Dissolved oxygen analyzer

1. Dissolved Oxygen Analyzer

2. 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.

3. 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.

4. 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.

5. Do
TX: XMT-A
Sensor : 499ADO-54

6. 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.

7. CALIBRATING TEMPERATURE

8. 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 known
At 25°C and 760 mm Hg, the equilibrium
solubility is 8.24 ppm.

9. Zero calibration

10. Span calibration using air

12. Span calibration against standard instrument:

13. Calibration by sample: