Dissolved Oxygen Sensors
Why measure dissolved oxygen online?
In biological processes like fermentation and cell culture dissolved oxygen is critical to support proper cell growth. In industrial processes, oxygen is often limited in applications where corrosion is of concern. Online oxygen measurement enables real-time control of dissolved oxygen at a predefined set point or enables the detection of a leak in the process that is resulting in oxygen infiltration.
Optical
Polarographic
Single Use Optical DO
Polarographic vs. Optical
For over 60 years polarographic sensors have been used to measure dissolved oxygen through the electrochemical reduction of oxygen with a noble metal. More recently optical oxygen has gained prominence by eliminating polarization time, electrolyte, and complicated maintenance, by requiring only a single replacement part.
Optical Oxygen Sensors
The VisiFamily of sensors have a blue LED in the sensor shaft and an oxygen sensitive dye (luminophore) fixed to a glass window in the replaceable sensor cap. The blue light excites the luminophore and then a photodetector in the sensor shaft senses the red light that is returned from the luminophore. A microprocessor in the sensor correlates the emitted blue light and the measured red light to partial pressure of oxygen. This measurement can then be output as a simulated nA signal, 4 - 20 mA, or digital protocol.
Polarographic "Clark Cell" Sensors
Classical Clark cell sensors have a platinum cathode and a silver anode that are separated from the sample by a gas permeable membrane. As oxygen diffuses across the membrane it is reduced resulting in a small nA current. The more oxygen the stronger the current. This current is then detected by a separate transmitter which amplifies the signal for process control.
Specific Applications
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App Note - Arc DO, pH, Cond in production scale fermentation process
GEA, a global equipment and process technology provider, integrates Arc sensor technology in fermentation plants at their GEA Diessel subsidiary in Hildesheim, Germany.
Resources
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Brochure - Optical Dissolved Oxygen
Download the dissolved oxygen brochure to explore the many benefits of making the switch to optical oxygen measurement. -
Guide - Dissolved Oxygen Measurement
This Hamilton guide provides the basic knowledge of dissolved oxygen measurement in a brief and understandable form. -
Brochure - BioPharma
Innovative solutions for process analytics in biotechnological and pharmaceutical industries.
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Does salinity impact my DO measurement?
Yes, salinity of a solution generally has an inverse relationship to the dissolved oxygen content within the solution. Arc sensors can have the salinity value of the liquid added in to correct the measurement by using ArcAir. -
Does temperature impact my oxygen measurement?
Yes, in a closed system, gas phase temperature is directly proportional to pressure and therefore measured oxygen content. In solution, temperature is inversely proportional to dissolved oxygen content. -
Does carbon dioxide sparging impact my DO measurement?
CO2 sparging will not impact the DO measurement using an optical DO sensor. CO2 can impact traditional polarographic DO sensors over time thus more frequent electrolyte replacement may be needed.
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Dissolved Oxygen Knowledge Base Articles
A summary page with links for all dissolved oxygen related articles -
Best Practices: Zero Point Oxygen Calibration
There are several methods to achieving a good zero point calibration of optical and polarographic DO sensors. This paper looks at each procedure in depth. -
How to Measure Sensor A-length
Sensor A-length determines the insertion depth of the sensor. This article shows how this measurement is made for various styles of sensors.
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