Daily maintenance, precautions, troubleshooting, and handling of zirconia oxygen analyzers

　　1. Why can't calibration be performed immediately after putting the instrument into use?

　　Answer: This is because: Within 24 hours of cold start-up, the indication is abnormal. After one day of use, calibrate with standard gas. This is because some adsorbed moisture or combustible substances may exist in the cold machine detector or newly installed detector. After the machine is heated, at high temperatures, this adsorbed moisture evaporates, and the combustible substances burn, consuming the reference air in the reference side battery, causing the oxygen content of the reference air to be lower than the normal value of 20.6%, resulting in a low detector signal, or even a negative signal, causing the measured oxygen content to be higher, or even greater than 20.6%. The measured value at this time is inaccurate. It should be waited until the moisture and combustible substances inside the detector are replaced with fresh air before accurate measurement can be made. Therefore, the zirconia detector needs to be heated for at least one day before calibration.

　　2. Why is it necessary to regularly calibrate the analyzer?

　　Answer: There are many interfering factors in the use of zirconia analyzers, such as aging of the zirconia tube, ash accumulation, and corrosion of the electrodes by SO2 and SO3. After running for a period of time, the performance of the instrument will gradually change, bringing errors to the measurement, so it is necessary to regularly calibrate the instrument! The calibration cycle is usually 1-3 months, depending on the use environment and usage of the instrument.

　　During calibration, pure N2 cannot be used as the zero point gas. Usually, the zero point gas should be 10% of the full scale; the range gas is 90% of the full scale; dry air is used as the range gas on site; the zero point gas uses 100PPM O2. This is because below 100PPM zero point, the standard gas error has too much influence on the instrument, and the calibration purge time is too long and it is not easy to purge in place; the measured value uses the downward extension line of the measurement linearity. Practice has proved that our choice is clear and effective!

　　3. Why should the instrument not be turned on and off easily?

　　Answer: There are two reasons: First, because the zirconia tube is a ceramic tube, although it has a certain resistance to thermal shock, during the on-off process, due to rapid cooling and heating, large temperature changes may cause the zirconia tube to break. Therefore, as little unnecessary on-off operation as possible should be done; second, the thermal expansion coefficient of the platinum electrode coated on the zirconia tube is inconsistent with that of the zirconia tube. After being used for a period of time, it is easy to fall off during the on-off process, causing the impedance inside the probe to increase, or even damaging the detector. Shutdown should be cautious!

　　4. Judgment of detector constant temperature

　　Answer: Enter the menu and check whether the detector temperature and voltage are consistent. This helps to determine whether the heating and temperature control system is normal. When the detector temperature is much higher than the constant temperature, it indicates that the thermocouple is open-circuited. Because the converter has a broken couple protection circuit, once the thermocouple is open-circuited, it will generate a millivolt signal to replace the thermocouple signal, causing the detector temperature display to be higher, and causing the heating power supply to be disconnected to protect the detector from being burned. At this time, although the temperature is very high, the electric furnace is not actually heating. Measuring the resistance at both ends of the measuring thermocouple (the lead must be disconnected) can verify this. The normal resistance of the thermocouple should be less than 20 ohms.

　　If the inspection finds that the temperature is lower than the constant value, this should consider that the heating is not performed, the heating wire is broken, or the temperature control system is malfunctioning or damaged.

　　5. Measured value is too high

　　Without considering the front-end factors, first consider whether the detector calibration port is leaking, and whether the flange screws are tightened; the instrument has not been calibrated for a long time or the calibration is incorrect.

　　6. Measured value is too low

　　The instrument needs calibration or needs to be recalibrated;

　　Incomplete combustion of the boiler, combustible gases in the flue gas;

　　7. Large fluctuation in measured value

　　Detector aging, high internal resistance, poor electrode contact;

　　High humidity or water droplets in the sample gas, vaporization in the detector;

　　8. Extreme drift of measured value, signal exceeding range

　　Detector components are damaged, such as zirconia tube breakage, open-circuit electrode leads, detector aging, temperature compensation resistor breakage (oxygen content 100%);

　　9. Causes and symptoms of probe aging

　　Generally, the probe aging we refer to refers to the aging of the zirconia detector, mainly manifested in the increase of internal resistance and the increase of background potential: (1) Increase of internal resistance

　　In practical applications, the increase in internal resistance caused by probe aging is more common. Internal resistance refers to the input resistance between the two ends of the signal line. It is the sum of the lead resistance, the interface resistance between the electrode and the zirconia, and the volume resistance of the zirconia. Therefore, electrode volatilization, electrode shedding, and the reverse stabilization of the zirconia electrolyte (from stable zirconia to unstable zirconia) will all cause an increase in internal resistance. Measuring the internal resistance of the detector can determine its aging condition. According to experience, when the internal resistance increases to near its usage limit, a large signal jump phenomenon will occur, and some reactions are slow response phenomena. For these detectors, the background potential is not necessarily very large.

　　(2) Increase in background potential

　　Background potential is the additional potential of the battery. There are two factors that cause the increase in background potential: one is a permanent factor, which is parasitic on the battery, such as the corrosive effect of SO2 and SO3, and the asymmetry of the battery; the other is a temporary factor, such as electrode ash and poor air convection. Once the conditions improve, the background potential can be reduced.

　　The increase in background potential often reflects the degree of detector aging. When the E0 value exceeds the maximum adjustment amount of the analyzer, it indicates that the detector is damaged.

　　For example:

　　A zirconia, the background potential at the time of leaving the factory is ±1mV, and its allowable change range is ±3mV. After half a year of use, the potential exceeds -3mV; after 18 months of use, it becomes: -29mV; this indicates that the detector has aged and needs to be replaced.

　　It should be noted that the aging of some detectors is manifested in the increase of background potential, while some detectors, although aged, do not have this phenomenon, so we need to carefully analyze and treat them. When the cause of the increase in background potential is caused by temporary factors, as the use time increases, it is possible that the background potential will first increase and then decrease.

　　The number of probes aging due to increased background potential is less than that due to increased internal resistance. A simple increase in background generally does not cause large signal fluctuations.

　　10. Precautions:

　　① The sample gas needs to be pressure-controlled; typically, the inlet pressure to the instrument should not exceed 0.05MPa;

　　② When calibrating with standard gas, the standard gas flow rate should be controlled between 150-300ml/min;

　　③ All gas lines entering the instrument must undergo strict leak detection, and this work must be performed system-wide every six months when the instrument is in normal operation;

　　④ Before the gas path enters the instrument, it must pass through a physical filter, 10u; if gas blockage is found, the filter screen (filter) can be checked first;

　　⑤ Regularly clean the analyzer's fan filter, once a quarter; in harsh environments, frequent cleaning is required to prevent instrument overheating due to poor ventilation;

　　⑥ The instrument's installation location should be level and away from vibration sources, to prevent errors caused by uneven sample convection due to an unlevel detector;

　　⑦ The environment around the analyzer requires good ventilation; avoid enclosed spaces to prevent measurement errors caused by unbalanced oxygen levels;

　　⑧ Flammable gases are strictly forbidden around the analyzer, as this will seriously affect the detector's accurate measurement;

　　⑨ Since detection is performed at high temperatures, if the gas to be measured contains H2, CO, or CH4, these substances will react with oxygen, consuming some oxygen, reducing the oxygen concentration, and causing measurement errors. Therefore, this factor should be considered accordingly when the instrument measures gases containing flammable substances, to avoid inaccurate measurements.

　　⑩ When measuring corrosive gases, active carbon filtration should be used first.

　　For other technical questions regarding the Zirconia Oxygen Analyzer, you can call for consultation: 18225808093