Working Principle
Dissolved oxygen analyzers mainly include film-coated electrodes and fluorescence dissolved oxygen analyzers. The measurement principle of the film-coated electrode dissolved oxygen meter is the electrochemical probe method: the cathode, anode, and electrolyte are separated from the outside world by an oxygen-sensitive film, a constant voltage is applied between the two electrodes, and electrons diffuse from the cathode to the anode, generating a diffusion current, the diffusion current is proportional to the concentration of dissolved oxygen in water at a certain temperature. Through the quantitative relationship between diffusion current and dissolved oxygen concentration, the current value is converted into dissolved oxygen value.
The measurement principle of the fluorescent dissolved oxygen meter is based on the quenching effect of oxygen molecules on fluorescent substances: the electrode shines a blue light on the material coated with fluorescent light to excite red light because oxygen molecules take away energy, so the time to excite red light And the intensity is inversely proportional to the concentration of oxygen molecules. Use the red light synchronized with the blue light as a reference to measure the phase difference between the two, compare the internal calibration value, and display the dissolved oxygen value after calculation and processing. The fluorescence dissolved oxygen meter has the characteristics of free maintenance and no oxygen consumption and is stable and undisturbed.
Factors Affecting Measurement Results
Dissolved oxygen mainly depends on the effects of temperature, atmospheric pressure, salinity in the water, and the flow rate of the water.
1. The influence of temperature
When the temperature is constant, the diffusion current generated by the dissolved oxygen electrode increases with the increase of the oxygen partial pressure in the water. When the oxygen partial pressure is constant, the output current of the dissolved oxygen electrode increases with the increase in temperature. When the temperature rises, the oxygen partial pressure in water decreases instead. The change in temperature directly affects the output of the electrode diffusion current. The temperature sensor in the electrode of the analyzer compensates for the temperature to eliminate the influence of temperature.
During the verification process, the reading time should be extended to balance the temperature of the film-covered electrode and the water temperature, thereby reducing the influence of temperature on the verification results.
2. The influence of atmospheric pressure
According to Henry's law, at isothermal pressure, the partial pressure of oxygen in water is the same as that in air. The solubility of oxygen is proportional to its partial pressure. When the atmospheric pressure changes, the dissolved oxygen content in the water also changes accordingly, which affects the output of the film-covered electrode. Atmospheric pressure is related to altitude. There are great differences between regions at different altitudes. , to compensate. The dissolved oxygen analyzer with a built-in barometer can be automatically compensated. If there is no barometer, it should be set according to the actual atmospheric pressure, so as to reduce the influence of atmospheric pressure on the test results.
3. The effect of salt content in water
The solubility of oxygen in water decreases with the increase in salt content, and the dissolved oxygen in salt water is significantly lower than that in tap water. When the temperature is constant, the dissolved oxygen decreases by about 1% for every 100mg/L increase in salt content. For accurate measurements, the effect of salt content on dissolved oxygen must be considered.
4. The influence of water velocity
Due to the consumption of oxygen by the film-covered electrode, the decrease in the oxygen concentration on the film-covered surface affects the measurement. For the accuracy of measurement, the measured value is relatively small when the water flow is still. During the measurement, the electrode should be swung at a constant speed in the water to increase the flow of water flowing through the membrane to compensate for the consumed oxygen. The swing should not be too violent, so as to avoid the oxygen exchange between air and water, which will affect the test results.
These influencing factors should be fully considered in the verification process so as not to affect the verification results.
Measurement Uncertainty
1 Overview
(1) Measurement basis: JJG291-2018 Dissolved Oxygen Meter Verification Regulations.
(2) Measurement environment conditions: temperature (20±5)°C, relative humidity less than 85%, and temperature fluctuation during verification shall not exceed ±2°C.
(3) Measurement standard: special test tank for dissolved oxygen meter, temperature fluctuation uncertainty U=0.03℃, k=2; digital thermometer, temperature indication error uncertainty U=0.10℃, k=2.
(4) Measured object: dissolved oxygen meter.
(5)Measurement method: adjust the temperature of the special test tank of the dissolved oxygen meter to the measurement temperature, put the electrode from the air into the special test tank at each measurement temperature point, read the indicated value after the indicated value is stable, and repeat the measurement 2 times, calculate the average value, and calculate the indication error according to the formula (1).
2. measurement model
In the formula:
Δc - indication error of dissolved oxygen concentration, mg/L;
- the average value measured by the instrument, mg/L;
cs - reference value of dissolved oxygen, obtained by interpolation method, mg/L;
3. Standard uncertainty components of each input quantity
(1) The source of the uncertainty of the input c is mainly the measurement repeatability of the dissolved oxygen meter, which can be obtained through continuous measurement, and the type A uncertainty evaluation method is used for evaluation.
For a dissolved oxygen meter, at a temperature of 20°C, measure the saturation content of dissolved oxygen in the water, and repeat 10 times to obtain a set of measurement columns. The displayed value of the dissolved oxygen meter is as follows (unit: mg/L):
Calculate the standard deviation of a single experiment according to Bessel's formula: s=0.031mg/L
In the actual measurement, the average value of two measurements is taken as the measurement result, so the standard uncertainty of the input quantity is:
(2) The standard uncertainty u(cs) of the input quantity cs
cs is the reference value of dissolved oxygen concentration calculated by the interpolation method provided by the verification regulations. According to the calculation method provided, its uncertainty mainly comes from the influence of temperature and atmospheric pressure.
①Uncertainty uT introduced by temperature
Uncertainty uT1 introduced by temperature fluctuation of special test tank for dissolved oxygen analyzer
According to the certificate, the uncertainty of the temperature fluctuation of the special test tank for the dissolved oxygen analyzer is U=0.10℃, k=2. Then the standard uncertainty introduced by the constant temperature water bath:
uT1=0.03/2=0.015℃
According to the regulations, at 20.0°C, the rate of change of dissolved oxygen concentration with the temperature at atmospheric pressure 1012hPa is 0.171mg L-1/°C, and the standard uncertainty introduced by temperature fluctuations is uT1=0.015℃×0.171mg L-1/℃=0.0026mg/L
●Uncertainty uT2 introduced by temperature fluctuation
According to the certificate, the uncertainty of the temperature indication error of the digital thermometer is U=0.10℃, k=2. Then the standard uncertainty introduced by the digital thermometer:
uT2=0.10/2=0.05℃
According to the regulations, at 20.0°C, the rate of change of dissolved oxygen concentration with temperature is 0.171mg L-1/°C when the atmospheric pressure is 1012hPa, and the standard uncertainty introduced by the temperature indication error is uT2=0.05℃×0.171mg L-1/℃=0.0086mg/L
●Uncertainty uT introduced by temperature
②Uncertainty up introduced by atmospheric pressure
According to the certificate, the expanded uncertainty of the barometer is 0.2kPa (k=2), then the standard uncertainty is up=0.2/2=0.1kPa=1hPa. According to the interpolation method, when the atmospheric pressure is 1012hPa at 20.0°C, the change rate of the dissolved oxygen concentration with the air pressure is 0.0092mg·L-1/hPa, then:
up=1×0.0092=0.0092mg/L
③Standard uncertainty u(cs) of the input quantity cs
4. Synthetic standard uncertainty
Calculation of the combined standard uncertainty:
5. Evaluation of expanded uncertainty
Taking k=2, the expansion is uncertain: U=kuc (Δc)=2×0.026=0.06mg/L
6. Reporting and expression of measurement uncertainty
The uncertainty of the measurement results of the indication error of the dissolved oxygen analyzer: U=0.06mg/L, k=2.
Precautions and handling methods during the verification process
1. Precautions for test preparation
(1) Before the verification, check whether the water storage sponge in the electrode calibration sleeve is soaked. If it is not soaked, add an appropriate amount of deionized water, wait until it is completely soaked, pour out the excess water in the calibration sleeve, remove the accumulated water, and perform full calibration.
(2) Check whether the electrolyte in the electrode is dry and whether there are bubbles, and whether the coating is complete and polluted. If the above problems occur, electrolytes should be added, cleaned, and the electrode coating should be replaced.
(3) The electrodes should be fully polarized before the test. Polarization balances the chemical system in the electrode, reduces the zero oxygen current, and stabilizes the electrode. If the polarization time is not enough, the verification result will be affected, so the polarized electrode should be placed in saturated air for a full calibration.
(4) Attention should be paid to the influence of atmospheric pressure during calibration, and compensation should be made according to the actual local atmospheric pressure. Deionized water is used as the test water during the test, and the error caused by the salinity has been eliminated, so the salinity compensation should be set to zero.
2. Matters needing attention in the verification process
(1) During the verification, the temperature of the calibrated saturated air and the temperature of the water should be relatively close (less than 10°C).
(2) Due to the influence of temperature, the reading time should be extended when changing from one temperature point to another temperature point during the verification, so that the electrode temperature and water temperature can be balanced before reading, so as to avoid large errors caused by temperature imbalance and thus affect the verification results.
(3) When the electrode is immersed in anaerobic water from the air and the displayed value is greater than 0.1mg/L after 15 minutes, the zero value of the electrode should be calibrated. If the electrode response time is long, the electrolyte or electrode coating should be replaced.
(4) During the test, there should not be any air bubbles on the film surface of the film-coated electrode, otherwise, the air bubbles will be read as dissolved oxygen in water, which will affect the test results.
(5) After replacing the electrolyte and the electrode coating, if the verification result is still unqualified, it may be that the cathode of the coating electrode is polluted. The cathode part is covered with oxides. It is necessary to polish the cathode with fine sandpaper below 1000 mesh or with the sandpaper that comes with the instrument. Rinse the inner electrode with deionized water and dry it with absorbent paper. After melting, perform the test again.
(6) After the above treatment, if the dissolved oxygen meter is still unqualified or the value display is unstable, it should be considered to replace the electrode and rule out whether is a problem with the electronic unit of the instrument.