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Factors that affect the measurement of dissolved oxygen

- Oct 25, 2017 -

The effect of temperature due to temperature changes, the diffusion coefficient and solubility of membrane oxygen will change, directly affect the oxygen electrode current output, often used to eliminate the influence of the temperature of the thermistor. With the increase of temperature, the diffusion coefficient increases, but the solubility decreases. The effect of temperature on solubility coefficient a can be estimated according to Henry's law, and the temperature can be estimated by the Leon Ni Lucius's law for the diffusion coefficient of the membrane beta.

The oxygen solubility coefficient: the solubility coefficient a is not only affected by the temperature, but also affected by the solution composition. Under the same oxygen partial pressure, the actual oxygen concentration of different components may also be different. According to Henry's law, the oxygen concentration is proportional to the partial pressure. For dilute solution, the change of solubility coefficient a is about 2%/ degrees centigrade.

The diffusion coefficient of film: according to Alen Ni Lucius's law, the relationship between the solubility coefficient and temperature of T: C=KPo2 exp (beta /T), which assumes that K and Po2 are constant, it can calculate the beta at 25 C 2.3%/ C. When the solubility coefficient a is calculated, the diffusion coefficient of the film can be calculated by the instrument indicator and the experimental analysis value, and the diffusion coefficient of the film is 25 1.5%/ at the temperature range of 0.

The influence of atmospheric pressure according to the Henry's law, the gas solubility and pressure is proportional to the. The oxygen partial pressure is related to the altitude of the area. The difference between the plateau area and the plain area can reach 20%, and the atmospheric pressure must be compensated according to the local atmospheric pressure before use. Some of the instrument with pressure meter, the calibration can be automatically corrected; some instrument is not configured barometer, according to the local meteorological station data provided in the calibration set, if the data is incorrect, will lead to large error.

The dissolved oxygen was significantly lower than that in tap water containing dissolved oxygen content in brine, salt solution, in order to accurately measure, must consider the effect of salinity on dissolved oxygen. When the temperature is constant, the dissolved oxygen decreases by about 1% when the salt content increases by 100mg/L. If the instrument is used in calibration, the salt content of the solution is low, and the salt content of the solution is high, which will lead to errors. In practical use, the salt content of the measuring medium must be analyzed so as to accurately measure and correct the compensation.

The flow rate of oxygen samples by film diffusion through the sample is slower than diffusion, must ensure that the full contact electrode film and solution. For circulation detection, the oxygen in the solution diffuses into the flow cell, causing oxygen loss in the solution near the membrane, resulting in diffusion interference and affecting the measurement. In order to measure accurately, the flow rate of the solution passing through the membrane should be increased to compensate for the lost oxygen diffusion. The minimum flow rate of the sample is 0.3m/s.

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