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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.
