Dissolved Oxygen Meter Knowledge Encyclopedia
How the dissolved oxygen meter works
Dissolved oxygen analyzers mostly use diaphragm electrodes as transducers to convert dissolved oxygen concentration (actually oxygen partial pressure) into electrical signals, which are then amplified and adjusted (including salinity and temperature compensation) and displayed by analog-to-digital conversion. There are two types of membrane electrodes for measuring dissolved oxygen: polarographic (Polarography) and galvanic cell (Galvanic Cell). Polarography: In the electrode, a gold (Au) ring or a platinum (Pt) gold ring is used as the cathode; silver-silver chloride (or mercury-mercurous chloride) is used as the anode. The electrolyte is potassium chloride solution. The outer surface of the cathode is covered with an oxygen-permeable film. The film can adopt breathable materials such as polytetrafluoroethylene, polyvinyl chloride, polyethylene, silicone rubber, etc. A polarization voltage of 0.5 to 1.5 volts is applied between the cathode and anode electrodes. Some have a polarization voltage of 0.7 volts. When dissolved oxygen permeates through the membrane and reaches the surface of the gold cathode, the following reaction occurs on the electrode.
The cathode is reduced: O2+2H2O+4e→4OHˉ; at the same time, the anode is oxidized: 4Clˉ+4Ag-4e→4AgCl; under normal circumstances, the value of the diffusion current i∞ generated by the above reduction-oxidation reaction is proportional to the concentration of dissolved oxygen . It can be represented by the following formula:
i∞=nFA(Pm/L)Cs In the formula: i∞-diffusion current in steady state n-number of gain and loss electrons; F-Faraday constant (96500 coulombs); A-cathode surface area (square centimeter); Pm-film penetration Coefficient (cm2/sec); L-film thickness (cm); Cs-dissolved oxygen concentration (ppm). When the electrode structure and film are determined, A, Pm, L, n, etc. in the formula are all constants. Let K= nFA(Pm/L), then in the above formula: i∞=KCs.
Therefore, it can be seen that as long as the diffusion current i∞ is measured, the dissolved oxygen concentration can be measured. In order to eliminate the influence of temperature, salinity and air pressure, each model uses its own technology to compensate. Galvanic Cell: When external oxygen molecules penetrate the film into the inner phase of the electrode and reach the three-phase interface of the cathode, the following reaction occurs.
The silver cathode is reduced: O2+2H2O+4e→4OHˉ At the same time, the lead anode is oxidized: 2Pb+2KOH+4OHˉ-4e→2KHPbO2+2H2O That is, oxygen is reduced to hydroxide ions on the silver cathode, and at the same time, the external circuit Gain electrons; the lead anode is replaced by potassium hydroxide
The solution corrodes to generate potassium hydrogen lead acid, and at the same time outputs electrons to the external circuit. After the external circuit is connected, a signal current passes through, and its value is proportional to the dissolved oxygen concentration.
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Calibration method of dissolved oxygen meter
Dissolved oxygen analyzers can generally be calibrated with standard solutions or on-site sampling.
(1) Standard solution calibration method of dissolved oxygen meter: Standard solution calibration generally adopts two-point calibration, that is, zero point calibration and range calibration. The zero point calibration solution can use 2% Na2SO3 solution. The range calibration solution can be selected according to the measurement range of the instrument: 4 M KCl solution (2 mg/L); 50% methanol solution (21.9 mg/L).
(2) On-site sampling and calibration method of dissolved oxygen meter (Winkler method): In the actual use of dissolved oxygen meter, the Winkler method is often used for on-site calibration of dissolved oxygen meter. When using this method, there are two situations: when sampling, the meter reading is M1, and the analytical value is A. When the meter is calibrated, the meter reading is still M1. At this time, it is only necessary to adjust the meter reading to be equal to A; when sampling, the meter reading is M1, the laboratory analysis value is A, and the meter reading is changed to M2 when the meter is calibrated. At this time, the meter reading cannot be adjusted to be equal to A, but the meter reading should be adjusted to 1MA×M2.
