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DO Sensors
Optical Oxygen Measurement with Built-in Electronics in a 12mm Format
Dirk Tillich, Director Marketing & Sales,
Business Unit Analytics
Hamilton Bonaduz AG
With Visiferm DO, Hamilton is the first company to offer a self-contained oxygen measurement in the typical 12 mm format similar to standard process pH electrodes and classical sterilizable oxygen sensors. Combined in the sensor shaft are high-temperature-resistant optical electronics, a microprocessor, a 4 - 20 mA analog output, a digital RS 485 interface with Mod Bus-protocol, and an ECS interface. The Electro-Chemical Sensor (ECS) interface allows Visiferm DO to be connected to existing classical measurement amplifiers (i.e. Yokogawa, Emerson, Knick or Mettler) designed for sterilizable oxygen sensors such as the Hamilton Oxyferm. Use the 4 - 20 mA analog output or the digital RS 485 interface integrated into the 12 mm shaft and an external measurement amplifier is unnecessary allowing measurement signals to be fed directly into a process control system.
Every beginning is difficult, as the proverb says. But for Hamilton Bonaduz, the decision to invest in the development of its own optical oxygen measurement for the demanding biotechnology and pharmaceutical industries was easy. After all, it is clear from other market segments how quickly and thoroughly optical oxygen measurement has established itself when compared to classical electrochemical, membrane-covered electrolyte containing sensors. The most impressive example is that of O2 measurement in wastewater. Now, roughly every second new measurement point in this area is equipped with an LDO, or light dissolved oxygen sensor. Hamilton has successfully manufactured steam-sterilizable. autoclavable,eIP compatible sensors for pH, oxidation/ reduction, conductivity and oxygen measurement for more than 10 years. These classical oxygen sensors are based, as is common for the industry on clark cell technology, in which oxygen that diffuses through a membrane is reduced on a precious metal. The involved electrons generate a very small current (nAs) which is converted to an oxygen measurement signal by a measurement amplifier. Such sensors have served well for decades, but the Visiferm optical DO sensors demonstrate a number of considerable advantages
"Sustained customer satisfaction" are among the first words Hamilton includes in its 50 year old family owned business philosophy. So it goes without saying that oxygen measurement offers, as always, the best possible measurement technology. The user of the Visiferm DOreceives more than a sensor based on another measurement principle. Visiferm DO is a symbiosis of sensor and measurement amplifier, yielding an intelligent sensor.
Technology That Sets New Standards
The designation "intelligent sensor" gains new meaning when considering the inte- . grated functions of Visiferm: • Ingenious measurement optics, stable to 130 "C,with a symmetrically oriented diagnostic and measurement design;
• Temperature-resistant electronics built into the 12mm shaft;
• Replaceable sensor cap contains the sensing element
• Digital or analog communication via a proven VP 8.0 connector head with PG 13.5 process thread connection;
• Monitoring of all sensor functions, including status diagnosis of the replaceable sensor cap, with corresponding signals via the 4-20 mA and digital interface. A history of the self-monitoring
is recorded in the sensor; and
• Configurable through the RS 485 interface with a notebook, PC or via the Mod-Bus connection from the process control system, or with Hamilton Visical or Visivisi modules.
Hamilton's Lightning Bright Optics
While other suppliers of optical oxygen sensors utilize sensitive light conductors, a single light channel or two different LEDs, Hamilton prefers a symmetrical design that is mechanically and thermally stable
The Visiferm DO Measurement Principles This unique design allows Hamilton to monitor the status of the blue LED with one of the photodiodes. The other photodiode with the red filter measures the oxygen dependent red light. The red light is generated on the luminophore through luminescence (fluorescence) after stimulation by the blue light. Electrons are excited to a higher energy level, and return to their original level after emission of red light.
When the luminophore comes into contact with elemental oxygen, the O2 molecules absorb the energy, resulting in reduced intensity of red light emission.
This difference in intensity is analyzed within the instrument's self-monitoring to pinpoint photo bleaching (bleaching of the luminophore).
For the measurement of oxygen concentration, the optical phase shift between blue and red light pulses is measured with high precision. The luminophore's excited electrons will remain in this state for some time. In the presence of oxygen they return to their ground state more quickly. Between the pulsing excitation of the luminophore with blue light and the emission of red light there is an oxygen-dependent time shift, which can be measured as an angle of phase. The entire measurement, calculation and output of the measured value occur inside the sensor.
It should be noted that Visiferm sensors measure the partial pressure of oxygen pOz just as classical sensors do; this can for be displayed as percent air saturation, concentration in mg/l, ppm or even as ppb.
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.
