cover. The sensor is specified for operating pressures up to 12 bar and temperatures up to 130°C.
Accurate Trace Measurements
Trace measurements with classical sensors are not considered particularly accurate, since in the absence of oxygen no O2 molecules can be reduced, as a result no current flows. The same happens when a cable break occurs. This is different with Visiferm: at low oxygen concentrations the greatest amount of red light is emitted. Thus the function of the sensor is easily monitored during trace measurement.
Operational Reliability Through Simple Maintenance
User friendliness is a critical concern for sensors. In particular simple maintenance, since for example in the case of a nightshift problem the personnel on hand must be able to carry out the necessary maintenance and calibration. Every procedure saved signifies an increase in operational reliability. Visiferm obliges here: if the sensor cover should need to be exchanged, this can be done as simply and easily as opening and closing a bottle of soda. The sensor cover twists off and a new one twists on. To achieve high measurement accuracy, calibration need only be done in air, or if necessary in nitrogen or carbon dioxide.
Areas of Application for Visiferm DO
Visiferm DO sensors have been evaluated in a variety of applications. The sensors were developed to be steam-sterilized and autoclaved without any troubles. Typical CIP cleaning is also tolerated very well. These properties along with the standard design form of a classical 12 mm sensor with a PC 13.5 thread make Visiferm DO superior for use in fermenters and other similar demanding applications.
Applications in addition to biotechnology include wastewater processing. In breweries Visiferm is already being used to monitor carbon dioxide recovery. Tests in the area of bottle filling are currently being conducted. Visiferm Do is already available in various shaft lengths: 120, 160,225, 325 and 425 mm.
