Fault handling and testing principle of new dissolved oxygen meter
Fault handling and testing principle of new dissolved oxygen meter The dissolved oxygen meter measures the amount of oxygen dissolved in an aqueous solution. Oxygen dissolves in the water through ambient air, air flow, and photosynthesis. It can be used to monitor processes that affect the rate of reaction, process efficiency or environment, such as aquaculture, biological reactions, environmental testing (lakes, streams, oceans), water/wastewater treatment, wine production. Instrument characteristics The dissolved oxygen meter measures the amount of oxygen dissolved in an aqueous solution. Oxygen dissolves in the water through ambient air, air flow, and photosynthesis. By breathing and decomposing, dissolved oxygen is consumed in water and is mainly supplemented by air and photosynthesis. The amount of oxygen in the water depends mainly on the temperature. The oxygen concentration in warm water is lower than that in cold water. However, excessive dissolved oxygen levels can be harmful to plants and animals. The dissolved oxygen electrode can be used to measure the dissolved oxygen content in the aqueous solution of the sample being tested in the field or in the laboratory. Since dissolved oxygen is one of the main indicators of the quality of water, the dissolved oxygen electrode can be widely used for the measurement of dissolved oxygen content in various occasions, especially aquaculture water, photosynthesis and respiration and on-site measurement. When assessing the ability of streams and lakes to support biological survival, a Biochemical Oxygen Demand Test (BOD) is performed to measure the oxygen-depleted aqueous solution containing organic matter and determine the dissolved oxygen concentration and the temperature of the sample aqueous solution. Relationship between. BDO-200A Chinese online dissolved oxygen meter is a highly intelligent online continuous monitor. It can be equipped with polarographic electrodes to automatically measure a wide range from ppb to ppm. It is a special instrument for measuring the oxygen content in liquids in boiler feed water, condensate, environmentally friendly sewage and other industries. The dissolved oxygen electrode uses a thin film to separate the platinum cathode, the silver anode, and the electrolyte from the outside. In general, the cathode is almost in direct contact with the film. Oxygen diffuses through the membrane at a rate proportional to its partial pressure. The greater the partial pressure of oxygen, the more oxygen is transmitted through the membrane. When dissolved oxygen continuously permeates through the membrane and penetrates into the cavity, it is reduced on the cathode to generate a current, which is displayed on the meter. Since this current is directly proportional to the dissolved oxygen concentration, the calibration meter only needs to convert the measured current into a concentration unit. The dissolved oxygen concentration is usually in mg/L (dissolved oxygen per liter of water) or ppm (parts per million). Some meters compare the calculated oxygen content to the observed concentration to give a percent saturation (O2% sat.) There are two ways to determine dissolved oxygen, polarographic and galvanic. Polarographic electrodes require instrument input and voltage to electrode polarization. Since the applied voltage may take 15 minutes to stabilize, the polarographic electrode is usually preheated before use to ensure proper polarization of the electrode. The two poles of the galvanic cell type are composed of two different metals capable of spontaneously generating a voltage. Since the galvanic cell voltage is generated spontaneously rather than externally, the galvanic cell electrode does not require the "preheating" required for polarographic electrode polarization. environmental impact Proper dissolved oxygen is essential for good water quality and all forms of life require oxygen. The natural stream purification process requires proper oxygen content to supply aerobic life forms. If the oxygen content in the water is less than 5.0 mg/L, it is difficult for aquatic organisms to survive, and the lower the concentration, the more difficult it is. If the oxygen content is less than 1-2 mg / L and lasts for several hours, it will cause a large number of deaths of aquatic organisms. Application Dissolved oxygen electrodes can be used to measure processes that affect the rate of reaction, process efficiency or the environment, such as aquaculture, biological reactions, environmental testing (lakes, streams, oceans), water/wastewater treatment, wine production. Temperature compensation For standard dissolved oxygen measurements, temperature affects the solubility and diffusion rate of oxygen, so temperature compensation must be performed. Salinity correction The presence of dissolved salts limits the amount of oxygen that is soluble in the water. The relationship between oxygen concentration and partial pressure varies with the salinity of each sample solution, so most instrument manufacturers offer artificially adjusted salinity to correct for changes due to different ion concentrations. Biochemical oxygen demand BOD testing is generally used in wastewater treatment plants where it is important to know the amount of oxygen that is consumed from the water when the microorganisms decompose organic matter. This test allows the water treatment plant to determine the effectiveness of the water treatment or the amount of contamination still present. The relative oxygen demand of wastewater, effluent, and sewage can be determined by measuring the amount of oxygen dissolved in the sample at the beginning and end of a particular culture period. The BOD can be calculated by measuring the dissolved oxygen (T1) of time 1 minus the dissolved oxygen (T2) of time 2; multiplying this value by the final sample volume (VF) and dividing by the initial sample volume (V). BOD (mg/L) = (T1 – T2)VF/V Troubleshooting When using polarographic electrodes, preheat for at least 15-30 minutes before calibration or measurement. To ensure that there are no air bubbles in the electrolyte of the membrane, the ASI membrane cap is designed to remove all air from the liquid chamber when the membrane head is installed. There should be no air bubbles left on the surface of the membrane, otherwise it will read the bubbles as an oxygen-saturated sample. Even if you are using a meter with automatic temperature compensation, calibrate the electrode at a temperature close to the sample solution. The electrodes should be calibrated in air with air as the 100% saturated dissolved oxygen standard point. Due to the oxygen consumption of the electrode, the concentration of oxygen on the surface of the probe will decrease instantaneously, so it is important to stir the solution during the measurement. If the membrane is damaged, replace it. Measuring principle Common dissolved oxygen meters use diaphragm electrodes as transducers to convert dissolved oxygen concentration (actually oxygen partial pressure) into electrical signals, and then amplified and adjusted (including salinity and temperature compensation), which are displayed by analog-digital conversion. . There are two types of membrane electrodes that are practical for dissolved oxygen meters: Polarography and Galvanic Cell. Polarography: In the electrode, a gold (Au) ring or a platinum (Pt) gold ring is used as a cathode; and silver-silver chloride (or mercury-mercurous chloride) is used as an anode. The electrolyte is a potassium chloride solution. The outer surface of the cathode is covered with an oxygen permeable film. The film may be a gas permeable material such as polytetrafluoroethylene, polyvinyl chloride, polyethylene or silicone rubber. 0.5~1.5 volts between the yin and yang electrodes Polarization voltage. Some polarization voltages are 0.7 volts. When dissolved oxygen penetrates the film to reach 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 conditions, the value of the diffusion current i∞ produced by the above reduction-oxidation reaction is proportional to the dissolved oxygen concentration. It can be expressed by the following formula: I∞=nFA(Pm/L)Cs Where: i∞-stabilized diffusion current N-loss electron number F-Faraday constant (96500 coulombs) A-cathode surface area (cm 2 ) Pm-film permeability coefficient (cm 2 / sec) L-film thickness (cm) Cs-dissolved oxygen concentration (ppm) After the electrode structure and the film are determined, A, Pm, L, n, and the like in the formula are constant. Let K = nFA(Pm/L), then Where: i∞=KCs. Therefore, it can be seen that the dissolved oxygen concentration can be measured as long as the diffusion current i∞ is measured. To eliminate the effects of temperature, salinity and barometric pressure, each model is compensated by its own technology. Galvanic Cell: When the external oxygen molecules pass through the film and enter the internal phase of the electrode to 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 electrons are simultaneously obtained from the external circuit; the lead anode is corroded by the potassium hydroxide solution to form potassium hydrogen leadate, and electrons are output to the external circuit. After the external circuit is turned on, a signal current is passed, and its value is proportional to the dissolved oxygen concentration. 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