Portable Dissolved Oxygen Meter Instructions
The electrode for the determination of dissolved oxygen consists of a membrane with a sensor attached and a built-in thermistor for temperature measurement and compensation. The permeable membrane of the electrode is a selective membrane, which separates the water sample to be tested from the sensor. Water and soluble substances cannot pass through, and only oxygen is allowed to pass through. When a voltage is supplied to the sensor, the oxygen passes through the film to undergo a reduction reaction, generating a weak diffusion current, and the dissolved oxygen concentration can be determined by measuring the current value.
Determination of Water Samples with Portable Dissolved Oxygen Meter
1. Electrode preparation
All newly purchased dissolved oxygen probes are dry, and must be filled with electrode filling solution before use, and then connected to the instrument.
The connection steps are as follows:
① Assemble the electrode according to the instrument manual.
② Add electrode filling solution to the electrode.
③ Gently spin the membrane onto the electrode.
④ Tap the edge of the electrode with your fingertips to ensure that there are no air bubbles in the electrode. To avoid damaging the film, do not tap the bottom of the film directly.
⑤ Make sure the rubber O-ring is correctly seated in the diaphragm cover.
⑥ With the sensor facing down, turn the membrane cover clockwise, and some electrolyte will overflow. When not in use, put on the supplied film protective cover.
2. Electrode polarization calibration process
The electrodes were polarized at a strength at a fixed voltage of approximately 800 mV. Electrode polarization is important to the reproducibility of the measurement results. With the electrodes properly polarized, the oxygen passing through the sensor membrane will dissolve in the electrolyte in the electrodes and be continuously consumed. If the polarization process is interrupted, the oxygen in the electrolyte will continue to increase until it reaches equilibrium with the dissolved oxygen in the external solution, if an unpolarized electrode is used, the measured value will be the sum of the dissolved oxygen in the external solution and the solute in the electrolyte , this result is wrong. When polarizing the electrode, cover the white plastic protective cap (remove during calibration and measurement).
① Press ON/OFF to turn on the instrument.
② The letter "COND" appears on the display, indicating that the electrode is automatically adjusted (polarized).
③ Wait for 20 minutes to ensure that the electrodes are stable.
④ The instrument will automatically polarize itself to an accurate saturation value. After about 1 minute, the display will display "100%" and small characters "SAMPLE", indicating that the polarization calibration has been completed. NOTE: Be sure to redo the polarization calibration whenever the electrode, membrane, or electrolyte is changed.
⑤ If you want to exit the calibration mode during the calibration process, press the CAL key again.
⑥ Press the RANGE key to switch the instrument from saturation percentage (%) to mg/L state (no need to re-calibrate).
3. Sample measurement
After the instrument is calibrated, immerse the electrode in the water sample to be tested, and at the same time ensure that the temperature sensing part is also immersed in the water sample. If you want to display the saturation percentage (%), press the RANGE key to switch to the saturation percentage (%) state. For accurate dissolved oxygen measurement, the minimum flow velocity of the water sample is required to be 0.3m/s, and the water flow will provide a proper circulation to ensure that the consumed oxygen is continuously replenished. Correct results cannot be obtained when the liquid is at rest. When performing field measurements, the electrodes can be shaken in parallel by hand. When measuring in the laboratory, it is recommended to use a magnetic stirrer to ensure that the water sample has a fixed flow rate (the electrodes of some instruments have a stirrer, just turn it on). This minimizes errors caused by oxygen in the air diffusing into the water sample. During each measurement, thermal equilibrium must be reached between the electrode and the water sample to be tested, and this process takes a certain amount of time (if the temperature difference is only a few degrees, it usually takes a few minutes).
Notes
1. In the mg/L state, the dissolved oxygen concentration can be directly read in mgm (ppm).
2. Oxygen saturation percentage reading (%) indicates the oxygen saturation ratio, taking the oxygen saturation percentage at 1 atmosphere as 100% reference.
3. Temperature reading: The lower right part of the display shows the temperature of the measured water sample. Before the measurement, the electrode must reach thermal equilibrium. Thermal equilibration generally takes a few minutes, and the greater the temperature difference between the environment and the sample, the longer it will take.
