Dissolved oxygen analyzer

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Dissolved Oxygen Analyzer

Transcript of Dissolved oxygen analyzer

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Dissolved Oxygen Analyzer

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Dissolved Oxygen: The air we breathe contains about 20% oxygen. Fish and other aquatic organisms require oxygen as well. The term Dissolved Oxygen (DO or D.O.) refers to the amount of free oxygen dissolved in water which is readily available to respiring aquatic organisms.

State water quality standards often express minimum concentrations of dissolved oxygen which must be maintained in order to support life as well as be of beneficial use. Levels of dissolved oxygen below 4-5 milligrams per liter affect fish health and levels below 2 milligrams per liter can be lethal to fish. Additionally, biochemical oxygen demand (BOD) is commonly used with reference to effluent discharges and is a common, environmental procedure for determining the extent to which oxygen within a sample can support microbial life. The test for BOD is especially important in waste water treatment.

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When an electrode of noble metal such as platinum or gold is made 0.6 to 0.8 V negative with respect to a suitable reference electrode such as AgAgCl in a neutral KCI solution , the oxygen dissolved in the liquid is reduce at the surface of the noble metal. 

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the negative voltage applied to the noble metal electrode (called the cathode) is increased, the current increases initially but soon it becomes saturated.

In this plateau region of the polarogram, the reaction of oxygen at the cathode is so fast that the rate of reaction is limited by the diffusion of oxygen to the cathode surface.

When the negative bias voltage is further increased, the current output of the electrode increases rapidly due to other reactions, mainly, the reduction of water to hydrogen.

If a fixed voltage in the plateau region (for example, - 0.6V) is applied to the cathode, the current output of the electrode can be linearly calibrated to the dissolved oxygen.

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Do TX: XMT-ASensor : 499ADO-54

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1.AUTOMATIC TEMPERATURE CORRECTION .

Membrane-covered amperometric sensors produce a current directly proportional to the rate the analyte (the substance being measured) diffuses through the membrane. The diffusion rate is proportional to the concentration of analyte and the temperature. As temperature increases, membrane permeability increases. Thus, an increase in temperature will cause the sensor current to increase even though the analyte level remained constant. A correction equation in the transmitter software automatically corrects for changes in membrane permeability. In automatic temperature correction, the transmitter uses the temperature measured by the sensor for the correction

2. MANUAL TEMPERATURE CORRECTION.

In manual temperature correction the transmitter uses the temperature entered by the user for the membrane permeability correction. It does not use the actual process temperature. Do NOT use manual temperature correction unless the measurement and calibration temperatures differ by no more than about 2°C. Manual temperature correction is useful only if the sensor temperature element has failed and areplacement sensor is not available.

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CALIBRATING TEMPERATURE

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The zero standard is necessary because oxygen sensors, even when no oxygen is present in the sample, generate a small current called the residual current. The transmitter compensates for the residual current by subtracting it from the measured current before converting the result to a dissolved oxygen value. New sensors require zeroing before being placed in service, and sensors should be zeroed whenever the electrolyte solution is replaced. The recommended zero standard is 5% sodium sulfite in water, although oxygen-free nitrogen can also be used.

CALIBRATING DO

The purpose of the full-scale standard is to establish the slope of the calibration curve. Because the solubility of atmospheric oxygen in water as a function of temperature and barometric pressure is well knownAt 25°C and 760 mm Hg, the equilibrium solubility is 8.24 ppm.

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Zero calibration

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Span calibration using air

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Span calibration against standard instrument:

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Calibration by sample:

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