Optical measurement of oxygen concentration in water

3x puncte

categorie: Engleza

nota: 9.93

nivel: Facultate

Oxygen analysis in sewage treatment plants Control and regulation of carbon degradation, nitrification and denitrification are dependent on knowing the oxygen concentration in the aeration tank. For sewage treatment plant operators, therefore, the question is not whether but simply how the oxygen concentration in the activated sludge can be continuously measured.

A characteristic a[...]
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Oxygen analysis in sewage treatment plants Control and regulation of carbon degradation, nitrification and denitrification are dependent on knowing the oxygen concentration in the aeration tank. For sewage treatment plant operators, therefore, the question is not whether but simply how the oxygen concentration in the activated sludge can be continuously measured.

A characteristic aspect of electrochemical methods of oxygen measurement is the inexorable degradation of the anode and the consumption of the electrolyte during use. Both processes inevitably cause the measured values to drift so that low-bias results are obtained.

These effects can only be kept within bounds by regular calibrations and changes of electrolyte. A completely new type of oxygen sensor was developed and launched in 2003: the HACH LANGE LDO. It is based on the luminescence of a luminophore, and measures the oxygen concentration by carrying out a purely physical time measurement. As the time measurement is drift free, the user does not need to calibrate the sensor.

The main disadvantages of electrochemical measuring cells have therefore been overcome. The most important characteristic of the optical measurement method is that stable and precise measured values can be obtained over long periods of time. And the maintenance required to ensure precise oxygen measurements has also been drastically reduced. Optical measurement method The optical method of measuring dissolved oxygen avoids the disadvantages of traditional electrochemical measurement methods.

The LDO principle is based on the physical phenomenon of luminescence. Some materials emit light when excited by a stimulus other than heat. In the case of the LDO principle, the stimulus is light. If a combination of a suitable luminophore and a suitable wavelength of excitation light is chosen, the intensity of the luminescence and the time it takes to fade are dependent on the oxygen concentration around the material.

The sensor cap with the luminophore coating on a transparent carrier material, and the probe body with a blue LED which emits the light that triggers the luminescence. A red LED which serves as the reference element, a photodiode and an electronic evaluation unit.
In operation, the sensor cap is screwed onto the sensor body and immersed in the water. Oxygen molecules from the analysis sample are in direct contact with the luminophore.

To carry out a measurement, the excitation LED transmits pulsed blue light. The energy-rich blue light enables highly precise measurements to be made. The light pulse (50 msec) passes through the transparent carrier material onto the luminophore, to which it transfers part of its radiant energy. This causes some of the electrons in the luminophore to jump from their basic energy level to a higher one. Within microseconds they then fall back to their original level via a number of intermediate levels, emitting the energy they lose as they do so in the form of red light (Fig. 2).

When oxygen molecules are in contact with the luminophore, two effects occurs: Firstly the oxygen molecules are able to absorb the energy of the higher level electrons and enable them to return to their basic energy level without emitting light. The higher the oxygen concentration, the greater the reduction in the intensity of the emitted red light. The oxygen molecules also cause "shocks" in the luminophore, so that electrons fall back from the higher energy level more quickly. The lifetime of the emitted red light is therefore shortened.
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