Performance comparison of primary fluorescence method and polarographic type online Dissolved oxygen sensor
Online Dissolved oxygen sensor can be divided into three forms according to the principle, namely: primary battery type, polarographic type, and fluorescence method. Today we will introduce the polarographic type and fluorescence method, and compare them from the following aspects.
1. Working principles of dissolved oxygen sensor
(1) The working principle of polarographic dissolved oxygen sensor
Two electrodes made of different metals are immersed in the electrolyte. When a polarization voltage of 0.6-0.8V is applied to the electrodes of the dissolved oxygen sensor, oxygen diffuses through the membrane into the electrolyte and forms a measurement circuit with the gold and silver electrodes. The cathode releases electrons and the anode accepts electrons, creating an electric current. The resulting current (or voltage) is proportional to the concentration of dissolved oxygen entering the electrode system.
(2) The working principle of the fluorescent dissolved oxygen sensor
The fluorescence dissolved oxygen sensor is based on the principle of fluorescence quenching. The blue light irradiates the fluorescent material to excite the fluorescent material and emit red light. Since oxygen molecules can take away energy (quenching effect), the time and intensity of the excited red light are inversely proportional to the concentration of oxygen molecules. By measuring the phase difference between the excitation red light and the reference light and comparing it with the internal calibration value, the concentration of oxygen molecules can be calculated.
2. Dissolved oxygen online sensor electrode composition
- Electrode composition of polarographic: silver anode; precious metal cathode, covered with semi-permeable membrane; electrolyte.
- Electrode composition of fluorescent: sensor, optical element, electronic element, probe.
3. Comparison of application scope
- Polarographic application range: wide application range, can measure ppb trace level and ppm level.
- Scope of application of fluorescence: since the optical dissolved oxygen sensor adopts a nonlinear calculation method (the fluorescence quenching time interval and dissolved oxygen concentration have a nonlinear relationship), only single-point verification and process calibration can be performed; it is recommended to use the measurement range 0.05-20 mg/L.
4. Contrast of use conditions
- Polarographic type: there is a minimum flow rate requirement for the measurement medium.
- Fluorescent type: there is no minimum flow rate requirement for the measurement medium.
5. Comparison of interference factors
- Interference factors of polarographic: PH, medium conductivity, oil, sulfide, iron, ferrous, carbonate and algae in the medium; if the electrolyte is polluted, it will cause the battery potential to decrease and drift, resulting in measurement errors.
- Interference factors of fluorescent: strong anti-interference ability — except for high concentration of chlorine dioxide, it is not easy to be affected by other interfering substances; the probe should avoid direct sunlight; the fluorescent cap retains its accuracy in any condition when scratched or partially polluted and is easy to clean.
6. Comparison of maintenance amount
- Maintenance of polarographic sensor: a good electrode needs maintenance such as replenishing the electrolyte, replacing the diaphragm, cleaning the electrode, etc. for about half a year.
- Maintenance of fluorescent sensor: low maintenance amount, no need to calibrate, replace electrolyte and permeable membrane; replace the fluorescent cap every 2~3 years.
To sum up, it is recommended to choose a fluorescent dissolved oxygen sensor on site, which is not only easy to use, but also convenient. OPS supplies fluorescent dissolved oxygen sensors and portable dissolved oxygen meters at factory price!