The measurement factors affecting the dissolved oxygen analyzer
1. The influence of atmospheric pressure on the measurement of dissolved oxygen by a measuring instrument
According to Henry's law, the solubility of a gas is directly proportional to its partial pressure. Oxygen partial pressure is related to the altitude of the region, with a difference of up to 20% between plateau and plain areas. Compensation must be made based on the local atmospheric pressure before use. Some instruments are equipped with a pressure gauge inside, which can be automatically calibrated during calibration; Some instruments are not equipped with a pressure gauge, and calibration should be based on the data provided by the local meteorological station. If the data is incorrect, it will lead to significant measurement errors.
2. The influence of temperature on the measurement of dissolved oxygen analyzer
Due to temperature changes, the diffusion coefficient of the membrane and the solubility of oxygen will both change, directly affecting the current output of the dissolved oxygen electrode. Thermistors are often used to eliminate the influence of temperature. As the temperature rises, the diffusion coefficient increases, but the solubility actually decreases. The effect of temperature on the solubility coefficient a can be estimated based on Henry's law, and the effect of temperature on the membrane diffusion coefficient β can be estimated based on Arrhenius's law.
(1) The solubility coefficient of oxygen: As the solubility coefficient a is not only affected by temperature, but also by the composition of the solution. At the same oxygen partial pressure, the actual oxygen concentration of different components may also be different. According to Henry's law, oxygen concentration is directly proportional to its partial pressure. For dilute solutions, the change in solubility coefficient a due to temperature change is about 2%/℃.
(2) The diffusion coefficient of the membrane: According to the Arrhenius law, the relationship between the solubility coefficient β and the temperature T is C=KPo2 · exp (- β/T). Assuming K and Po2 are constants, β can be calculated to be 2.3%/℃ at 25 ℃. After calculating the solubility coefficient a, the membrane can be calculated by comparing instrument indications and laboratory analysis values
The diffusion coefficient (omitting the calculation process here) of the membrane is 1.5%/℃ at 25 ℃.
3. The influence of sample flow rate on dissolved oxygen analyzer
The flow rate of the sample is slower for oxygen to diffuse through the membrane than through the sample, and it is necessary to ensure complete contact between the electrode membrane and the solution. For the flow detection method, oxygen in the solution will diffuse into the flow cell, causing a loss of oxygen in the solution near the membrane, resulting in diffusion interference and affecting the measurement. For accurate measurement, the flow rate of the solution flowing through the membrane should be increased to compensate for the diffusion loss of oxygen. The minimum flow rate of the sample is 0.3m/s.
4. The influence of salt content in solution on the measurement of dissolved oxygen analyzer
The dissolved oxygen in saline water is significantly lower than that in tap water. In order to accurately measure, the influence of salt content on dissolved oxygen must be considered. Under constant temperature, for every 100mg/L increase in salt content, dissolved oxygen decreases by approximately 1%. If the solution used for calibration of the instrument has a low salt content, but the actual measured solution has a high salt content, it will also lead to errors. In practical use, it is necessary to analyze the salt content of the measuring medium in order to accurately measure and compensate correctly.
