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Press Release June 2005
Measuring Technology for Dissolved
Oxygen in Aqueous Media
By Peter Lindmüller
For many organisms living in water, dissolved oxygen constitutes the
critical basis for their survival in the medium. This spectrum ranges
from fish farming and monitoring the status of surface and river water to
the bacteria in a sewage treatment plant that clean the wastewater.
There are two important aspects to measuring dissolved oxygen: on the one
hand, certain minimum values may not be undershot in terms of limit values
in order to safeguard the existing and desired population. On the other hand,
specified concentrations of oxygen must be maintained in the medium as
economically as possible for process control.
Amperometric sensors
The standard method used up to now for the online measurement of dissolved
oxygen involves the use of an electrochemical cell. This amperometric sensor
supplies a current which is linearly dependent on the oxygen partial pressure
of the medium. Oxygen gets to the cathode of the electrochemical cell by
passing through a gas-permeable membrane. Here it is reduced by releasing
electrons.
Optical sensors
In these types of sensors, such as the Oxymax W COS61 oxygen sensor from
Endress+Hauser, an optically transparent carrier separates the optical analysis
within the sensor from the fluid. A sensor layer open to diffusion is fitted on
the carrier on the fluid side. This layer contains fluorescence-active
molecules. These molecules emit a red light after being excited by a short-
Measuring Technology for Dissolved Oxygen
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Press Release April 2005
wave light pulse. The intensity and duration of this light depend on the
oxygen partial pressure present. The time-based dependency is used to
measure the oxygen contents.
Various measuring principles for different applications
The amperometric sensors in place to date have been proven in use for years.
The sensors have a linear behavior and very wide measuring ranges can be
achieved from traces to supersaturated solutions. This method of
measurement makes a high degree of accuracy possible which the optical
measuring principle cannot provide. Chemical handling (electrolyte) and the
membrane cap, which is mechanically sensitive, are the critical elements of
amperometric sensors. Following commissioning and servicing, the sensor
needs a polarization time until stable measured values are generated. Process
elements such as H2S or NH3 contaminate the sensor and the measured values
become incorrect.
The optical process can demonstrate its merits particularly with regard to the
critical aspects of the amperometric sensors. It is insensitive to inference from
components such as H2S, there is no electrolyte and the sensor cap is easy to
use. Optical oxygen sensors such as COS61 from Endress+Hauser do not
have any running-in period and are ready for use practically immediately. The
response time is very short due to the short diffusion path of the oxygen
molecules from the medium to the fluorescence-active layer.
Maintenance intervals and response behavior
In municipal wastewater treatment with storm water tanks or combination
tanks, the two technologies are on a par from the point of view of measured
value generation. The balance tips in favor of the optical sensor as soon as the
maintenance intervals of the amperometric sensors have to be shortened
considerably as a result of contamination, H2S etc.. Depending on the
application, the additional advantages of the optical process come into their
own. This is the rapid response time in single-batch reactor (SBR)
applications. In industrial applications, the chemical compatibility and
stability against abrasion are the elements that distinguish the sensor here.
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Special attention should be given to cleaning: depending on the process and
mounting location, loading occurs due to high grease and fat levels or through
the growth of microorganisms. In such instances, cyclic cleaning is essential
regardless of the technology used.
Comparing the application-related process requirements with the performance
data of the amperometric and optical sensors demonstrates that the practicable
measuring range of the optical process of 0 .. 20 mg/l perfectly suits the
requirements in water and wastewater treatment. The accuracy that can be
achieved is acceptable in these applications. Furthermore, the fact that the
optical system is a low-maintenance system that is very easy to use makes it
all the more interesting from an operating perspective.
Further transparency is provided when the "measuring point over operating
life" costs are assessed. If you observe the investment costs per measuring
point and the costs for maintenance and consumable materials, the
amperometric system will appear to be the most cost-effective solution for
rotational maintenance. As soon as the maintenance intervals (electrolyte
replacement and calibration) become considerably shorter for reasons specific
to the system, the maintenance-specific benefits of the optical system come to
the fore. The costs and effort involved in cleaning have not been included as
these are the same for both processes.
Both technologies have different performance focal points. The optical
oxygen sensor perfectly complements the existing measuring technology.
(Pictures:)
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Optical oxygen sensor COS61
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Optical oxygen sensor COS61 – detail
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Measuring principle of optical oxygen sensor with fluorescense
quenching
Please mention the following address below the article. Not the address of
Endress+Hauser Conducta.
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Press Release April 2005
For further information visit:
www.endress.com
or contact
Nikolaus Krüger
Director Marketing
Endress+Hauser Holding AG
Kägenstrasse 7
CH-4153 Reinach
e-mail: [email protected]
Tel.: +41 61 715 77 59
Fax: +41 61 715 77 62
Please send the sales leads and a specimen copy to the address of
Endress+Hauser Conducta. Thank you.
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