On-line dissolved oxygen (DO) analyzer measurement principle

An oxygen analyzer measurement principle The solubility of oxygen in water depends on temperature, pressure and dissolved salts in water. Dissolved oxygen analyzer sensing part is composed of gold electrode (cathode) and silver electrode (anode) and potassium chloride or potassium hydroxide electrolyte, oxygen diffusion through the membrane into the electrolyte and the gold electrode and silver electrode constitute a measurement loop. When a polarization voltage of 0.6 to 0.8 V is applied to the electrodes of the dissolved oxygen analyzer, oxygen diffuses through the membrane, the cathode releases electrons, and the anode accepts electrons to generate an electric current. The entire reaction process is: anode Ag Cl→AgCl 2 e−cathode O 2 2H 2 O 4e→4OH—According to Faraday's law: The current flowing through the electrode of the dissolved oxygen analyzer is proportional to the partial pressure of oxygen, and the linear relationship between the current and the oxygen concentration is obtained when the temperature is constant.

The two dissolved oxygen content of the expression of dissolved oxygen content has 3 different ways: oxygen partial pressure (mmHg); percent saturation (%); oxygen concentration (mg/L or 10-6), these three methods of essence There is no difference.

(1) Partial Pressure Representation: Oxygen partial pressure representation is the most basic and most essential representation. According to Henry's law, P=(Po2 PH2O)×0.209, where P is the total pressure; Po2 is the partial pressure of oxygen (mmHg); PH2O is the partial pressure of water vapor; 0.209 is the content of oxygen in the air.

(2) Percent saturation notation: Since the aeration fermentation is very complicated, the partial pressure of oxygen cannot be calculated. In this case, the expression of percent saturation is most appropriate. For example, when the dissolved oxygen is set at 100% during calibration and 0% at zero oxygen, the dissolved oxygen content during the reaction is the percentage of the calibration.

(3) Oxygen Concentration Representation: According to Henry's law, the oxygen concentration is proportional to its partial pressure, ie: C=Po2×a, where C is oxygen concentration (mg/L); Po2 is oxygen partial pressure (mmHg); a is Solubility coefficient (mg/mmHg·L). The solubility coefficient a is not only related to the temperature but also to the composition of the solution. For a constant temperature aqueous solution, a is a constant, then the oxygen concentration can be measured. The oxygen concentration notation is not commonly used in the fermentation industry, but it is expressed in terms of oxygen concentration in sewage treatment, drinking water, and other processes.

Three factors affect the measurement of dissolved oxygen The solubility of oxygen depends on temperature, pressure, and dissolved salts in water. In addition, diffusion of oxygen through the solution is faster than diffusion through the membrane. If the flow rate is too slow, interference will occur.

1. Influence of temperature As the temperature changes, the diffusion coefficient of the film and the solubility of oxygen will change, directly affecting the current output of the dissolved oxygen electrode. The thermistor is often used to eliminate the influence of temperature. As the temperature rises, the diffusion coefficient increases and the solubility decreases. The influence of temperature on the solubility coefficient a can be estimated according to Henry's law, and the temperature on the membrane diffusion coefficient β can be estimated by Arrhenius' law. (1) Oxygen solubility coefficient: Since the solubility coefficient a is not only influenced by the temperature, but also by the composition of the solution. The actual oxygen concentration of different components may also be different at the same oxygen partial pressure. According to Henry's Law, it is known that the oxygen concentration is proportional to the partial pressure thereof, and for dilute solutions, the temperature change solubility coefficient a changes by about 2%/°C. (2) Diffusion coefficient of membrane: According to Arrhenius' law, the relationship between solubility coefficient β and temperature T is: C=KPo2·exp(-β/T), where assuming that K and Po2 are constants, it can be calculated. β is 2.3%/°C at 25°C. When the solubility coefficient a is calculated, the diffusion coefficient of the membrane can be calculated by comparison between the instrument indication and the assay analysis value (the calculation process is omitted here). The diffusion coefficient of the membrane is 1.5%/°C at 25°C.

2. Influence of atmospheric pressure According to Henry's law, the solubility of gas is proportional to its partial pressure. The partial pressure of oxygen is related to the altitude of the area. The difference between the plateau area and the plain area can reach 20%. Before use, compensation must be made according to the local atmospheric pressure. Some instruments have a barometer inside, which can be automatically calibrated at the time of calibration. Some instruments are not equipped with a barometer. The calibration time must be set according to the data provided by the local weather station. If the data is incorrect, it will result in a large measurement error.

3. Salt content in solution The dissolved oxygen in salt water is significantly lower than that in tap water. In order to measure accurately, the effect of salt content on dissolved oxygen must be considered. When the temperature is constant, the dissolved oxygen decreases by about 1% for every 100 mg/L increase in salt content. If the salt content of the solution used by the meter at the time of calibration is low, and the salt content of the actually measured solution is high, it will also cause errors. In actual use, the salt content of the measuring medium must be analyzed in order to accurately measure and correctly compensate.

4. The flow rate of oxygen through the membrane diffusion is slower than diffusion through the sample, and it must be ensured that the electrode membrane is in complete contact with the solution. For the flow-through detection method, oxygen in the solution will diffuse into the flow cell, causing loss of oxygen in the solution near the membrane, causing diffusion interference and affecting the measurement. For accurate measurements, the flow of the solution through the membrane should be increased to compensate for the oxygen lost by diffusion, and the minimum sample flow rate is 0.3 m/s.